Patent Application
20210065723
The present invention relates to audio coding/decoding and, particularly, to audio coding using Intelligent Gap Filling (IGF).
Audio coding is the domain of signal compression that deals with exploiting redundancy and irrelevancy in audio signals using psychoacoustic knowledge. Today audio codecs typically need around 60 kbps/channel for perceptually transparent coding of almost any type of audio signal. Newer codecs are aimed at reducing the coding bitrate by exploiting spectral similarities in the signal using techniques such as bandwidth extension (BWE). A BWE scheme uses a low bitrate parameter set to represent the high frequency (HF) components of an audio signal. The HF spectrum is filled up with spectral content from low frequency (LF) regions and the spectral shape, tilt and temporal continuity adjusted to maintain the timbre and color of the original signal. Such BWE methods enable audio codecs to retain good quality at even low bitrates of around 24 kbps/channel.
Storage or transmission of audio signals is often subject to strict bitrate constraints. In the past, coders were forced to drastically reduce the transmitted audio bandwidth when only a very low bitrate was available.
Modern audio codecs are nowadays able to code wide-band signals by using bandwidth extension (BWE) methods [1]. These algorithms rely on a parametric representation of the high-frequency content (HF)—which is generated from the waveform coded low-frequency part (LF) of the decoded signal by means of transposition into the HF spectral region (“patching”) and application of a parameter driven post processing. In BWE schemes, the reconstruction of the HF spectral region above a given so-called cross-over frequency is often based on spectral patching. Typically, the HF region is composed of multiple adjacent patches and each of these patches is sourced from band-pass (BP) regions of the LF spectrum below the given cross-over frequency. State-of-the-art systems efficiently perform the patching within a filterbank representation, e.g. Quadrature Mirror Filterbank (QMF), by copying a set of adjacent subband coefficients from a source to the target region.
Another technique found in today's audio codecs that increases compression efficiency and thereby enables extended audio bandwidth at low bitrates is the parameter driven synthetic replacement of suitable parts of the audio spectra. For example, noise-like signal portions of the original audio signal can be replaced without substantial loss of subjective quality by artificial noise generated in the decoder and scaled by side information parameters. One example is the Perceptual Noise Substitution (PNS) tool contained in MPEG-4 Advanced Audio Coding (AAC) [5].
A further provision that also enables extended audio bandwidth at low bitrates is the noise filling technique contained in MPEG-D Unified Speech and Audio Coding (USAC) [7]. Spectral gaps (zeroes) that are inferred by the dead-zone of the quantizer due to a too coarse quantization, are subsequently filled with artificial noise in the decoder and scaled by a parameter-driven post-processing.
Another state-of-the-art system is termed Accurate Spectral Replacement (ASR) [2-4]. In addition to a waveform codec, ASR employs a dedicated signal synthesis stage which restores perceptually important sinusoidal portions of the signal at the decoder. Also, a system described in [5] relies on sinusoidal modeling in the HF region of a waveform coder to enable extended audio bandwidth having decent perceptual quality at low bitrates. All these methods involve transformation of the data into a second domain apart from the Modified Discrete Cosine Transform (MDCT) and also fairly complex analysis/synthesis stages for the preservation of HF sinusoidal components.
Furthermore, even though the typical audio core coders operate in the spectral domain, the core decoder nevertheless generates a time domain signal which is then, again, converted into a spectral domain by the filter bank 1326 functionality. This introduces additional processing delays, may introduce artifacts due to tandem processing of firstly transforming from the spectral domain into the frequency domain and again transforming into typically a different frequency domain and, of course, this also may use a substantial amount of computation complexity and thereby electric power, which is specifically an issue when the bandwidth extension technology is applied in mobile devices such as mobile phones, tablet or laptop computers, etc.
Current audio codecs perform low bitrate audio coding using BWE as an integral part of the coding scheme. However, BWE techniques are restricted to replace high frequency (HF) content only. Furthermore, they do not allow perceptually important content above a given cross-over frequency to be waveform coded. Therefore, contemporary audio codecs either lose HF detail or timbre when the BWE is implemented, since the exact alignment of the tonal harmonics of the signal is not taken into consideration in most of the systems.
Another shortcoming of the current state of the art BWE systems is the need for transformation of the audio signal into a new domain for implementation of the BWE (e.g. transform from MDCT to QMF domain). This leads to complications of synchronization, additional computational complexity and increased memory requirements.
In case of two-channel pairs, there basically exist several channel representations such as a joint channel representation or a separate channel representation. A well-known joint representation is a mid/side representation where the mid channel is the sum of the left and right channel and where the side channel is the difference between the left and right channel.
Another representation is a downmix channel and a residual channel and an additional prediction coefficient that allows to recreate the left and right channel from the downmix and the residual. The separate representation would be, in this case, the separate channel left and right or generally, the first channel and the second channel.
Furthermore, a situation exists, where a source range for gap filling operations might show a strong correlation, while the target range does not show this strong correlation. When the source range is, in this embodiment, encoded using a first stereo representation such as a mid/side representation in order to reduce the bitrate for the core frequency portion, then a wrong two-channel image is generated for the reconstruction portion or target range. When, on the other hand, the source range does not show any correlation or only has a small correlation and the target range has a small correlation or no correlation, then again a straightforward gap filling operation would result in artifacts.
An embodiment may have an audio decoder for generating a decoded two-channel signal, the audio decoder including: a parametric decoder configured for providing parametric data for a second set of second spectral portions and configured for providing a two-channel identification for a second spectral portion of the second set of second spectral portions identifying either a first two-channel representation for the second spectral portion of the second set of second spectral portions or a second different two-channel representation for the second spectral portion of the second set of second spectral portions; and a frequency regenerator configured for regenerating the second spectral portion of the second set of second spectral portions depending on a first spectral portion of a first set of first spectral portions, the parametric data for the second spectral portion of the second set of second spectral portions and the two-channel identification for the second spectral portion of the second set of second spectral portions to acquire a regenerated second spectral portion of the second set of second spectral portions, wherein the decoded two-channel signal includes the regenerated second spectral portion of the second set of second spectral portions.
Another embodiment may have an audio encoder for encoding a two-channel audio signal to acquire an encoded two-channel audio signal, including: a spectral analyzer configured for providing an indication of a first set of first spectral portions of a spectral representation of the two-channel audio signal to be encoded with a first spectral resolution and an indication of a second set of second spectral portions of the spectral representation of the two-channel audio signal to be encoded with a second spectral resolution, the second spectral resolution being smaller than the first spectral resolution, the second set of second spectral portions being in a reconstruction band; a two-channel analyzer configured for analyzing the second spectral portions of the second set of second spectral portions of the spectral representation of the two-channel audio signal within the reconstruction band to determine a two-channel identification for each second spectral portion of the second set of second spectral portions either identifying a first two-channel representation for the second spectral portion of the second set of second spectral portions or a different second two-channel representation for the second spectral portion of the second set of second spectral portions to acquire the two-channel identifications for the second spectral portions of the second set of second spectral portions; a core encoder configured for encoding the first set of first spectral portions of the spectral representation of the two-channel audio signal using the first spectral resolution to provide an encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal; and a parameter encoder configured for parametrically encoding the second spectral portions of the second set of second spectral portions in the reconstruction band using the second spectral resolution, wherein the parameter encoder is configured for calculating parametric data on the second spectral portions of the second set of second spectral portions using either the first two-channel representation for the second spectral portion of the second set of second spectral portions or the second two-channel representation for the second spectral portion of the second set of second spectral portions as determined by the two-channel analyzer to acquire an encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, wherein the encoded two-channel audio signal includes the encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal, the encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, and the two-channel identifications for the second spectral portions of the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal.
According to another embodiment, a method of generating a decoded two-channel signal may have the steps of: providing parametric data for a second set of second spectral portions and providing a two-channel identification for a second spectral portion of the second set of second spectral portions identifying either a first two-channel representation for the second spectral portion of the second set of second spectral portions or a second different two-channel representation for the second spectral portion of the second set of second spectral portion; and regenerating the second spectral portion of the second set of second spectral portions depending on a first spectral portion of a first set of first spectral portions, the parametric data for the second spectral portion of the second set of second spectral portions and the two-channel identification for the second spectral portion of the second set of second spectral portions to acquire a regenerated second spectral portion of the second set of second spectral portions, wherein the decoded two-channel signal includes the regenerated second spectral portion of the second set of second spectral portions.
According to another embodiment, a method of encoding a two-channel audio signal to acquire an encoded two-channel audio signal may have the steps of: providing an indication of a first set of first spectral portions of a spectral representation of the two-channel audio signal to be encoded with a first spectral resolution and an indication of a second set of second spectral portions of the spectral representation of the two-channel audio signal to be encoded with a second spectral resolution, the second spectral resolution being smaller than the first spectral resolution, the second set of second spectral portions being in a reconstruction band; analyzing the second spectral portions of the second set of second spectral portions of the spectral representation of the two-channel audio signal within the reconstruction band to determine a two-channel identification for each second spectral portion of the second set of second spectral portions either identifying a first two-channel representation for the second spectral portion of the second set of second spectral portions or a different second two-channel representation for the second spectral portion of the second set of second spectral portions to acquire the two-channel identifications for the second spectral portions of the second set of second spectral portions; core encoding the first set of first spectral portions of the spectral representation of the two-channel audio signal using the first spectral resolution to provide an encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal; and parametrically encoding the second spectral portions of the second set of second spectral portions in the reconstruction band using the second spectral resolution, wherein the parametrically encoding includes calculating parametric data on the second spectral portions of the second set of second spectral portions using either the first two-channel representation for the second spectral portion of the second set of second spectral portions or the second two-channel representation for the second spectral portion of the second set of second spectral portions as determined by the analyzing to acquire an encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, wherein the encoded two-channel audio signal includes the encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal, the encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, and the two-channel identifications for the second spectral portions of the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal.
Another embodiment may have a non-transitory digital storage medium having a computer program stored thereon to perform, when the computer program is run by a computer, a method of generating a decoded two-channel signal, the method having the steps of: providing parametric data for a second set of second spectral portions and providing a two-channel identification for a second spectral portion of the second set of second spectral portions identifying either a first two-channel representation for the second spectral portion of the second set of second spectral portions or a second different two-channel representation for the second spectral portion of the second set of second spectral portion; and regenerating the second spectral portion of the second set of second spectral portions depending on a first spectral portion of a first set of first spectral portions, the parametric data for the second spectral portion of the second set of second spectral portions and the two-channel identification for the second spectral portion of the second set of second spectral portions to acquire a regenerated second spectral portion of the second set of second spectral portions, wherein the decoded two-channel signal includes the regenerated second spectral portion of the second set of second spectral portions.
Another embodiment may have a non-transitory digital storage medium having a computer program stored thereon to perform, when the computer program is run by a computer, a method of encoding a two-channel audio signal to acquire an encoded two-channel audio signal, the method having the steps of: providing an indication of a first set of first spectral portions of a spectral representation of the two-channel audio signal to be encoded with a first spectral resolution and an indication of a second set of second spectral portions of the spectral representation of the two-channel audio signal to be encoded with a second spectral resolution, the second spectral resolution being smaller than the first spectral resolution, the second set of second spectral portions being in a reconstruction band; analyzing the second spectral portions of the second set of second spectral portions of the spectral representation of the two-channel audio signal within the reconstruction band to determine a two-channel identification for each second spectral portion of the second set of second spectral portions either identifying a first two-channel representation for the second spectral portion of the second set of second spectral portions or a different second two-channel representation for the second spectral portion of the second set of second spectral portions to acquire the two-channel identifications for the second spectral portions of the second set of second spectral portions; core encoding the first set of first spectral portions of the spectral representation of the two-channel audio signal using the first spectral resolution to provide an encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal; and parametrically encoding the second spectral portions of the second set of second spectral portions in the reconstruction band using the second spectral resolution, wherein the parametrically encoding includes calculating parametric data on the second spectral portions of the second set of second spectral portions using either the first two-channel representation for the second spectral portion of the second set of second spectral portions or the second two-channel representation for the second spectral portion of the second set of second spectral portions as determined by the analyzing to acquire an encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, wherein the encoded two-channel audio signal includes the encoded core representation for the first set of first spectral portions of the spectral representation of the two-channel audio signal, the encoded parametric representation for the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal, and the two-channel identifications for the second spectral portions of the second set of second spectral portions in the reconstruction band of the spectral representation of the two-channel audio signal.
The present invention is based on the finding that the correlation situation is not only important for the source range but is also important for the target range. Furthermore, the present invention acknowledges the situation that different correlation situations can occur in the source range and the target range. When, for example, a speech signal with high frequency noise is considered, the situation can be that the low frequency band comprising the speech signal with a small number of overtones is highly correlated in the left channel and the right channel, when the speaker is placed in the middle. The high frequency portion, however, can be strongly uncorrelated due to the fact that there might be a different high frequency noise on the left side compared to another high frequency noise or no high frequency noise on the right side. Thus, when a straightforward gap filling operation would be performed that ignores this situation, then the high frequency portion would be correlated as well, and this might generate serious spatial segregation artifacts in the reconstructed signal. In order to address this issue, parametric data for a reconstruction band or, generally, for the second set of second spectral portions which have to be reconstructed using a first set of first spectral portions is calculated to identify either a first or a second different two-channel representation for the second spectral portion or, stated differently, for the reconstruction band. On the encoder side, a two-channel identification is, therefore calculated for the second spectral portions, i.e., for the portions, for which, additionally, energy information for reconstruction bands is calculated. A frequency regenerator on the decoder side then regenerates a second spectral portion depending on a first portion of the first set of first spectral portions, i.e., the source range and parametric data for the second portion such as spectral envelope energy information or any other spectral envelope data and, additionally, dependent on the two-channel identification for the second portion, i.e., for this reconstruction band under reconsideration.
The two-channel identification is advantageously transmitted as a flag for each reconstruction band and this data is transmitted from an encoder to a decoder and the decoder then decodes the core signal as indicated by advantageously calculated flags for the core bands. Then, in an implementation, the core signal is stored in both stereo representations (e.g. left/right and mid/side) and, for the IGF frequency tile filling, the source tile representation is chosen to fit the target tile representation as indicated by the two-channel identification flags for the intelligent gap filling or reconstruction bands, i.e., for the target range.
It is emphasized that this procedure not only works for stereo signals, i.e., for a left channel and the right channel but also operates for multi-channel signals. In the case of multi-channel signals, several pairs of different channels can be processed in that way such as a left and a right channel as a first pair, a left surround channel and a right surround as the second pair and a center channel and an LFE channel as the third pair. Other pairings can be determined for higher output channel formats such as 7.1, 11.1 and so on.
A further aspect is based on the finding that the problems related to the separation of the bandwidth extension on the one hand and the core coding on the other hand can be addressed and overcome by performing the bandwidth extension in the same spectral domain in which the core decoder operates. Therefore, a full rate core decoder is provided which encodes and decodes the full audio signal range. This does not require the need for a downsampler on the encoder side and an upsampler on the decoder side. Instead, the whole processing is performed in the full sampling rate or full bandwidth domain. In order to obtain a high coding gain, the audio signal is analyzed in order to find a first set of first spectral portions which has to be encoded with a high resolution, where this first set of first spectral portions may include, in an embodiment, tonal portions of the audio signal. On the other hand, non-tonal or noisy components in the audio signal constituting a second set of second spectral portions are parametrically encoded with low spectral resolution. The encoded audio signal then only may use the first set of first spectral portions encoded in a waveform-preserving manner with a high spectral resolution and, additionally, the second set of second spectral portions encoded parametrically with a low resolution using frequency “tiles” sourced from the first set. On the decoder side, the core decoder, which is a full band decoder, reconstructs the first set of first spectral portions in a waveform-preserving manner, i.e., without any knowledge that there is any additional frequency regeneration. However, the so generated spectrum has a lot of spectral gaps. These gaps are subsequently filled with the inventive Intelligent Gap Filling (IGF) technology by using a frequency regeneration applying parametric data on the one hand and using a source spectral range, i.e., first spectral portions reconstructed by the full rate audio decoder on the other hand.
In further embodiments, spectral portions, which are reconstructed by noise filling only rather than bandwidth replication or frequency tile filling, constitute a third set of third spectral portions. Due to the fact that the coding concept operates in a single domain for the core coding/decoding on the one hand and the frequency regeneration on the other hand, the IGF is not only restricted to fill up a higher frequency range but can fill up lower frequency ranges, either by noise filling without frequency regeneration or by frequency regeneration using a frequency tile at a different frequency range.
Furthermore, it is emphasized that an information on spectral energies, an information on individual energies or an individual energy information, an information on a survive energy or a survive energy information, an information a tile energy or a tile energy information, or an information on a missing energy or a missing energy information may comprise not only an energy value, but also an (e.g. absolute) amplitude value, a level value or any other value, from which a final energy value can be derived. Hence, the information on an energy may e.g. comprise the energy value itself, and/or a value of a level and/or of an amplitude and/or of an absolute amplitude.
A further aspect is based on the finding that the audio quality of the reconstructed signal can be improved through IGF since the whole spectrum is accessible to the core encoder so that, for example, perceptually important tonal portions in a high spectral range can still be encoded by the core coder rather than parametric substitution. Additionally, a gap filling operation using frequency tiles from a first set of first spectral portions which is, for example, a set of tonal portions typically from a lower frequency range, but also from a higher frequency range if available, is performed. For the spectral envelope adjustment on the decoder side, however, the spectral portions from the first set of spectral portions located in the reconstruction band are not further post-processed by e.g. the spectral envelope adjustment. Only the remaining spectral values in the reconstruction band which do not originate from the core decoder are to be envelope adjusted using envelope information. Advantageously, the envelope information is a full band envelope information accounting for the energy of the first set of first spectral portions in the reconstruction band and the second set of second spectral portions in the same reconstruction band, where the latter spectral values in the second set of second spectral portions are indicated to be zero and are, therefore, not encoded by the core encoder, but are parametrically coded with low resolution energy information.
It has been found that absolute energy values, either normalized with respect to the bandwidth of the corresponding band or not normalized, are useful and very efficient in an application on the decoder side. This especially applies when gain factors have to be calculated based on a residual energy in the reconstruction band, the missing energy in the reconstruction band and frequency tile information in the reconstruction band.
Furthermore, it is advantageous that the encoded bitstream not only covers energy information for the reconstruction bands but, additionally, scale factors for scale factor bands extending up to the maximum frequency. This ensures that for each reconstruction band, for which a certain tonal portion, i.e., a first spectral portion is available, this first set of first spectral portion can actually be decoded with the right amplitude. Furthermore, in addition to the scale factor for each reconstruction band, an energy for this reconstruction band is generated in an encoder and transmitted to a decoder. Furthermore, it is advantageous that the reconstruction bands coincide with the scale factor bands or in case of energy grouping, at least the borders of a reconstruction band coincide with borders of scale factor bands.
A further aspect is based on the finding that certain impairments in audio quality can be remedied by applying a signal adaptive frequency tile filling scheme. To this end, an analysis on the encoder-side is performed in order to find out the best matching source region candidate for a certain target region. A matching information identifying for a target region a certain source region together with optionally some additional information is generated and transmitted as side information to the decoder. The decoder then applies a frequency tile filling operation using the matching information. To this end, the decoder reads the matching information from the transmitted data stream or data file and accesses the source region identified for a certain reconstruction band and, if indicated in the matching information, additionally performs some processing of this source region data to generate raw spectral data for the reconstruction band. Then, this result of the frequency tile filling operation, i.e., the raw spectral data for the reconstruction band, is shaped using spectral envelope information in order to finally obtain a reconstruction band that comprises the first spectral portions such as tonal portions as well. These tonal portions, however, are not generated by the adaptive tile filling scheme, but these first spectral portions are output by the audio decoder or core decoder directly.
The adaptive spectral tile selection scheme may operate with a low granularity. In this implementation, a source region is subdivided into typically overlapping source regions and the target region or the reconstruction bands are given by non-overlapping frequency target regions. Then, similarities between each source region and each target region are determined on the encoder-side and the best matching pair of a source region and the target region are identified by the matching information and, on the decoder-side, the source region identified in the matching information is used for generating the raw spectral data for the reconstruction band.
For the purpose of obtaining a higher granularity, each source region is allowed to shift in order to obtain a certain lag where the similarities are maximum. This lag can be as fine as a frequency bin and allows an even better matching between a source region and the target region.
Furthermore, in addition of only identifying a best matching pair, this correlation lag can also be transmitted within the matching information and, additionally, even a sign can be transmitted. When the sign is determined to be negative on the encoder-side, then a corresponding sign flag is also transmitted within the matching information and, on the decoder-side, the source region spectral values are multiplied by “−1” or, in a complex representation, are “rotated” by 180 degrees.
A further implementation of this invention applies a tile whitening operation. Whitening of a spectrum removes the coarse spectral envelope information and emphasizes the spectral fine structure which is of foremost interest for evaluating tile similarity. Therefore, a frequency tile on the one hand and/or the source signal on the other hand are whitened before calculating a cross correlation measure. When only the tile is whitened using a predefined procedure, a whitening flag is transmitted indicating to the decoder that the same predefined whitening process shall be applied to the frequency tile within IGF.
Regarding the tile selection, it is advantageous to use the lag of the correlation to spectrally shift the regenerated spectrum by an integer number of transform bins. Depending on the underlying transform, the spectral shifting may use addition corrections. In case of odd lags, the tile is additionally modulated through multiplication by an alternating temporal sequence of −1/1 to compensate for the frequency-reversed representation of every other band within the MDCT. Furthermore, the sign of the correlation result is applied when generating the frequency tile.
Furthermore, it is advantageous to use tile pruning and stabilization in order to make sure that artifacts created by fast changing source regions for the same reconstruction region or target region are avoided. To this end, a similarity analysis among the different identified source regions is performed and when a source tile is similar to other source tiles with a similarity above a threshold, then this source tile can be dropped from the set of potential source tiles since it is highly correlated with other source tiles. Furthermore, as a kind of tile selection stabilization, it is advantageous to keep the tile order from the previous frame if none of the source tiles in the current frame correlate (better than a given threshold) with the target tiles in the current frame.
A further aspect is based on the finding that an improved quality and reduced bitrate specifically for signals comprising transient portions as they occur very often in audio signals is obtained by combining the Temporal Noise Shaping (TNS) or Temporal Tile Shaping (TTS) technology with high frequency reconstruction. The TNS/TTS processing on the encoder-side being implemented by a prediction over frequency reconstructs the time envelope of the audio signal. Depending on the implementation, i.e., when the temporal noise shaping filter is determined within a frequency range not only covering the source frequency range but also the target frequency range to be reconstructed in a frequency regeneration decoder, the temporal envelope is not only applied to the core audio signal up to a gap filling start frequency, but the temporal envelope is also applied to the spectral ranges of reconstructed second spectral portions. Thus, pre-echoes or post-echoes that would occur without temporal tile shaping are reduced or eliminated. This is accomplished by applying an inverse prediction over frequency not only within the core frequency range up to a certain gap filling start frequency but also within a frequency range above the core frequency range. To this end, the frequency regeneration or frequency tile generation is performed on the decoder-side before applying a prediction over frequency. However, the prediction over frequency can either be applied before or subsequent to spectral envelope shaping depending on whether the energy information calculation has been performed on the spectral residual values subsequent to filtering or to the (full) spectral values before envelope shaping.
The TTS processing over one or more frequency tiles additionally establishes a continuity of correlation between the source range and the reconstruction range or in two adjacent reconstruction ranges or frequency tiles.
In an implementation, it is advantageous to use complex TNS/TTS filtering. Thereby, the (temporal) aliasing artifacts of a critically sampled real representation, like MDCT, are avoided. A complex TNS filter can be calculated on the encoder-side by applying not only a modified discrete cosine transform but also a modified discrete sine transform in addition to obtain a complex modified transform. Nevertheless, only the modified discrete cosine transform values, i.e., the real part of the complex transform is transmitted. On the decoder-side, however, it is possible to estimate the imaginary part of the transform using MDCT spectra of preceding or subsequent frames so that, on the decoder-side, the complex filter can be again applied in the inverse prediction over frequency and, specifically, the prediction over the border between the source range and the reconstruction range and also over the border between frequency-adjacent frequency tiles within the reconstruction range.
The inventive audio coding system efficiently codes arbitrary audio signals at a wide range of bitrates. Whereas, for high bitrates, the inventive system converges to transparency, for low bitrates perceptual annoyance is minimized. Therefore, the main share of available bitrate is used to waveform code just the perceptually most relevant structure of the signal in the encoder, and the resulting spectral gaps are filled in the decoder with signal content that roughly approximates the original spectrum. A very limited bit budget is consumed to control the parameter driven so-called spectral Intelligent Gap Filling (IGF) by dedicated side information transmitted from the encoder to the decoder.
Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which:
Typically, a first spectral portion such as 306 of
The decoder further comprises a frequency regenerator 116 for regenerating a reconstructed second spectral portion having the first spectral resolution using a first spectral portion. The frequency regenerator 116 performs a tile filling operation, i.e., uses a tile or portion of the first set of first spectral portions and copies this first set of first spectral portions into the reconstruction range or reconstruction band having the second spectral portion and typically performs spectral envelope shaping or another operation as indicated by the decoded second representation output by the parametric decoder 114, i.e., by using the information on the second set of second spectral portions. The decoded first set of first spectral portions and the reconstructed second set of spectral portions as indicated at the output of the frequency regenerator 116 on line 117 is input into a spectrum-time converter 118 configured for converting the first decoded representation and the reconstructed second spectral portion into a time representation 119, the time representation having a certain high sampling rate.
The spectral analyzer/tonal mask 226 separates the output of TNS block 222 into the core band and the tonal components corresponding to the first set of first spectral portions 103 and the residual components corresponding to the second set of second spectral portions 105 of
Advantageously, the analysis filterbank 222 is implemented as an MDCT (modified discrete cosine transform filterbank) and the MDCT is used to transform the signal 99 into a time-frequency domain with the modified discrete cosine transform acting as the frequency analysis tool.
The spectral analyzer 226 advantageously applies a tonality mask. This tonality mask estimation stage is used to separate tonal components from the noise-like components in the signal. This allows the core coder 228 to code all tonal components with a psycho-acoustic module. The tonality mask estimation stage can be implemented in numerous different ways and is advantageously implemented similar in its functionality to the sinusoidal track estimation stage used in sine and noise-modeling for speech/audio coding [8, 9] or an HILN model based audio coder described in [10]. Advantageously, an implementation is used which is easy to implement without the need to maintain birth-death trajectories, but any other tonality or noise detector can be used as well.
The IGF module calculates the similarity that exists between a source region and a target region. The target region will be represented by the spectrum from the source region. The measure of similarity between the source and target regions is done using a cross-correlation approach. The target region is split into nTar non-overlapping frequency tiles. For every tile in the target region, nSrc source tiles are created from a fixed start frequency. These source tiles overlap by a factor between 0 and 1, where 0 means 0% overlap and 1 means 100% overlap. Each of these source tiles is correlated with the target tile at various lags to find the source tile that best matches the target tile. The best matching tile number is stored in tileNum[idx_tar], the lag at which it best correlates with the target is stored in xcorr_lag[idx_tar][idx_src] and the sign of the correlation is stored in xcorr_sign[idx_tar][idx_src]. In case the correlation is highly negative, the source tile needs to be multiplied by −1 before the tile filling process at the decoder. The IGF module also takes care of not overwriting the tonal components in the spectrum since the tonal components are preserved using the tonality mask. A band-wise energy parameter is used to store the energy of the target region enabling us to reconstruct the spectrum accurately.
This method has certain advantages over the classical SBR [1] in that the harmonic grid of a multi-tone signal is preserved by the core coder while only the gaps between the sinusoids is filled with the best matching “shaped noise” from the source region. Another advantage of this system compared to ASR (Accurate Spectral Replacement) [2-4] is the absence of a signal synthesis stage which creates the important portions of the signal at the decoder. Instead, this task is taken over by the core coder, enabling the preservation of important components of the spectrum. Another advantage of the proposed system is the continuous scalability that the features offer. Just using tileNum[idx_tar] and xcorr_lag=0, for every tile is called gross granularity matching and can be used for low bitrates while using variable xcorr_lag for every tile enables us to match the target and source spectra better.
In addition, a tile choice stabilization technique is proposed which removes frequency domain artifacts such as trilling and musical noise.
In case of stereo channel pairs an additional joint stereo processing is applied. This is useful because for a certain destination range the signal can a highly correlated panned sound source. In case the source regions chosen for this particular region are not well correlated, although the energies are matched for the destination regions, the spatial image can suffer due to the uncorrelated source regions. The encoder analyses each destination region energy band, typically performing a cross-correlation of the spectral values and if a certain threshold is exceeded, sets a joint flag for this energy band. In the decoder the left and right channel energy bands are treated individually if this joint stereo flag is not set. In case the joint stereo flag is set, both the energies and the patching are performed in the joint stereo domain. The joint stereo information for the IGF regions is signaled similar the joint stereo information for the core coding, including a flag indicating in case of prediction if the direction of the prediction is from downmix to residual or vice versa.
The energies can be calculated from the transmitted energies in the L/R-domain.
midNrg[k]=leftNrg[k]+rightNrg[k];
sideNrg[k]=leftNrg[k]−rightNrg[k];
with k being the frequency index in the transform domain.
Another solution is to calculate and transmit the energies directly in the joint stereo domain for bands where joint stereo is active, so no additional energy transformation is needed at the decoder side.
The source tiles are created according to the Mid/Side-Matrix:
midTile[k]=0.5·(leftTile[k]+rightTile[k])
sideTile[k]=0.5·(leftTile[k]−rightTile[k])
Energy adjustment:
midTile[k]=midTile[k]*midNrg[k];
sideTile[k]=sideTile[k]*sideNrg[k];
Joint stereo->LR transformation:
If no additional prediction parameter is coded:
leftTile[k]=midTile[k]+sideTile[k]
rightTile[k]=midTile[k]−sideTile[k]
If an additional prediction parameter is coded and if the signalled direction is from mid to side:
sideTile[k]=sideTile[k]−predictionCoeff·midTile[k]
leftTile[k]=midTile[k]+sideTile[k]
rightTile[k]=midTile[k]−sideTile[k]
If the signalled direction is from side to mid:
midTile1[k]=midTile[k]−prediction Coeff·sideTile[k]
leftTile[k]=mdTile1[k]−sideTile[k]
rightTile[k]=midTile1[k]+sideTile[k]
This processing ensures that from the tiles used for regenerating highly correlated destination regions and panned destination regions, the resulting left and right channels still represent a correlated and panned sound source even if the source regions are not correlated, preserving the stereo image for such regions.
In other words, in the bitstream, joint stereo flags are transmitted that indicate whether L/R or M/S as an example for the general joint stereo coding shall be used. In the decoder, first, the core signal is decoded as indicated by the joint stereo flags for the core bands. Second, the core signal is stored in both L/R and M/S representation. For the IGF tile filling, the source tile representation is chosen to fit the target tile representation as indicated by the joint stereo information for the IGF bands.
Temporal Noise Shaping (TNS) is a standard technique and part of AAC [11-13]. TNS can be considered as an extension of the basic scheme of a perceptual coder, inserting an optional processing step between the filterbank and the quantization stage. The main task of the TNS module is to hide the produced quantization noise in the temporal masking region of transient like signals and thus it leads to a more efficient coding scheme. First, TNS calculates a set of prediction coefficients using “forward prediction” in the transform domain, e.g. MDCT. These coefficients are then used for flattening the temporal envelope of the signal. As the quantization affects the TNS filtered spectrum, also the quantization noise is temporarily flat. By applying the invers TNS filtering on decoder side, the quantization noise is shaped according to the temporal envelope of the TNS filter and therefore the quantization noise gets masked by the transient.
IGF is based on an MDCT representation. For efficient coding, advantageously long blocks of approx. 20 ms have to be used. If the signal within such a long block contains transients, audible pre- and post-echoes occur in the IGF spectral bands due to the tile filling.
This pre-echo effect is reduced by using TNS in the IGF context. Here, TNS is used as a temporal tile shaping (TTS) tool as the spectral regeneration in the decoder is performed on the TNS residual signal. The TTS prediction coefficients that may be used are calculated and applied using the full spectrum on encoder side as usual. The TNS/TTS start and stop frequencies are not affected by the IGF start frequency fIGFstart of the IGF tool. In comparison to the legacy TNS, the TTS stop frequency is increased to the stop frequency of the IGF tool, which is higher than fIGFstart. On decoder side the TNS/TTS coefficients are applied on the full spectrum again, i.e. the core spectrum plus the regenerated spectrum plus the tonal components from the tonality map (see
In legacy decoders, spectral patching on an audio signal corrupts spectral correlation at the patch borders and thereby impairs the temporal envelope of the audio signal by introducing dispersion. Hence, another benefit of performing the IGF tile filling on the residual signal is that, after application of the shaping filter, tile borders are seamlessly correlated, resulting in a more faithful temporal reproduction of the signal.
In an inventive encoder, the spectrum having undergone TNS/TTS filtering, tonality mask processing and IGF parameter estimation is devoid of any signal above the IGF start frequency except for tonal components. This sparse spectrum is now coded by the core coder using principles of arithmetic coding and predictive coding. These coded components along with the signaling bits form the bitstream of the audio.
Advantageously, the high resolution is defined by a line-wise coding of spectral lines such as MDCT lines, while the second resolution or low resolution is defined by, for example, calculating only a single spectral value per scale factor band, where a scale factor band covers several frequency lines. Thus, the second low resolution is, with respect to its spectral resolution, much lower than the first or high resolution defined by the line-wise coding typically applied by the core encoder such as an AAC or USAC core encoder.
Regarding scale factor or energy calculation, the situation is illustrated in
Particularly, when the core encoder is under a low bitrate condition, an additional noise-filling operation in the core band, i.e., lower in frequency than the IGF start frequency, i.e., in scale factor bands SCB1 to SCB3 can be applied in addition. In noise-filling, there exist several adjacent spectral lines which have been quantized to zero. On the decoder-side, these quantized to zero spectral values are re-synthesized and the re-synthesized spectral values are adjusted in their magnitude using a noise-filling energy such as NF2 illustrated at 308 in
Advantageously, the bands, for which energy information is calculated coincide with the scale factor bands. In other embodiments, an energy information value grouping is applied so that, for example, for scale factor bands 4 and 5, only a single energy information value is transmitted, but even in this embodiment, the borders of the grouped reconstruction bands coincide with borders of the scale factor bands. If different band separations are applied, then certain re-calculations or synchronization calculations may be applied, and this can make sense depending on the certain implementation.
Advantageously, the spectral domain encoder 106 of
In the audio encoder of
Then, at the output of block 422, a quantized spectrum is obtained corresponding to what is illustrated in
The set to zero blocks 410, 418, 422, which are provided alternatively to each other or in parallel are controlled by the spectral analyzer 424. The spectral analyzer advantageously comprises any implementation of a well-known tonality detector or comprises any different kind of detector operative for separating a spectrum into components to be encoded with a high resolution and components to be encoded with a low resolution. Other such algorithms implemented in the spectral analyzer can be a voice activity detector, a noise detector, a speech detector or any other detector deciding, depending on spectral information or associated metadata on the resolution requirements for different spectral portions.
Subsequently, reference is made to
As illustrated at 301 in
Advantageously, an IGF operation, i.e., a frequency tile filling operation using spectral values from other portions can be applied in the complete spectrum. Thus, a spectral tile filling operation can not only be applied in the high band above an IGF start frequency but can also be applied in the low band. Furthermore, the noise-filling without frequency tile filling can also be applied not only below the IGF start frequency but also above the IGF start frequency. It has, however, been found that high quality and high efficient audio encoding can be obtained when the noise-filling operation is limited to the frequency range below the IGF start frequency and when the frequency tile filling operation is restricted to the frequency range above the IGF start frequency as illustrated in
Advantageously, the target tiles (TT) (having frequencies greater than the IGF start frequency) are bound to scale factor band borders of the full rate coder. Source tiles (ST), from which information is taken, i.e., for frequencies lower than the IGF start frequency are not bound by scale factor band borders. The size of the ST should correspond to the size of the associated TT. This is illustrated using the following example. TT[0] has a length of 10 MDCT Bins. This exactly corresponds to the length of two subsequent SCBs (such as 4+6). Then, all possible ST that are to be correlated with TT[0], have a length of 10 bins, too. A second target tile IT[1] being adjacent to TT[0] has a length of 15 bins I (SCB having a length of 7+8). Then, the ST for that have a length of 15 bins rather than 10 bins as for TT[0].
Should the case arise that one cannot find a TT for an ST with the length of the target tile (when e.g. the length of TT is greater than the available source range), then a correlation is not calculated and the source range is copied a number of times into this TT (the copying is done one after the other so that a frequency line for the lowest frequency of the second copy immediately follows—in frequency—the frequency line for the highest frequency of the first copy), until the target tile TT is completely filled up.
Subsequently, reference is made to
Then, the first spectral portion of the reconstruction band such as 307 of
In this context, it is very important to evaluate the high frequency reconstruction accuracy of the present invention compared to HE-AAC. This is explained with respect to scale factor band 7 in
In an implementation, the spectral analyzer is also implemented to calculating similarities between first spectral portions and second spectral portions and to determine, based on the calculated similarities, for a second spectral portion in a reconstruction range a first spectral portion matching with the second spectral portion as far as possible. Then, in this variable source range/destination range implementation, the parametric coder will additionally introduce into the second encoded representation a matching information indicating for each destination range a matching source range. On the decoder-side, this information would then be used by a frequency tile generator 522 of
Furthermore, as illustrated in
As illustrated, the encoder operates without downsampling and the decoder operates without upsampling. In other words, the spectral domain audio coder is configured to generate a spectral representation having a Nyquist frequency defined by the sampling rate of the originally input audio signal.
Furthermore, as illustrated in
As outlined, the spectral domain audio decoder 112 is configured so that a maximum frequency represented by a spectral value in the first decoded representation is equal to a maximum frequency included in the time representation having the sampling rate wherein the spectral value for the maximum frequency in the first set of first spectral portions is zero or different from zero. Anyway, for this maximum frequency in the first set of spectral components a scale factor for the scale factor band exists, which is generated and transmitted irrespective of whether all spectral values in this scale factor band are set to zero or not as discussed in the context of
The invention is, therefore, advantageous that with respect to other parametric techniques to increase compression efficiency, e.g. noise substitution and noise filling (these techniques are exclusively for efficient representation of noise like local signal content) the invention allows an accurate frequency reproduction of tonal components. To date, no state-of-the-art technique addresses the efficient parametric representation of arbitrary signal content by spectral gap filling without the restriction of a fixed a-priory division in low band (LF) and high band (HF).
Embodiments of the inventive system improve the state-of-the-art approaches and thereby provides high compression efficiency, no or only a small perceptual annoyance and full audio bandwidth even for low bitrates.
The general system consists of
A first step towards a more efficient system is to remove the need for transforming spectral data into a second transform domain different from the one of the core coder. As the majority of audio codecs, such as AAC for instance, use the MDCT as basic transform, it is useful to perform the BWE in the MDCT domain also. A second requirement for the BWE system would be the need to preserve the tonal grid whereby even HF tonal components are preserved and the quality of the coded audio is thus superior to the existing systems. To take care of both the above mentioned requirements for a BWE scheme, a new system is proposed called Intelligent Gap Filling (IGF).
In
Advantageously, a complex valued TNS filter or TTS filter is calculated. This is illustrated in
On the decoder-side, the encoded data is input into a demultiplexer 720 to separate IGF side information on the one hand, TTS side information on the other hand and the encoded representation of the first set of first spectral portions.
Then, block 724 is used for calculating a complex spectrum from one or more real-valued spectra. Then, both the real-valued and the complex spectra are input into block 726 to generate reconstructed frequency values in the second set of second spectral portions for a reconstruction band. Then, on the completely obtained and tile filled full band frame, the inverse TTS operation 728 is performed and, on the decoder-side, a final inverse complex MDCT operation is performed in block 730. Thus, the usage of complex TNS filter information allows, when being applied not only within the core band or within the separate tile bands but being applied over the core/tile borders or the tile/tile borders automatically generates a tile border processing, which, in the end, reintroduces a spectral correlation between tiles. This spectral correlation over tile borders is not obtained by only generating frequency tiles and performing a spectral envelope adjustment on this raw data of the frequency tiles.
Embodiments or the inventive audio coding system use the main share of available bitrate to waveform code only the perceptually most relevant structure of the signal in the encoder, and the resulting spectral gaps are filled in the decoder with signal content that roughly approximates the original spectrum. A very limited bit budget is consumed to control the parameter driven so-called spectral Intelligent Gap Filling (IGF) by dedicated side information transmitted from the encoder to the decoder.
Storage or transmission of audio signals is often subject to strict bitrate constraints. In the past, coders were forced to drastically reduce the transmitted audio bandwidth when only a very low bitrate was available. Modern audio codecs are nowadays able to code wide-band signals by using bandwidth extension (BWE) methods like Spectral Bandwidth Replication (SBR) [1]. These algorithms rely on a parametric representation of the high-frequency content (HF)—which is generated from the waveform coded low-frequency part (LF) of the decoded signal by means of transposition into the HF spectral region (“patching”) and application of a parameter driven post processing. In BWE schemes, the reconstruction of the HF spectral region above a given so-called cross-over frequency is often based on spectral patching. Typically, the HF region is composed of multiple adjacent patches and each of these patches is sourced from band-pass (BP) regions of the LF spectrum below the given cross-over frequency. State-of-the-art systems efficiently perform the patching within a filterbank representation by copying a set of adjacent subband coefficients from a source to the target region.
If a BWE system is implemented in a filterbank or time-frequency transform domain, there is only a limited possibility to control the temporal shape of the bandwidth extension signal. Typically, the temporal granularity is limited by the hop-size used between adjacent transform windows. This can lead to unwanted pre- or post-echoes in the BWE spectral range.
From perceptual audio coding, it is known that the shape of the temporal envelope of an audio signal can be restored by using spectral filtering techniques like Temporal Envelope Shaping (TNS) [14]. However, the TNS filter known from state-of-the-art is a real-valued filter on real-valued spectra. Such a real-valued filter on real-valued spectra can be seriously impaired by aliasing artifacts, especially if the underlying real transform is a Modified Discrete Cosine Transform (MDCT).
The temporal envelope tile shaping applies complex filtering on complex-valued spectra, like obtained from e.g. a Complex Modified Discrete Cosine Transform (CMDCT). Thereby, aliasing artifacts are avoided.
The temporal tile shaping consists of
The invention extends state-of-the-art technique known from audio transform coding, specifically Temporal Noise Shaping (TNS) by linear prediction along frequency direction, for the use in a modified manner in the context of bandwidth extension.
Further, the inventive bandwidth extension algorithm is based on Intelligent Gap Filling (IGF), but employs an oversampled, complex-valued transform (CMDCT), as opposed to the IGF standard configuration that relies on a real-valued critically sampled MDCT representation of a signal. The CMDCT can be seen as the combination of the MDCT coefficients in the real part and the MDST coefficients in the imaginary part of each complex-valued spectral coefficient.
Although the new approach is described in the context of IGF, the inventive processing can be used in combination with any BWE method that is based on a filter bank representation of the audio signal.
In this novel context, linear prediction along frequency direction is not used as temporal noise shaping, but rather as a temporal tile shaping (TTS) technique. The renaming is justified by the fact that tile filled signal components are temporally shaped by TTS as opposed to the quantization noise shaping by TNS in state-of-the-art perceptual transform codecs.
So the basic encoding scheme works as follows:
Here, the basic decoding scheme works as follows:
Note that, alternatively, the order of TTS synthesis and IGF post-processing can also be reversed in the decoder if TTS analysis and IGF parameter estimation are consistently reversed in the encoder.
For efficient transform coding, advantageously so-called “long blocks” of approx. 20 ms have to be used to achieve reasonable transform gain. If the signal within such a long block contains transients, audible pre- and post-echoes occur in the reconstructed spectral bands due to tile filling.
The main task of the TTS module is to confine these unwanted signal components in close vicinity around a transient and thereby hide them in the temporal region governed by the temporal masking effect of human perception. Therefore, the TTS prediction coefficients that may be used are calculated and applied using “forward prediction” in the CMDCT domain.
In an embodiment that combines TTS and IGF into a codec it is important to align certain TTS parameters and IGF parameters such that an IGF tile is either entirely filtered by one TTS filter (flattening or shaping filter) or not. Therefore, all TTSstart[ . . . ] or TTSstop[ . . . ] frequencies shall not be comprised within an IGF tile, but rather be aligned to the respective fIGF . . . frequencies.
The TTS stop frequency is adjusted to the stop frequency of the IGF tool, which is higher than fIGFstart. If TTS uses more than one filter, it has to be ensured that the cross-over frequency between two TTS filters has to match the IGF split frequency. Otherwise, one TTS sub-filter will run over fIGFstart resulting in unwanted artifacts like over-shaping.
In the implementation variant depicted in
In the alternative implementation variant, the order of IGF post-processing and TTS is reversed. In the decoder, this means that the energy adjustment by IGF post-processing is calculated subsequent to TTS filtering and thereby is the final processing step before the synthesis transform. Therefore, regardless of different TTS filter gains being applied to one tile during coding, the final energy is adjusted correctly by the IGF processing.
On decoder-side, the TTS filter coefficients are applied on the full spectrum again, i.e. the core spectrum extended by the regenerated spectrum. The application of the TTS may be used for forming the temporal envelope of the regenerated spectrum to match the envelope of the original signal again. So the shown pre-echoes are reduced. In addition, it still temporally shapes the quantization noise in the signal below fIGFstart as usual with legacy TNS.
In legacy coders, spectral patching on an audio signal (e.g. SBR) corrupts spectral correlation at the patch borders and thereby impairs the temporal envelope of the audio signal by introducing dispersion. Hence, another benefit of performing the IGF tile filling on the residual signal is that, after application of the TTS shaping filter, tile borders are seamlessly correlated, resulting in a more faithful temporal reproduction of the signal.
The result of the accordingly processed signal is shown in
Furthermore, as discussed,
Finally, as illustrated in
Advantageously, the prediction filter 704 comprises a filter information calculator configured for using the spectral values of the spectral representation for calculating the filter information. Furthermore, the prediction filter is configured for calculating the spectral residual values using the same spectral values of the spectral representation used for calculating the filter information.
Advantageously, the TTS filter 704 is configured in the same way as known for conventional-technology audio encoders applying the TNS tool in accordance with the AAC standard.
Subsequently, a further implementation using two-channel decoding is discussed in the context of
In an implementation, the first two-channel representation can be a left/right (UR) representation and the second two-channel representation is a joint stereo representation. However, other two-channel representations apart from left/right or M/S or stereo prediction can be applied and used for the present invention.
It is emphasized that the two channels of the two-channel representation can be two stereo channels such as the left channel and the right channel. However, the signal can also be a multi-channel signal having, for example, five channels and a sub-woofer channel or having even more channels. Then, a pair-wise two-channel processing as discussed in the context of
Then, block 836 applies a frequency tile generation using, as in input, a source range ID and additionally using as an input a two-channel ID for the target range. Based on the two-channel ID for the target range, the frequency tile generator accesses the storage 834 and receives the two-channel representation of the source range matching with the two-channel ID for the target range input into the frequency tile generator at 835. Thus, when the two-channel ID for the target range indicates joint stereo processing, then the frequency tile generator 836 accesses the storage 834 in order to obtain the joint stereo representation of the source range indicated by the source range ID 833.
The frequency tile generator 836 performs this operation for each target range and the output of the frequency tile generator is so that each channel of the channel representation identified by the two-channel identification is present. Then, an envelope adjustment by an envelope adjuster 838 is performed. The envelope adjustment is performed in the two-channel domain identified by the two-channel identification. To this end, envelope adjustment parameters may be used and these parameters are either transmitted from the encoder to the decoder in the same two-channel representation as described. When, the two-channel identification in the target range to be processed by the envelope adjuster has a two-channel identification indicating a different two-channel representation than the envelope data for this target range, then a parameter transformer 840 transforms the envelope parameters into the two-channel representation that may be used. When, for example, the two-channel identification for one band indicates joint stereo coding and when the parameters for this target range have been transmitted as L/R envelope parameters, then the parameter transformer calculates the joint stereo envelope parameters from the L/R envelope parameters as described so that the correct parametric representation is used for the spectral envelope adjustment of a target range.
In another advantageous embodiment the envelope parameters are already transmitted as joint stereo parameters when joint stereo is used in a target band.
When it is assumed that the input into the envelope adjuster 838 is a set of target ranges having different two-channel representations, then the output of the envelope adjuster 838 is a set of target ranges in different two-channel representations as well. When, a target range has a joined representation such as M/S, then this target range is processed by a representation transformer 842 for calculating the separate representation that may be used for a storage or transmission to loudspeakers. When, however, a target range already has a separate representation, signal flow 844 is taken and the representation transformer 842 is bypassed. At the output of block 842, a two-channel spectral representation being a separate two-channel representation is obtained which can then be further processed as indicated by block 846, where this further processing may, for example, be a frequency/time conversion or any other processing that may be used.
Advantageously, the second spectral portions correspond to frequency bands, and the two-channel identification is provided as an array of flags corresponding to the table of
In an embodiment, only the reconstruction range starting with the IGF start frequency 309 of
In a further embodiment, the source band identification and the target band identification can be adaptively determined by a similarity analysis. However, the inventive two-channel processing can also be applied when there is a fixed association of a source range to a target range. A source range can be used for recreating a, with respect to frequency, broader target range either by a harmonic frequency tile filling operation or a copy-up frequency tile filling operation using two or more frequency tile filling operations similar to the processing for multiple patches known from high efficiency AAC processing.
Furthermore, a two-channel analyzer 864 is provided for analyzing the second set of second spectral portions to determine a two-channel identification identifying either a first two-channel representation or a second two-channel representation.
Depending on the result of the two-channel analyzer, a band in the second spectral representation is either parameterized using the first two-channel representation or the second two-channel representation, and this is performed by a parameter encoder 868. The core frequency range, i.e., the frequency band below the IGF start frequency 309 of
Furthermore, it is advantageous that the audio encoder comprises a bandwise transformer 862. Based on the decision of the two-channel analyzer 862, the output signal of the time spectrum converter 862 is transformed into a representation indicated by the two-channel analyzer and, particularly, by the two-channel ID 835. Thus, an output of the bandwise transformer 862 is a set of frequency bands where each frequency band can either be in the first two-channel representation or the second different two-channel representation. When the present invention is applied in full band, i.e., when the source range and the reconstruction range are both processed by the bandwise transformer, the spectral analyzer 860 can analyze this representation. Alternatively, however, the spectral analyzer 860 can also analyze the signal output by the time spectrum converter as indicated by control line 861. Thus, the spectral analyzer 860 can either apply the advantageous tonality analysis on the output of the bandwise transformer 862 or the output of the time spectrum converter 860 before having been processed by the bandwise transformer 862. Furthermore, the spectral analyzer can apply the identification of the best matching source range for a certain target range either on the result of the bandwise transformer 862 or on the result of the time-spectrum converter 860.
Subsequently, reference is made to
Modern state of the art audio coders apply various techniques to minimize the amount of data representing a given audio signal. Audio coders like USAC [1] apply a time to frequency transformation like the MDCT to get a spectral representation of a given audio signal. These MDCT coefficients are quantized exploiting the psychoacoustic aspects of the human hearing system. If the available bitrate is decreased the quantization gets coarser introducing large numbers of zeroed spectral values which lead to audible artifacts at the decoder side. To improve the perceptual quality, state of the art decoders fill these zeroed spectral parts with random noise. The IGF method harvests tiles from the remaining non zero signal to fill those gaps in the spectrum. It is crucial for the perceptual quality of the decoded audio signal that the spectral envelope and the energy distribution of spectral coefficients are preserved. The energy adjustment method presented here uses transmitted side information to reconstruct the spectral MDCT envelope of the audio signal.
Within eSBR [15] the audio signal is downsampled at least by a factor of two and the high frequency part of the spectrum is completely zeroed out [1, 17]. This deleted part is replaced by parametric techniques, eSBR, on the decoder side. eSBR implies the usage of an additional transform, the QMF transformation which is used to replace the empty high frequency part and to resample the audio signal [17]. This adds both computational complexity and memory consumption to an audio coder.
The USAC coder [15] offers the possibility to fill spectral holes (zeroed spectral lines) with random noise but has the following downsides: random noise cannot preserve the temporal fine structure of a transient signal and it cannot preserve the harmonic structure of a tonal signal.
The area where eSBR operates on the decoder side was completely deleted by the encoder [1]. Therefore eSBR is prone to delete tonal lines in high frequency region or distort harmonic structures of the original signal. As the QMF frequency resolution of eSBR is very low and reinsertion of sinusoidal components is only possible in the coarse resolution of the underlying filterbank, the regeneration of tonal components in eSBR in the replicated frequency range has very low precision.
eSBR uses techniques to adjust energies of patched areas, the spectral envelope adjustment [1]. This technique uses transmitted energy values on a QMF frequency time grid to reshape the spectral envelope. This state of the art technique does not handle partly deleted spectra and because of the high time resolution it is either prone to need a relatively large amount of bits to transmit appropriate energy values or to apply a coarse quantization to the energy values.
The method of IGF does not need an additional transformation as it uses the legacy MDCT transformation which is calculated as described in [15].
The energy adjustment method presented here uses side information generated by the encoder to reconstruct the spectral envelope of the audio signal. This side information is generated by the encoder as outlined below:
fIGFstart and FIGFstop are user given parameters.
The calculated values from step c) and d) are lossless encoded and transmitted as side information with the bit stream to the decoder.
The decoder receives the transmitted values and uses them to adjust the spectral envelope.
Let {circumflex over (x)}∈N be the MDCT transformed, real valued spectral representation of a windowed audio signal of window-length 2N. This transformation is described in [16]. The encoder optionally applies TNS on {circumflex over (x)}.
In [16, 4.6.2] a partition of {circumflex over (x)} in scale-factor bands is described. Scale-factor bands are a set of a set of indices and are denoted in this text with scb.
The limits of each scbk with k=0, 1, 2, . . . max_sfb are defined by an array swb_offset (16, 4.6.2), where swb_offset[k] and swb_offset[k+1]−1 define first and last index for the lowest and highest spectral coefficient line contained in scbk. We denote the scale-factor band
scb
k:={swb_offset[k],1+swb_offset[k],2+swb_offset[k], . . . ,swb_offset[k+1]−1}
If the IGF tool is used by the encoder, the user defines an IGF start frequency and an IGF stop frequency. These two values are mapped to the best fitting scale-factor band index igfStartSfb and igfStopSfb. Both are signaled in the bit stream to the decoder.
[16] describes both a long block and short block transformation. For long blocks only one set of spectral coefficients together with one set of scale-factors is transmitted to the decoder. For short blocks eight short windows with eight different sets of spectral coefficients are calculated. To save bitrate, the scale-factors of those eight short block windows are grouped by the encoder.
In case of IGF the method presented here uses legacy scale factor bands to group spectral values which are transmitted to the decoder:
Where k=igfStartSfb, 1+igfStartSfb, 2+igfStartSfb, . . . , igfEndSfb.
For quantizing
Ê
k
=nINT(4 log2(Ek))
is calculated. All values Êk are transmitted to the decoder.
We assume that the encoder decides to group num_window_group scale-factor sets.
We denote with w this grouping-partition of the set {0, 1, 2, . . . , 7} which are the indices of the eight short windows. wl denotes the l-th subset of w, where l denotes the index of the window group, 0≤l<num_window_group.
For short block calculation the user defined IGF start/stop frequency is mapped to appropriate scale-factor bands. However, for simplicity one denotes for short blocks k=igfStartSfb, 1+igfStartSfb, 2+igfStartSfb, . . . , igfEndSfb as well.
The IGF energy calculation uses the grouping information to group the values Ek,l:
For quantizing
Ê
k,l
=nINT(4 log2(Ek,l))
is calculated. All values Êk,l are transmitted to the decoder.
The above-mentioned encoding formulas operate using only real-valued MDCT coefficients {circumflex over (x)}. To obtain a more stable energy distribution in the IGF range, that is, to reduce temporal amplitude fluctuations, an alternative method can be used to calculate the values Ek:
Let {circumflex over (x)}r∈N be the MDCT transformed, real valued spectral representation of a windowed audio signal of window-length 2N, and {circumflex over (x)}i∈
N the real valued MDST transformed spectral representation of the same portion of the audio signal. The MDST spectral representation {circumflex over (x)}i could be either calculated exactly or estimated from {circumflex over (x)}r. ĉ:=({circumflex over (x)}r,{circumflex over (x)}i)∈
N denotes the complex spectral representation of the windowed audio signal, having {circumflex over (x)}r as its real part and {circumflex over (x)}i as its imaginary part. The encoder optionally applies TNS on {circumflex over (x)}r and {circumflex over (x)}i.
Now the energy of the original signal in the IGF range can be measured with
The real- and complex-valued energies of the reconstruction band, that is, the tile which should be used on the decoder side in the reconstruction of the IGF range scbk, is calculated with:
where trk is a set of indices—the associated source tile range, in dependency of scbk. In the two formulae above, instead of the index set scbk, the set
Calculate
now a more stable version of Ek is calculated, since a calculation of Ek with MDCT values only is impaired by the fact that MDCT values do not obey Parseval's theorem, and therefore they do not reflect the complete energy information of spectral values. Êk is calculated as above.
As noted earlier, for short blocks we assume that the encoder decides to group num_window_group scale-factor sets. As above, wl denotes the l-th subset of w, where l denotes the index of the window group, 0≤l<num_window_group.
Again, the alternative version outlined above to calculate a more stable version of Ek,l could be calculated. With the defines of ĉ:=({circumflex over (x)}r,{circumflex over (x)}j)∈N, {circumflex over (x)}r∈
N being the MDCT transformed and {circumflex over (x)}i∈
N being the MDST transformed windowed audio signal of length 2N, calculate
Analogously calculate
and proceed with the factor fk,l
which is used to adjust the previously calculated Erk,l:
E
k,l=√{square root over (fk,lErk,l)}
Ek,l is calculated as above.
The procedure of not only using the energy of the reconstruction band either derived from the complex reconstruction band or from the MDCT values, but also using an energy information from the source range provides an improver energy reconstruction.
Specifically, the parameter calculator 1006 is configured to calculate the energy information for the reconstruction band using information on the energy of the reconstruction band and additionally using information on an energy of a source range to be used for reconstructing the reconstruction band.
Furthermore, the parameter calculator 1006 is configured to calculate an energy information (Eok) on the reconstruction band of a complex spectrum of the original signal, to calculate a further energy information (Erk) on a source range of a real valued part of the complex spectrum of the original signal to be used for reconstructing the reconstruction band, and wherein the parameter calculator is configured to calculate the energy information for the reconstruction band using the energy information (Eok) and the further energy information (Erk).
Furthermore, the parameter calculator 1006 is configured for determining a first energy information (Eok) on a to be reconstructed scale factor band of a complex spectrum of the original signal, for determining a second energy information (Etk) on a source range of the complex spectrum of the original signal to be used for reconstructing the to be reconstructed scale factor band, for determining a third energy information (Erk) on a source range of a real valued part of the complex spectrum of the original signal to be used for reconstructing the to be reconstructed scale factor band, for determining a weighting information based on a relation between at least two of the first energy information, the second energy information, and the third energy information, and for weighting one of the first energy information and the third energy information using the weighting information to obtain a weighted energy information and for using the weighted energy information as the energy information for the reconstruction band.
Examples for the calculations are the following, but many other may appear to those skilled in the art in view of the above general principle:
All these examples acknowledge the fact that although only real MDCT values are processed on the decoder side, the actual calculation is—due to the overlap and add—of the time domain aliasing cancellation procedure implicitly made using complex numbers. However, particularly the determination 918 of the tile energy information of the further spectral portions 922, 923 of the reconstruction band 920 for frequency values different from the first spectral portion 921 having frequencies in the reconstruction band 920 relies on real MDCT values. Hence, the energy information transmitted to the decoder will typically be smaller than the energy information Eok on the reconstruction band of the complex spectrum of the original signal. For example for case C above, this means that the factor f_k (weighting information) will be smaller than 1.
On the decoder side, if the IGF tool is signaled as ON, the transmitted values Ek are obtained from the bit stream and shall be dequantized with
for all k=igfStartSfb, 1+igfStartSfb, 2+igfStartSfb, . . . , igfEndSfb.
A decoder dequantizes the transmitted MDCT values to x∈N and calculates the remaining survive energy:
where k is in the range as defined above.
We denote
The IGF get subband method (not described here) is used to fill spectral gaps resulting from a coarse quantization of MDCT spectral values at encoder side by using non zero values of the transmitted MDCT. x will additionally contain values which replace all previous zeroed values. The tile energy is calculated by:
where k is in the range as defined above.
The energy missing in the reconstruction band is calculated by:
mE
k
:=|scb
k
|E
k
2
−sE
k
And the gain factor for adjustment is obtained by:
The spectral envelope adjustment using the gain factor is:
x
i
:=g′x
i
for all i∈
This reshapes the spectral envelope of x to the shape of the original spectral envelope {circumflex over (x)}.
With short window sequence all calculations as outlined above stay in principle the same, but the grouping of scale-factor bands are taken into account. We denote as Ek,l the dequantized, grouped energy values obtained from the bit stream. Calculate
The index j describes the window index of the short block sequence.
For low bitrate applications a pairwise grouping of the values Ek is possible without losing too much precision. This method is applied only with long blocks:
where k=igfStartSfb, 2+igfStartSfb, 4+igfStartSfb, . . . , igfEndSfb.
Again, after quantizing all values Ek>>1 are transmitted to the decoder.
The frequency regenerator 906 further comprises a calculator 914 for a missing energy in the reconstruction band, and the calculator 914 operates using the individual energy for the reconstruction band and the survive energy generated by block 912. Furthermore, the frequency regenerator 906 comprises a spectral envelope adjuster 916 for adjusting the further spectral portions in the reconstruction band based on the missing energy information and the tile energy information generated by block 918.
Reference is made to
Subsequently, a certain example with real numbers is discussed. The remaining survive energy as calculated by block 912 is, for example, five energy units and this energy is the energy of the exemplarily indicated four spectral lines in the first spectral portion 921.
Furthermore, the energy value E3 for the reconstruction band corresponding to scale factor band 6 of
Based on the missing energy divided by the tile energy tEk, a gain factor of 0.79 is calculated. Then, the raw spectral lines for the second spectral portions 922, 923 are multiplied by the calculated gain factor. Thus, only the spectral values for the second spectral portions 922, 923 are adjusted and the spectral lines for the first spectral portion 921 are not influenced by this envelope adjustment. Subsequent to multiplying the raw spectral values for the second spectral portions 922, 923, a complete reconstruction band has been calculated consisting of the first spectral portions in the reconstruction band, and consisting of spectral lines in the second spectral portions 922, 923 in the reconstruction band 920.
Advantageously, the source range for generating the raw spectral data in bands 922, 923 is, with respect to frequency, below the IGF start frequency 309 and the reconstruction band 920 is above the IGF start frequency 309.
Furthermore, it is advantageous that reconstruction band borders coincide with scale factor band borders. Thus, a reconstruction band has, in one embodiment, the size of corresponding scale factor bands of the core audio decoder or are sized so that, when energy pairing is applied, an energy value for a reconstruction band provides the energy of two or a higher integer number of scale factor bands. Thus, when is assumed that energy accumulation is performed for scale factor band 4, scale factor band 5 and scale factor band 6, then the lower frequency border of the reconstruction band 920 is equal to the lower border of scale factor band 4 and the higher frequency border of the reconstruction band 920 coincides with the higher border of scale factor band 6.
Subsequently,
Subsequently, reference is made to
The audio encoder advantageously has scale factor bands with different frequency bandwidths, i.e., with a different number of spectral values. Therefore, the parametric calculator comprise a normalizer 1012 for normalizing the energies for the different bandwidth with respect to the bandwidth of the specific reconstruction band. To this end, the normalizer 1012 receives, as inputs, an energy in the band and a number of spectral values in the band and the normalizer 1012 then outputs a normalized energy per reconstruction/scale factor band.
Furthermore, the parametric calculator 1006a of
In case the audio encoder is performing the grouping of two or more short windows, this grouping is applied for the energy information as well. When the core encoder performs a grouping of two or more short blocks, then, for these two or more blocks, only a single set of scale factors is calculated and transmitted. On the decoder-side, the audio decoder then applies the same set of scale factors for both grouped windows.
Regarding the energy information calculation, the spectral values in the reconstruction band are accumulated over two or more short windows. In other words, this means that the spectral values in a certain reconstruction band for a short block and for the subsequent short block are accumulated together and only single energy information value is transmitted for this reconstruction band covering two short blocks. Then, on the decoder-side, the envelope adjustment discussed with respect to
The corresponding normalization is then again applied so that even though any grouping in frequency or grouping in time has been performed, the normalization easily allows that, for the energy value information calculation on the decoder-side, only the energy information value on the one hand and the amount of spectral lines in the reconstruction band or in the set of grouped reconstruction bands has to be known.
In state-of-the-art BWE schemes, the reconstruction of the HF spectral region above a given so-called cross-over frequency is often based on spectral patching. Typically, the HF region is composed of multiple adjacent patches and each of these patches is sourced from band-pass (BP) regions of the LF spectrum below the given cross-over frequency. Within a filterbank representation of the signal such systems copy a set of adjacent subband coefficients out of the LF spectrum into the target region. The boundaries of the selected sets are typically system dependent and not signal dependent. For some signal content, this static patch selection can lead to unpleasant timbre and coloring of the reconstructed signal.
Other approaches transfer the LF signal to the HF through a signal adaptive Single Side Band (SSB) modulation. Such approaches are of high computational complexity compared to [1] since they operate at high sampling rate on time domain samples. Also, the patching can get unstable, especially for non-tonal signals (e.g. unvoiced speech), and thereby state-of-the-art signal adaptive patching can introduce impairments into the signal.
The inventive approach is termed Intelligent Gap Filling (IGF) and, in its advantageous configuration, it is applied in a BWE system based on a time-frequency transform, like e.g. the Modified Discrete Cosine Transform (MDCT). Nevertheless, the teachings of the invention are generally applicable, e.g. analogously within a Quadrature Mirror Filterbank (QMF) based system.
An advantage of the IGF configuration based on MDCT is the seamless integration into MDCT based audio coders, for example MPEG Advanced Audio Coding (AAC). Sharing the same transform for waveform audio coding and for BWE reduces the overall computational complexity for the audio codec significantly.
Moreover, the invention provides a solution for the inherent stability problems found in state-of-the-art adaptive patching schemes.
The proposed system is based on the observation that for some signals, an unguided patch selection can lead to timbre changes and signal colorations. If a signal that is tonal in the spectral source region (SSR) but is noise-like in the spectral target region (STR), patching the noise-like STR by the tonal SSR can lead to an unnatural timbre. The timbre of the signal can also change since the tonal structure of the signal might get misaligned or even destroyed by the patching process.
The proposed IGF system performs an intelligent tile selection using cross-correlation as a similarity measure between a particular SSR and a specific STR. The cross-correlation of two signals provides a measure of similarity of those signals and also the lag of maximal correlation and its sign. Hence, the approach of a correlation based tile selection can also be used to precisely adjust the spectral offset of the copied spectrum to become as close as possible to the original spectral structure.
The fundamental contribution of the proposed system is the choice of a suitable similarity measure, and also techniques to stabilize the tile selection process. The proposed technique provides an optimal balance between instant signal adaption and, at the same time, temporal stability. The provision of temporal stability is especially important for signals that have little similarity of SSR and STR and therefore exhibit low cross-correlation values or if similarity measures are employed that are ambiguous. In such cases, stabilization prevents pseudo-random behavior of the adaptive tile selection.
For example, a class of signals that often poses problems for state-of-the-art BWE is characterized by a distinct concentration of energy to arbitrary spectral regions, as shown in
An important step of the new approach is to define a set of tiles amongst which the subsequent similarity based choice can take place. First, the tile boundaries of both the source region and the target region have to be defined in accordance with each other. Therefore, the target region between the IGF start frequency of the core coder fIGFstart and a highest available frequency fIGFstop is divided into an arbitrary integer number nTar of tiles, each of these having an individual predefined size. Then, for each target tile tar[idx_tar], a set of equal sized source tiles src[idx_src] is generated. By this, the basic degree of freedom of the IGF system is determined. The total number of source tiles nSrc is determined by the bandwidth of the source region,
bw
src=(fIGFstart−fIGFmin)
where fIGFmin is the lowest available frequency for the tile selection such that an integer number nSrc of source tiles fits into Bwsrc. The minimum number of source tiles is 0.
To further increase the degree of freedom for selection and adjustment, the source tiles can be defined to overlap each other by an overlap factor between 0 and 1, where 0 means no overlap and 1 means 100% overlap. The 100% overlap case implicates that only one or no source tiles is available.
For a target tile, the cross correlation is computed with various source tiles at lags up xcorr_maxLag bins. For a given target tile idx_tar and a source tile idx_src, the xcorr_val[idx_tar][idx_src] gives the maximum value of the absolute cross correlation between the tiles, whereas xcorr_lag[idx_tar][idx_src] gives the lag at which this maximum occurs and xcorr_sign[idx_tar][idx_src] gives the sign of the cross correlation at xcorr_lag[idx_tar][idx_src].
The parameter xcorr_lag is used to control the closeness of the match between the source and target tiles. This parameter leads to reduced artifacts and helps better to preserve the timbre and color of the signal.
In some scenarios it may happen that the size of a specific target tile is bigger than the size of the available source tiles. In this case, the available source tile is repeated as often as needed to fill the specific target tile completely. It is still possible to perform the cross correlation between the large target tile and the smaller source tile in order to get the best position of the source tile in the target tile in terms of the cross correlation lag xcorr_lag and sign xcorr_sign.
The cross correlation of the raw spectral tiles and the original signal may not be the most suitable similarity measure applied to audio spectra with strong formant structure. Whitening of a spectrum removes the coarse envelope information and thereby emphasizes the spectral fine structure, which is of foremost interest for evaluating tile similarity. Whitening also aids in an easy envelope shaping of the STR at the decoder for the regions processed by IGF. Therefore, optionally, the tile and the source signal is whitened before calculating the cross correlation.
In other configurations, only the tile is whitened using a predefined procedure. A transmitted “whitening” flag indicates to the decoder that the same predefined whitening process shall be applied to the tile within IGF.
For whitening the signal, first a spectral envelope estimate is calculated. Then, the MDCT spectrum is divided by the spectral envelope. The spectral envelope estimate can be estimated on the MDCT spectrum, the MDCT spectrum energies, the MDCT based complex power spectrum or power spectrum estimates. The signal on which the envelope is estimated will be called base signal from now on.
Envelopes calculated on MDCT based complex power spectrum or power spectrum estimates as base signal have the advantage of not having temporal fluctuation on tonal components.
If the base signal is in an energy domain, the MDCT spectrum has to be divided by the square root of the envelope to whiten the signal correctly.
There are different methods of calculating the envelope:
Advantageously, the last approach is chosen. For applications that may use low computational complexity, some simplification can be done to the whitening of an MDCT spectrum: First the envelope is calculated by means of a moving average. This only needs two processor cycles per MDCT bin. Then in order to avoid the calculation of the division and the square root, the spectral envelope is approximated by 2n, where n is the integer logarithm of the envelope. In this domain the square root operation simply becomes a shift operation and furthermore the division by the envelope can be performed by another shift operation.
After calculating the correlation of each source tile with each target tile, for all nTar target tiles the source tile with the highest correlation is chosen for replacing it. To match the original spectral structure best, the lag of the correlation is used to modulate the replicated spectrum by an integer number of transform bins. In case of odd lags, the tile is additionally modulated through multiplication by an alternating temporal sequence of −1/1 to compensate for the frequency-reversed representation of every other band within the MDCT.
So the total amount of side information to transmit form the encoder to the decoder could consists of the following data:
Tile pruning and stabilization is an important step in the IGF. Its need and advantages are explained with an example, assuming a stationary tonal audio signal like e.g. a stable pitch pipe note. Logic dictates that least artifacts are introduced if, for a given target region, source tiles are selected from the same source region across frames. Even though the signal is assumed to be stationary, this condition would not hold well in every frame since the similarity measure (e.g. correlation) of another equally similar source region could dominate the similarity result (e.g. cross correlation). This leads to tileNum[nTar] between adjacent frames to vacillate between two or three very similar choices. This can be the source of an annoying musical noise like artifact.
In order to eliminate this type of artifacts, the set of source tiles shall be pruned such that the remaining members of the source set are maximally dissimilar. This is achieved over a set of source tiles
S={s
1
,s
2
, . . . s
n}
as follows. For any source tile si, we correlate it with all the other source tiles, finding the best correlation between si and sj and storing it in a matrix Sx. Here Sx[i][j] contains the maximal absolute cross correlation value between si and sj. Adding the matrix Sx along the columns, gives us the sum of cross correlations of a source tile si with all the other source tiles T.
T[i]=Sx[i][1]+Sx[i][2] . . . +Sx[i][n]
Here T represents a measure of how well a source is similar to other source tiles. If, for any source tile i,
T>threshold
source tile i can be dropped from the set of potential sources since it is highly correlated with other sources. The tile with the lowest correlation from the set of tiles that satisfy the condition in equation 1 is chosen as a representative tile for this subset. This way, we ensure that the source tiles are maximally dissimilar to each other.
The tile pruning method also involves a memory of the pruned tile set used in the preceding frame. Tiles that were active in the previous frame are retained in the next frame also if alternative candidates for pruning exist.
Let tiles s3, s4 and s5 be active out of tiles {s1, s2 . . . , s5} in frame k, then in frame k+1 even if tiles s1, s3 and s2 are contending to be pruned with s3 being the maximally correlated with the others, s3 is retained since it was a useful source tile in the previous frame, and thus retaining it in the set of source tiles is beneficial for enforcing temporal continuity in the tile selection. This method is advantageously applied if the cross correlation between the source i and target j, represented as Tx[i][j] is high
An additional method for tile stabilization is to retain the tile order from the previous frame k-1 if none of the source tiles in the current frame k correlate well with the target tiles. This can happen if the cross correlation between the source i and target j, represented as Tx[i][j] is very low for all i, j
For example, if
T
x[i][j]<0.6
a tentative threshold being used now, then
tileNum[nTar]k=tileNum[nTar]k−1
for all nTar of this frame k.
The above two techniques greatly reduce the artifacts that occur from rapid changing set tile numbers across frames. Another added advantage of this tile pruning and stabilization is that no extra information needs to be sent to the decoder nor is a change of decoder architecture needed. This proposed tile pruning is an elegant way of reducing potential musical noise like artifacts or excessive noise in the tiled spectral regions.
Furthermore, the audio decoder comprises a parametric decoder 1104 for generating a second decoded representation of a second set of second spectral portions having a second spectral resolution being lower than the first spectral resolution. Furthermore, a frequency regenerator 1106 is provided which receives, as a first input 1101, decoded first spectral portions and as a second input at 1103 the parametric information including, for each target frequency tile or target reconstruction band a source range information. The frequency regenerator 1106 then applies the frequency regeneration by using spectral values from the source range identified by the matching information in order to generate the spectral data for the target range. Then, the first spectral portions 1101 and the output of the frequency regenerator 1107 are both input into a spectrum-time converter 1108 to finally generate the decoded audio signal.
Advantageously, the audio decoder 1102 is a spectral domain audio decoder, although the audio decoder can also be implemented as any other audio decoder such as a time domain or parametric audio decoder.
As indicated at
MID_WHITENING and STRONG_WHITENING refer to different whitening filters (1122) that may differ in the way the envelope is calculated (as described before).
The decoder-side frequency regenerator can be controlled by a source range ID 1121 when only a coarse spectral tile selection scheme is applied. When, however, a fine-tuned spectral tile selection scheme is applied, then, additionally, a source range lag 1119 is provided. Furthermore, provided that the correlation calculation provides a negative result, then, additionally, a sign of the correlation can also be applied to block 1120 so that the page data spectral lines are each multiplied by “−1” to account for the negative sign.
Thus, the present invention as discussed in
The encoded source ranges are transmitted to a decoder together with matching information for the target ranges so that the decoder illustrated in
The parameter calculator 1134 is configured for calculating similarities between first spectral portions and second spectral portions and for determining, based on the calculated similarities, for a second spectral portion a matching first spectral portion matching with the second spectral portion. Advantageously, matching results for different source ranges and target ranges as illustrated in
As discussed, a fine granularity is obtained by comparing a target region with a source region without any lag to the source region and the same source region, but with a certain lag. These lags are applied in the cross-correlation calculator 1140 of
Furthermore, it is advantageous to perform a source and/or target ranges whitening illustrated at block 1142. This block 1142 then provides a whitening flag to the bitstream which is used for controlling the decoder-side switch 1123 of
Furthermore, the parameter calculator 1134 is configured for performing a source tile pruning 1146 by reducing the number of potential source ranges in that a source patch is dropped from a set of potential source tiles based on a similarity threshold. Thus, when two source tiles are similar more or equal to a similarity threshold, then one of these two source tiles is removed from the set of potential sources and the removed source tile is not used anymore for the further processing and, specifically, cannot be selected by the tile selector 1144 or is not used for the cross-correlation calculation between different source ranges and target ranges as performed in block 1140.
Different implementations have been described with respect to different figures.
All these different aspects can be of inventive use independent of each other, but, additionally, can also be applied together as basically illustrated in
Although some aspects have been described in the context of an apparatus for encoding or decoding, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus.
Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a non-transitory storage medium such as a digital storage medium, for example a floppy disc, a Hard Disk Drive (HDD), a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.
Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.
Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may, for example, be stored on a machine readable carrier.
Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.
In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.
A further embodiment of the inventive method is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitory.
A further embodiment of the invention method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may, for example, be configured to be transferred via a data communication connection, for example, via the internet.
A further embodiment comprises a processing means, for example, a computer or a programmable logic device, configured to, or adapted to, perform one of the methods described herein.
A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.
A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver.
In some embodiments, a programmable logic device (for example, a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are advantageously performed by any hardware apparatus.
While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.
Geneva, 2010.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13177346 | Jul 2013 | EP | regional |
| 13177348 | Jul 2013 | EP | regional |
| 13177350 | Jul 2013 | EP | regional |
| 13177353 | Jul 2013 | EP | regional |
| 13189366 | Oct 2013 | EP | regional |
This application is a continuation of copending U.S. patent application Ser. No. 16/156,683, filed Oct. 10, 2018, which in turn is a divisional of copending U.S. patent application Ser. No. 15/000,902, filed Jan. 19, 2016, which in turn is a continuation of copending International Application No. PCT/EP2014/065106, filed Jul. 15, 2014, all of which are incorporated herein by reference in its entirety, and additionally claims priority from European Applications Nos. EP13177353, filed Jul. 22, 2013, EP13177350, filed Jul. 22, 2013, EP13177348, filed Jul. 22, 2013, EP13177346, filed Jul. 22, 2013, and EP13189366, filed Oct. 18, 2013, all of which are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15000902 | Jan 2016 | US |
| Child | 16156683 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16156683 | Oct 2018 | US |
| Child | 17094791 | US | |
| Parent | PCT/EP2014/065106 | Jul 2014 | US |
| Child | 15000902 | US |
