Patent Application
20160035329
The present document relates to audio coding systems which make use of a harmonic transposition method for high frequency reconstruction (HFR), and to digital effect processors, e.g. so-called exciters, where generation of harmonic distortion adds brightness to the processed signal. In particular, the present document relates to low complexity methods for implementing high frequency reconstruction.
In the patent document WO 98/57436 the concept of transposition was established as a method to recreate a high frequency band from a lower frequency band of an audio signal. A substantial saving in bitrate can be obtained by using this concept in audio coding. In an HFR based audio coding system, a low bandwidth signal, also referred to as the low frequency component of a signal, is presented to a core waveform coder, and the higher frequencies, also referred to as the high frequency component of the signal, are regenerated using signal transposition and additional side information of very low bitrate describing the target spectral shape of the high frequency component at the decoder side. For low bitrates, where the bandwidth of the core coded signal, i.e. the low band signal or low frequency component, is narrow, it becomes increasingly important to recreate a high band signal, i.e. a high frequency component, with perceptually pleasant characteristics. The harmonic transposition defined in the patent document WO 98/57436 performs well for complex musical material in a situation with low cross over frequency, i.e. in a situation of a low upper frequency of the low band signal. The principle of a harmonic transposition is that a sinusoid with frequency ω is mapped to a sinusoid with frequency Tω, where T>1 is an integer defining the order of the transposition, i.e. the transposition order. In contrast to this, a single sideband modulation (SSB) based HFR maps a sinusoid with frequency ω to a sinusoid with frequency ω+Δω, where Δω is a fixed frequency shift. Given a core signal with low bandwidth, i.e. a low band signal with a low upper frequency, a dissonant ringing artifact will typically result from the SSB transposition, which may therefore be disadvantageous compared to harmonic transposition.
In order to reach improved audio quality and in order to synthesize the required bandwidth of the high band signal, harmonic HFR methods typically employ several orders of transposition. In order to implement a plurality of transpositions of different transposition order, prior art solutions require a plurality of filter banks either in the analysis stage or the synthesis stage or in both stages. Typically, a different filter bank is required for each different transposition order. Moreover, in situations where the core waveform coder operates at a lower sampling rate than the sampling rate of the final output signal, there is typically an additional need to convert the core signal to the sampling rate of the output signal, and this upsampling of the core signal is usually achieved by adding yet another filter bank. All in all, the computationally complexity increases significantly with an increasing number of different transposition orders.
The present invention provides a method for reducing the complexity of harmonic HFR methods by means of enabling the sharing of an analysis and synthesis filter bank pair by several harmonic transposers, or by one or several harmonic transposers and an upsampler. The proposed frequency domain transposition may comprise the mapping of nonlinearly modified subband signals from an analysis filter bank into selected subbands of a synthesis filter bank. The nonlinear operation on the subband signals may comprise a multiplicative phase modification. Furthermore, the present invention provides various low complexity designs of HFR systems.
According to one aspect, a system configured to generate a high frequency component of a signal from a low frequency component of the signal is described. The system may comprise an analysis filter bank configured to provide a set of analysis subband signals from the low frequency component of the signal; wherein the set of analysis subband signals typically comprises at least two analysis subband signals. The analysis filter bank may have a frequency resolution of Δf and a number LA of analysis subbands, with LA>1, where k is an analysis subband index with k=0, . . . , LA−1. In particular, the analysis filter bank may be configured to provide a set of complex valued analysis subband signals comprising magnitude samples and phase samples.
The system may further comprise a nonlinear processing unit configured to determine a set of synthesis subband signals from the set of analysis subband signals using a transposition order P; wherein the set of synthesis subband signals typically comprises a portion of the set of analysis subband signals phase shifted by an amount derived from the transposition order P. In other words, the set of synthesis subband signals may be determined based on a portion of the set of analysis subband signals phase shifted by an amount derived from the transposition order P. The phase shifting of an analysis subband signal may be achieved by multiplying the phase samples of the analysis subband signal by the amount derived from transposition factor P. As such, the set of synthesis subband signals may correspond to a portion or a subset of the set of analysis subband signals, wherein the phases of the subband samples have been multiplied by an amount derived from the transposition order. In particular, the amount derived from the transposition order may be a fraction of the transposition order.
The system may comprise a synthesis filter bank configured to generate the high frequency component of the signal from the set of synthesis subband signals. The synthesis filter bank may have a frequency resolution of FΔf; with F being a resolution factor, e.g. an integer value, with F≧1; and a number LS of synthesis subbands, with LS>0, where n is a synthesis subband index with n=0, . . . , LS−1. The transposition order P may be different from the resolution factor F. The analysis filter bank may employ an analysis time stride ΔtA and the synthesis filter bank may employ a synthesis time stride ΔtS; and the analysis time stride ΔtA and the synthesis time stride ΔtS may be equal.
The nonlinear processing unit may be configured to determine a synthesis subband signal of the set of synthesis subband signals based on an analysis subband signal of the set of analysis subband signals phase shifted by the transposition order P; or based on a pair of analysis subband signals from the set of analysis subband signals wherein a first member of the pair of subband signals is phase shifted by a factor P′ and a second member of the pair is phase shifted by a factor P″, with P′+P″=P. The above operations may be performed on a sample of the synthesis and analysis subband signals. In other words, a sample of a synthesis subband signal may be determined based on a sample of an analysis subband signal phase shifted by the transposition order P; or based on a pair of samples from a corresponding pair of analysis subband signals, wherein a first sample of the pair of samples is phase shifted by a factor P′ and a second sample of the pair is phase shifted by a factor P″.
The nonlinear processing unit may be configured to determine an nth synthesis subband signal of the set of synthesis subband signals from a combination of the kth analysis subband signal and a neighboring (k+1)th analysis subband signal of the set of analysis subband signals. In particular, the nonlinear processing unit may be configured to determine a phase of the nth synthesis subband signal as the sum of a shifted phase of the kth analysis subband signal and a shifted phase of the neighboring (k+1)th analysis subband signal. Alternatively or in addition, the nonlinear processing unit may be configured to determine a magnitude of the nth synthesis subband signal as the product of an exponentiated magnitude of the kth analysis subband signal and an exponentiated magnitude of the neighboring (k+1)th analysis subband signal.
The analysis subband index k of the analysis subband signal contributing to the synthesis subband with synthesis subband index n may be given by the integer obtained by truncating the expression
A remainder r of such truncating operation may be given by
In such cases, the nonlinear processing unit may be configured to determine the phase of the nth synthesis subband signal as the sum of the phase of the kth analysis subband signal shifted by P(1−r) and the phase of the neighboring (k+1)th analysis subband signal shifted by P(r). In particular, the nonlinear processing unit may be configured to determine the phase of the nth synthesis subband signal as the sum of the phase of the kth analysis subband signal multiplied by P(1−r) and the phase of the neighboring (k+1)th analysis subband signal multiplied by P(r). Alternatively or in addition, the nonlinear processing unit may be configured to determine the magnitude of the nth synthesis subband signal as the product of the magnitude of the kth analysis subband signal raised to the power of (1−r) and the magnitude of the neighboring (k+1)th analysis subband signal raised to the power of r.
In an embodiment, the analysis filter bank and the synthesis filter bank may be evenly stacked such that a center frequency of an analysis subband is given by kΔf and a center frequency of a synthesis subband is given by nFΔf. In another embodiment, the analysis filter bank and the synthesis filter bank may be oddly stacked such that a center frequency of an analysis subband is given by (k+½)Δf and a center frequency of a synthesis subband is given by (n+½)FΔf; and the difference between the transposition order P and the resolution factor F is even.
According to another aspect, a system configured to generate a high frequency component of a signal from a low frequency component of the signal is described. The system may comprise an analysis filter bank configured to provide a set of analysis subband signals from the low frequency component of the signal; wherein the set of analysis subband signals comprises at least two analysis subband signals.
The system may further comprise a first nonlinear processing unit configured to determine a first set of synthesis subband signals from the set of analysis subband signals using a first transposition order P1; wherein the first set of synthesis subband signals is determined based on a portion of the set of analysis subband signals phase shifted by an amount derived from the first transposition order P1. The system may also comprise a second nonlinear processing unit configured to determine a second set of synthesis subband signals from the set of analysis subband signals using a second transposition order P2; wherein the second set of synthesis subband signals is determined based on a portion of the set of analysis subband signals phase shifted by an amount derived from the second transposition order P2; wherein the first transposition order P1 and the second transposition order P2 are different. The first and second nonlinear processing unit may be configured according to any of the features and aspects outlined in the present document.
The system may further comprise a combining unit configured to combine the first and the second set of synthesis subband signals; thereby yielding a combined set of synthesis subband signals. Such combining may be performed by combining, e.g. adding and/or averaging, synthesis subband signals from the first and the second set which correspond to the same frequency ranges. In other words, the combining unit may be configured to superpose synthesis subband signals of the first and the second set of synthesis subband signals corresponding to overlapping frequency ranges. In addition, the system may comprise a synthesis filter bank configured to generate the high frequency component of the signal from the combined set of synthesis subband signals.
According to a further aspect, a system configured to generate a high frequency component of a signal from a low frequency component of the signal is described. The system may comprise an analysis filter bank having a frequency resolution of Δf. The analysis filter bank may be configured to provide a set of analysis subband signals from the low frequency component of the signal. The system may comprise a nonlinear processing unit configured to determine a set of intermediate synthesis subband signals having a frequency resolution of PΔf from the set of analysis subband signals using a transposition order P; wherein the set of intermediate synthesis subband signals comprises a portion of the set of analysis subband signals, phase shifted by the transposition order P. In particular, the nonlinear processing unit may multiply the phase of complex analysis subband signals by the transposition order. It should be noted that the transposition order P may be e.g. the transposition order P or P1 or P2 outlined above.
The nonlinear processing unit may be configured to interpolate one or more intermediate synthesis subband signals to determine a synthesis subband signal of a set of synthesis subband signals having a frequency resolution of FΔf; with F being the resolution factor, with F≧1. In an embodiment two or more intermediate synthesis subband signals are interpolated. The transposition order P may be different from the frequency resolution F.
The system may comprise a synthesis filter bank having a frequency resolution of FΔf. The synthesis filter bank may be configured to generate the high frequency component of the signal from the set of synthesis subband signals.
The systems described in the present document may further comprise a core decoder configured to convert an encoded bit stream into the low frequency component of the signal; wherein the core decoder may be based on a coding scheme being one of: Dolby E, Dolby Digital, AAC, HE-AAC. The system may comprise a multi-channel analysis quadrature mirror filter bank, referred to as QMF bank, configured to convert the high frequency component and/or the low frequency component into a plurality of QMF subband signals; and/or a high frequency reconstruction processing module configured to modify the QMF subband signals; and/or a multi-channel synthesis QMF bank configured to generate a modified high frequency component from the modified QMF subband signals. The systems may also comprise a downsampling unit upstream of the analysis filter bank configured to reduce a sampling rate of the low frequency component of the signal; thereby yielding a low frequency component at a reduced sampling rate.
According to another aspect, a system configured to generate a high frequency component of a signal at a second sampling frequency from a low frequency component of the signal at a first sampling frequency is described. In particular, the signal comprising the low and the high frequency component may be at the second sampling frequency. The second sampling frequency may be R times the first sampling frequency, wherein R≧1. The system may comprise a harmonic transposer of order T configured to generate a modulated high frequency component from the low frequency component; wherein the modulated high frequency component may comprise or may be determined based on a spectral portion of the low frequency component transposed to a T times higher frequency range. The modulated high frequency component may be at the first sampling frequency multiplied by a factor S; wherein T>1 and S≦R. In other words, the modulated high frequency component may be at a sampling frequency which is lower than the second sampling frequency. In particular, the modulated high frequency component may be critically (or close to critically) sampled.
The system may comprise an analysis quadrature mirror filter bank, referred to as QMF bank, configured to map the modulated high frequency component into at least one of X QMF subbands; wherein X is a multiple of S; thereby yielding at least one QMF subband signal; and/or a high frequency reconstruction module configured to modify the at least one QMF subband signal, e.g. scale one or more QMF subband signals; and/or a synthesis QMF bank configured to generate the high frequency component from the at least one modified QMF subband signal.
The harmonic transposer may comprise any of the features and may be configured to perform any of the method steps outlined in the present document. In particular, the harmonic transposer may comprise an analysis filter bank configured to provide a set of analysis subband signals from the low frequency component of the signal. The harmonic transposer may comprise a nonlinear processing unit associated with the transposition order T and configured to determine a set of synthesis subband signals from the set of analysis subband signals by altering a phase of the set of analysis subband signals. As outlined above, the altering of the phase may comprise multiplying the phase of complex samples of the analysis subband signals. The harmonic transposer may comprise a synthesis filter bank configured to generate the modulated high frequency component of the signal from the set of synthesis subband signals.
The low frequency component may have a bandwidth B. The harmonic transposer may be configured to generate a set of synthesis subband signals which embraces or spans a frequency range (T−1)*B up to T*B. In such cases, the harmonic transposer may be configured to modulate the set of synthesis subband signals into a baseband centered around the zero frequency, thereby yielding the modulated high frequency component. Such modulation may be performed by highpass filtering a time domain signal generated from a set of subband signals including the set of synthesis subband signals and by subsequent modulation and/or downsampling of the filtered time domain signal. Alternatively or in addition, such modulation may be performed by directly generating a modulated time domain signal from the set of synthesis subband signals. This may be achieved by using a synthesis filter bank of a smaller than nominal size. For example, if the synthesis filter bank has a nominal size of L and the frequency range from (T−1)*B up to T*B corresponds to synthesis subband indices from k0 to k1, the synthesis subband signals may be mapped to subband indices from 0 to k1−k0 in a k1−k0 (<L) size synthesis filter bank, i.e. a synthesis filter bank having a size k1−k0 which is smaller than L.
The system may comprise downsampling means upstream of the harmonic transposer configured to provide a critically (or close to critically) downsampled low frequency component at the first sampling frequency divided by a downsampling factor Q from the low frequency component of the signal. In such cases, the different sampling frequencies in the system may be divided by the downsampling factor Q. In particular, the modulated high frequency component may be at the first sampling frequency multiplied by a factor S and divided by the downsampling factor Q. The size of the analysis QMF bank X may be a multiple of S/Q.
According to a further aspect, a method for generating a high frequency component of a signal from a low frequency component of the signal is described. The method may comprise the step of providing a set of analysis subband signals from the low frequency component of the signal using an analysis filter bank having a frequency resolution of Δf; wherein the set of analysis subband signals comprises at least two analysis subband signals. The method may further comprise the step of determining a set of synthesis subband signals from the set of analysis subband signals using a transposition order P; wherein the set of synthesis subband signals is determined based on a portion of the set of analysis subband signals phase shifted by an amount derived from the transposition order P. Furthermore, the method may comprise the step of generating the high frequency component of the signal from the set of synthesis subband signals using a synthesis filter bank (504) having a frequency resolution of FΔf; with F being a resolution factor, with F≧1; wherein the transposition order P is different from the resolution factor F.
According to another aspect, a method for generating a high frequency component of a signal from a low frequency component of the signal is described. The method may comprise the step of providing a set of analysis subband signals from the low frequency component of the signal; wherein the set of analysis subband signals may comprise at least two analysis subband signals. The method may comprise the step of determining a first set of synthesis subband signals from the set of analysis subband signals using a first transposition order P1; wherein the first set of synthesis subband signals comprises a portion of the set of analysis subband signals phase shifted by an amount derived from the first transposition order P1. Furthermore, the method may comprise the step of determining a second set of synthesis subband signals from the set of analysis subband signals using a second transposition order P2; wherein the second set of synthesis subband signals comprises a portion of the set of analysis subband signals phase shifted by an amount derived by the second transposition order P2. The first transposition order P1 and the second transposition order P2 may be different. The first and the second set of synthesis subband signals may be combined to yield a combined set of synthesis subband signals and the high frequency component of the signal may be generated from the combined set of synthesis subband signals.
According to another aspect a method for generating a high frequency component of a signal from a low frequency component of the signal is described. The method may comprise the step of providing a set of analysis subband signals having a frequency resolution of Δf from the low frequency component of the signal. The method may further comprise the step of determining a set of intermediate synthesis subband signals having a frequency resolution of PΔf from the set of analysis subband signals using a transposition order P; wherein the set of intermediate synthesis subband signals comprises a portion of the set of analysis subband signals phase shifted by the transposition order P. One or more intermediate synthesis subband signals may be interpolated to determine a synthesis subband signal of a set of synthesis subband signals having a frequency resolution of FΔf; with F being a resolution factor, with F≧1; wherein the transposition order P2 may be different from the frequency resolution F. The high frequency component of the signal may be generated from the set of synthesis subband signals.
According to a further aspect, a method for generating a high frequency component of a signal at a second sampling frequency from a low frequency component of the signal at a first sampling frequency is described. The second sampling frequency may be R times the first sampling frequency, with R≧1. The method may comprise the step of generating a modulated high frequency component from the low frequency component by applying harmonic transposition of order T; wherein the modulated high frequency component comprises a spectral portion of the low frequency component transposed to a T times higher frequency range; wherein the modulated high frequency component is at the first sampling frequency multiplied by a factor S; wherein T>1 and S≦R. In an embodiment, S<R.
According to another aspect, a set-top box for decoding a received signal comprising at least an audio signal is described. The set-top box may comprise a system for generating the high frequency component of the audio signal from the low frequency component of the audio signal. The system may comprise any of the aspects and features outlined in the present document.
According to another aspect, a software program is described. The software program may be adapted for execution on a processor and for performing any of the aspects and method steps outlined in the present document when carried out on a computing device.
According to a further aspect, a storage medium is described. The storage medium may comprise a software program adapted for execution on a processor and for performing any of the aspects and method steps outlined in the present document when carried out on a computing device.
According to another aspect, a computer program product is described. The computer program product may comprise executable instructions for performing any of the aspects and method steps outlined in the present document when executed on a computer.
It should be noted that the embodiments and aspects described in this document may be arbitrarily combined. In particular, it should be noted that the aspects and features outlined in the context of a system are also applicable in the context of the corresponding method and vice versa. Furthermore, it should be noted that the disclosure of the present document also covers other claim combinations than the claim combinations which are explicitly given by the back references in the dependent claims, i.e., the claims and their technical features can be combined in any order and any formation.
The present invention will now be described by way of illustrative examples, not limiting the scope or spirit of the invention, with reference to the accompanying drawings, in which:
b and 5c illustrate examples for the mapping of subband signals for a multiple transposer scheme according to
a illustrates example effects of harmonic transposition on a signal in the frequency domain;
b and 12c illustrate example methods for combining overlapping and non-overlapping transposed signals;
The below-described embodiments are merely illustrative for the principles of the present invention for efficient combined harmonic transposition. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.
Typically, each filter bank has a physical frequency resolution measured in Hertz and a time stride parameter measured in seconds. These two parameters, i.e. the frequency resolution and the time stride, define the discrete-time parameters of the filter bank given the chosen sampling rate. By choosing the physical time stride parameters, i.e. the time stride parameter measured in time units e.g. seconds, of the analysis and synthesis filter banks to be identical, an output signal of the transposer 100 may be obtained which has the same sampling rate as the input signal. Furthermore, by omitting the nonlinear processing 102 a perfect reconstruction of the input signal at the output may be achieved. This requires a careful design of the analysis and synthesis filter banks. On the other hand, if the output sampling rate is chosen to be different from the input sampling rate, a sampling rate conversion may be obtained. This mode of operation may be necessary, e.g. when applying signal transposition where the desired output bandwidth is larger than the half of the input sampling rate, i.e. when the desired output bandwidth exceeds the Nyqvist frequency of the input signal.
It should be noted that each transposer 201-1, 201-2, . . . , 201-P requires an analysis and a synthesis filter bank as depicted in
It should be noted that if the synthesis filter banks 303-1, 303-2, . . . , 303-P corresponding to the different transposition orders operate at different sampling rates, e.g. by using different degrees of bandwidth expansion, the time domain output signals of the different synthesis filter banks 303-1, 303-2, . . . , 303-P need to be differently resampled in order to align the P output signals to the same time grid, prior to their summation in combiner 304.
As already indicated above, the nonlinear processing 102 typically provides a number of subbands at the output which corresponds to the number of subbands at the input. The non-linear processing 102 typically modifies the phase and/or the amplitude of the subband or the subband signal according to the underlying transposition order T. By way of example a subband at the input is converted to a subband at the output with T times higher frequency, i.e. a subband at the input to the nonlinear processing 102, i.e. the analysis subband, [(k−½)Δf,(k+½)Δf] may be transposed to a subband at the output of the nonlinear processing 102, i.e. the synthesis subband, [(k−½)TΔf,(k+½)TΔf], wherein k is a subband index number and Δf if the frequency resolution of the analysis filter bank. In order to allow for the use of common analysis filter banks 501 and common synthesis filter banks 504, one or more of the advanced processing units 502-1, 502-2, . . . , 502-P may be configured to provide a number of output subbands which is different from the number of input subbands. In an embodiment, the number of input subbands into an advanced processing unit 502-1, 502-2, . . . , 502-P may be roughly F/T times the number of output subbands, where T is the transposition order of the advanced processing unit and F is a filter bank resolution factor introduced below.
In the following, the principles of advanced nonlinear processing in the nonlinear processing units 502-1, 502-2, . . . , 502-P will be outlined. For this purpose, it is assumed that
Furthermore, it is assumed that the filter banks are evenly stacked, i.e. the subband with index zero is centered around the zero frequency, such that the analysis filter bank center frequencies are given by kΔf where the analysis subband index k=0, 1, . . . LA−1 and LA is the number of subbands of the analysis filter bank. The synthesis filter bank center frequencies are given by nFΔf where the synthesis subband index n=0, 1, . . . LS−1 and LS is the number of subbands of the synthesis filter bank.
When performing a conventional transposition of integer order T≧1 as shown in
θS(k)=TθA(k), (1)
where θA (k) is the phase of a sample of the analysis subband k and θS (k) is the phase of a sample of the synthesis subband k. The magnitude or amplitude of a sample of the subband may be kept unmodified or may be increased or decreased by a constant gain factor. Due to the fact that T is an integer, the operation of equation (1) is independent of the definition of the phase angle.
If the resolution factor F is selected to be equal to the transposition order T, i.e. F=T, then the frequency resolution of the synthesis filter bank, i.e. FΔf, depends on the transposition order T. Consequently, it is necessary to use different filter banks for different transposition orders T either in the analysis or synthesis stage. This is due to the fact that the transposition order T defines the quotient of physical frequency resolutions, i.e. the quotient of the frequency resolution Δf of the analysis filter bank and the frequency resolution FΔf of the synthesis filter bank.
In order to be able to use a common analysis filter bank 501 and a common synthesis filter bank 504 for a plurality of different transposition orders T, it is proposed to set the frequency resolution of the synthesis filter bank 504 to FΔf, i.e. it is proposed to make the frequency resolution of the synthesis filter bank 504 independent of the transposition order T. Then the question arises of how to implement a transposition of order T when the resolution factor F, i.e. the quotient F of the physical frequency resolution of the analysis and synthesis filter bank, does not necessarily obey the relation F=T.
As outlined above, a principle of harmonic transposition is that the input to the synthesis filter bank subband n with center frequency nFΔf is determined from an analysis subband at a T time lower center frequency, i.e. at the center frequency nFΔf/T. The center frequencies of the analysis subbands are identified through the analysis subband index k as kΔf. Both expressions for the center frequency of the analysis subband index, i.e. nFΔf/T and kΔf, may be equated. Taking into account that the index n is an integer value, the expression
is a rational number which can be expressed as the sum of an integer analysis subband index k and a remainder rε{0, 1/T, 2/T, . . . (T−1)/T} such that
As such, it may be stipulated that the input to a synthesis subband with synthesis subband index n may be derived, using a transposition of order T, from the analysis subband or subbands k with the index given by equation (2). In view of the fact that
is a rational number, the remainder r may be unequal to 0 and the value k+r may be greater than the analysis subband index k and smaller than the analysis subband index k+1. Consequently, the input to a synthesis subband with synthesis subband index n should be derived, using a transposition of order T, from the analysis subbands with the analysis subband index k and k+1, wherein k is given by equation (2).
As an outcome of the above analysis, the advanced nonlinear processing performed in a nonlinear processing unit 502-1, 502-2, . . . , 502-P may comprise, in general, the step of considering two neighboring analysis subbands with index k and k+1 in order to provide the output for synthesis subband n. For a transposition order T, the phase modification performed by the nonlinear processing unit 502-1, 502-2, . . . , 502-P may therefore be defined by the linear interpolation rule,
θS(n)=T(1−r)θA(k)+TrθA(k+1), (3)
where θA(k) is the phase of a sample of the analysis subband k, θA(k+1) is the phase of a sample of the analysis subband k+1, and θS(k) is the phase of a sample of the synthesis subband n. I.e. if the remainder r is close to zero, i.e. if the value k+r is close to k, then the main contribution of the phase of the synthesis subband sample is derived from the phase of the analysis subband sample of subband k. On the other hand, if the remainder r is close to one, i.e. if the value k+r is close to k+1, then the main contribution of the phase of the synthesis subband sample is derived from the phase of the analysis subband sample of subband k+1. It should be noted that the phase multipliers T(1−r) and T r are both integers such that the phase modifications of equation (3) are well defined and independent of the definition of the phase angle.
Concerning the magnitudes of the subband samples, the following geometrical mean value may be selected for the determination of the magnitude of the synthesis subband samples,
a
S(n)=aA(k)(1−r)aA(k+1)r, (4)
where aS (n) denotes the magnitude of a sample of the synthesis subband n, aA(k) denotes the magnitude of a sample of the analysis subband k and aA (k+1) denotes the magnitude of a sample of the analysis subband k+1.
For the case of an oddly stacked filter bank where the analysis filter bank center frequencies are given by (k+½)Δf with k=0, 1, . . . LA−1 and the synthesis filter bank center frequencies are given by (n+½)FΔf with n=0, 1, . . . LS−1, a corresponding equation to equation (2) may derived by equating the transposed synthesis filter bank center frequency
and the analysis filter bank center frequency (k+½)Δf. Assuming an integer index k and a remainder rε[0,1] the following equation for oddly stacked filter banks can be derived:
It can be seen that if T−F, i.e. the difference between the transposition order and the resolution factor, is even, T(1−r) and T r are both integers and the interpolation rules of equations (3) and (4) can be used.
The mapping of analysis subbands into synthesis subbands is illustrated in
In the illustrated case, equation (2) may be written as
Consequently, for a transposition order T=1, an analysis subband with an index k is mapped to a corresponding synthesis subband n and the remainder r is always zero. This can be seen in
In case of a transposition order T=2, the remainder r takes on the values 0 and ½ and a source bin is mapped to a plurality of target bins. When reversing the perspective, it may be stated that each target bin 532, 535 receives a contribution from up to two source bins. This can be seen in
The similar situation for the case of F=2, where equation (2) may be written as
is depicted in
In case of a transposition order T=3, the remainder r takes on the values 0, 1/3, and 2/3 and a source bin is mapped to a plurality of target bins. When reversing the perspective, it may be stated that each target bin 542, 545 receives a contribution from up to two source bins. This can be seen in
A further interpretation of the above advanced nonlinear processing may be as follows. The advanced nonlinear processing may be understood as a combination of a transposition of a given order T and a subsequent mapping of the transposed subband signals to a frequency grid defined by the common synthesis filter bank, i.e. by a frequency grid FΔf. In order to illustrate this interpretation, reference is made again to
In summary, a nonlinear processing method has been described which allows the determination of contributions to a synthesis subband by means of the transposition of several analysis subbands. The nonlinear processing method enables the use of single common analysis and synthesis subband filter banks for different transposition orders, thereby significantly reducing the computational complexity of multiple harmonic transposers.
In the following various embodiments of multiple harmonic transposers or multiple harmonic transposer systems are described. In audio source coding/decoding systems employing HFR (high frequency reconstruction), such as SBR (spectral band replication) specified e.g. in WO 98/57436 which is incorporated by reference, a typical scenario is that the core decoder, i.e. the decoder of a low frequency component of an audio signal, outputs a time domain signal to the HFR module or HFR system, i.e. the module or system performing the reconstruction of the high frequency component of the audio signal. The low frequency component may have a bandwidth which is lower than half the bandwidth of the original audio signal comprising the low frequency component and the high frequency component. Consequently, the time domain signal comprising the low frequency component, also referred to as the low band signal, may be sampled at half the sampling rate of the final output signal of the audio coding/decoding system. In such cases, the HFR module will have to effectively resample the core signal, i.e. the low band signal, to twice the sampling frequency in order to facilitate the core signal to be added to the output signal. Hence, the so-called bandwidth extension factor applied by the HFR module equals 2.
After generation of a high frequency component, also referred to as the HFR generated signal, the HFR generated signal is dynamically adjusted to match the HFR generated signal as close as possible to the high frequency component of the original signal, i.e. to the high frequency component of the originally encoded signal. This adjustment is typically performed by a so-called HFR processor by means of transmitted side information. The transmitted side information may comprise information on the spectral envelope of the high frequency component of the original signal and the adjustment of the HFR generated signal may comprise the adjustment of the spectral envelope of the HRF generated signal.
In order to perform the adjustment of the HFR generated signal according to the transmitted side information, the HFR generated signal is analyzed by a multichannel QMF (Quadrature Mirror Filter) bank which provides spectral QMF subband signals of the HFR generated signal. Subsequently, the HFR processor performs the adjustment of the HFR generated signal on the spectral QMF subband signals obtained from analysis QMF banks. Eventually, the adjusted QMF subband signals are synthesized in a synthesis QMF bank. In order to perform a modification of the sampling frequency, e.g. in order to double the sampling frequency from the sampling frequency of the low band signal to the sampling frequency of the output signal of the audio coding/decoding system, the number of analysis QMF bands may be different from the number of synthesis QMF bands. In an embodiment, the analysis QMF bank generates 32 QMF subband signals and the synthesis QMF bank processes 64 QMF subbands, thereby providing a doubling of the sampling frequency. It should be noted that typically the analysis and/or synthesis filter banks of the transposer generate several hundred analysis and/or synthesis subbands, thereby providing a significantly higher frequency resolution than the QMF banks.
An example of a process for the generation of a high frequency component of a signal is illustrated in the HFR system 600 of
The impact of transposition by an order T=2 on a signal at a sampling frequency fs is shown in the frequency diagrams illustrated in
As can be seen in
As has been outlined above, the transposer modules 602-2, . . . , 602-P produce time domain signals of different sampling rates, i.e. sampling rates 2fs, . . . , Pfs, respectively. The resampling of the output signals of the transposer modules 602-2, . . . , 602-P is achieved by “inserting” or discarding subband channels in the following corresponding QMF analysis banks 603-1, . . . , 603-P. In other words, the resampling of the output signals of the transposer modules 602-2, . . . , 602-P may be achieved by using a different number of QMF subbands in the subsequent respective analysis QMF banks 603-1, . . . , 603-P and the synthesis QMF bank 605. Hence, the output QMF subband signals from the QMF banks 602-2, . . . , 602-P may need to be fitted into the 64 channels finally being transmitted to the synthesis QMF bank 605. This fitting or mapping may be achieved by mapping or adding the 32 QMF subband signals coming from the 32 channel analysis QMF bank 603-1 to the first 32 channels, i.e. the 32 lower frequency channels, of the synthesis or inverse QMF bank 605. This effectively results in a signal which is filtered by the analysis QMF bank 603-1 to be upsampled by a factor 2. All the subband signals coming from the 64 channel analysis QMF bank 603-2 may be mapped or added directly to the 64 channels of the inverse QMF bank 605. In view of the fact that the analysis QMF bank 603-2 is of exactly the same size as the synthesis QMF bank 605, the respective transposed signal will not be resampled. The QMF banks 603-3, . . . , 603-P have a number of output QMF subband signals which exceeds 64 subband signals. In such cases, the lower 64 channels may be mapped to or added to the 64 channels of the synthesis QMF bank 605. The upper remaining channels may be discarded. As an outcome of the use of a 32·P channel analysis QMF bank 603-P, the signal which is filtered by QMF bank 603-P will be downsampled a factor P/2. Consequently, this resampling depending on the transposition order P will result in all transposed signals having the same sampling frequency.
In other words, it is desirable that the subband signals have the same sampling rates when fed to the HFR processing module 604, even though the transposer modules 602-2, . . . , 602-P produce time domain signals of different sampling rates. This may be achieved by using different sizes of the analysis QMF banks 603-3, . . . , 603-P, where the size typically is 32T, with T being the transposition factor or transposition order. Since the HFR processing module 604 and the synthesis QMF bank 605 typically operate on 64 subband signals, i.e. twice the size of analysis QMF bank 603-1, all subband signals from the analysis QMF banks 603-3, . . . , 603-P with subband indices exceeding this number may be discarded. This can be done since the output signals of the transposers 602-2, . . . , 602-P may actually cover frequency ranges above the Nyqvist frequency fs of the output signal. The remaining subband signals, i.e. the subband signals that have been mapped to the subbands of the synthesis QMF bank 605, may be added to generate frequency overlapping transposed signals (see
As indicated above, in typical embodiments, a plurality of transposers 602-2, . . . , 602-P are used to generate the high frequency component of the output signal of the HFR module 600. It is assumed that the input signal to the transposers 602-2, . . . , 602-P, i.e. the low frequency component of the output signal, has a bandwidth of B Hz and a sampling rate fs and the output signal of the HRF module 600 has a sampling rate 2fs. Consequently, the high frequency component may cover the frequency range [B,fs]. Each of the transposers 602-2, . . . , 602-P may provide a contribution to the high frequency component, wherein the contributions may be overlapping and/or non-overlapping.
c illustrates a similar scenario to
b and 12c illustrate the extreme scenarios of completely overlapping output signals of the transposers 602-2, . . . , 602-P and of completely non-overlapping output signals of the transposers 602-2, . . . , 602-P. It should be noted that mixed scenarios with partly overlapping output signals are possible. Moreover, it should be noted that the two scenarios of
A more efficient implementation of the system of
This means that the output from the downsampler 706 and the output from the transposers 702-2, . . . , 702-P are critically sampled. The output signal of the 2nd order transposer 702-2 would have a sampling frequency fs/Q which is identical to the output signal of the downsampler 706. However, it should be noted that the signal from the 2nd order transposer 702-2 is actually a highpass signal with a bandwidth of fs/(2Q) which is modulated to the baseband, since the transposer 702-2 is configured such that it only synthesizes a transposed frequency range from approximately B to 2B Hz.
For transposers of larger order, e.g. transposer 702-P, at least two likely scenarios are possible. The first scenario is that the transposed signals are overlapping, i.e. the lower frequency part of the Pth order transposed signal is overlapping with the frequency range of the transposed signal of order P−1 (see
which corresponds to a signal covering the frequency interval from fs/(2Q) (highest frequency of lowband signal) up to the Nyqvist frequency fs. The other scenario is that the transposed signals are non-overlapping. In this case S=1, and all transposed signals have identical sampling frequencies, albeit covering different non-overlapping frequency ranges in the output signal of the inverse QMF bank 705, i.e. in the output signal of the HFR system 700 (see
The effect of the described subsampling or downsampling on an output signal of the core decoder 701 having a bandwidth B Hz is illustrated in
Such a critically sampled signal is illustrated in the frequency diagram 1320. This critically sampled signal with sampling frequency fs/Q is passed to the transposer 702-2 where it is segmented into analysis subbands. Such a segmented signal is illustrated in frequency diagram 1330. Subsequently, nonlinear processing is performed on the analysis subband signals which results in a stretching of the analysis subbands to T=2 times higher frequency ranges and a sampling frequency 2fs/Q. This is illustrated in frequency diagram 1340, which alternatively may be viewed as the frequency diagram 1330 with scaled frequency axis. It should be noted that only a subset of the transposed subbands will typically be considered in the HFR processing module 704. These relevant transposed subbands are indicated in frequency diagram 1340 as the hatched subbands which cover the frequency range [B,2B]. Only the hatched subbands may need to be considered in the transposer synthesis filter bank, and hence the relevant range can be modulated down to the baseband and the signal may be downsampled by a factor 2 to a sampling frequency of fs/Q. This is illustrated in frequency diagram 1360, where it can be seen that the signal covering a frequency range [B,2B] has been modulated into the baseband range [0,B]. The fact that the modulated signal actually covers the higher frequency range [B,2B] is illustrated by the reference signs “B” and “2B”.
It should be noted that the illustrated steps of transposition (shown in frequency diagram 1340) and the subsequent modulation into the baseband (shown in frequency diagram 1360) are only shown for illustrative purposes. Both operations may be performed by assigning the hatched subbands (shown in frequency diagram 1340) to the synthesis subbands of a synthesis filter bank having half the number of subbands as the analysis filter bank. As an outcome of such mapping operation, the output signal shown in frequency diagram 1360, which is modulated into the baseband, i.e. which is centered around the zero frequency, may be obtained. In the non-overlapping scenario, the synthesis filter bank size is reduced with respect to the analysis filter bank in order to enable the achievable downsampling factor which is given by the ratio between the full frequency range [0,PB] which may be covered by the output signal of a Pth order transposer 703-P and the actual frequency range [(P−1)B, PB] covered by the output signal of the Pth order transposer 703-P, i.e. the factor P.
In a similar manner to
As already indicated above, it should be noted that the intermediate signals within the transposer 706-P, i.e. notably the signals shown in the frequency diagrams 1340, 1440, 1540, 1640 are not physical signals present in the HFR system shown in
It should be noted that in the example outlined above, the output signal from the core decoder 701 may possibly already be critically sampled with the sampling rate fs/Q when entering the HFR module 700. This can be accomplished, e.g., by using a smaller synthesis transform size than the nominal size in the core decoder 701. In this scenario, computational complexity is decreased because of the smaller synthesis transform used in the core decoder 701 and because of the obsolete downsampler 706.
Another measure for improving the efficiency of an HFR system, is to combine the individual transposers 602-2, . . . , 602-P of
As outlined in the context of
As outlined in the context of
In embodiments, the QMF bank analyzing the core coder signal, i.e. the analysis QMF bank 803-1 of
In a similar manner to the downsampling described in the context of
As mentioned above, the multiple transposers 802, 902, 1002, and 1102 illustrated in
With the examples outlined in the context of
The filter bank, or transform sizes, Na and Ns may be related as
and the hopsizes, or signal strides, δa and δs may be related as
δs=Wδa. (8)
The maximally decimated, or critically sampled, transposer building block 170 may have either the input signal to the analysis filter bank 172, or the output from the synthesis filter bank 174, or both, covering exclusively the spectral bandwidth relevant for the subsequent processing, such as the HFR processing unit 704 of
A plurality of the building blocks 170 may be combined and configured such that a critically sampled transposer system of several transposition orders is obtained. In such a system, one or more of the modules 171-174 of the building block 170 may be shared between the building blocks using different transposition orders. Typically, a system using a common analysis filter bank 301, as outlined in the context of
In the present document, a multiple transposition scheme and system has been described which allows the use of a common analysis filter bank and a common synthesis filter bank. In order to enable the use of a common analysis and synthesis filter bank, an advanced nonlinear processing scheme has been described which involves the mapping from multiple analysis subbands to a synthesis subband. As a result of using a common analysis filter bank and a common synthesis filter bank, the multiple transposition scheme may be implemented at reduced computational complexity compared to conventional transposition schemes. In other words, the computational complexity of harmonic HFR methods is greatly reduced by means of enabling the sharing of an analysis and synthesis filter bank pair for several harmonic transposers, or by one or several harmonic transposers in combination with an upsampler.
Furthermore, various configurations of HFR modules comprising multiple transposition have been described. In particular, configurations of HFR modules at reduced complexity have been described which manipulate critically downsampled signals. The outlined methods and systems may be employed in various decoding devices, e.g. in multimedia receivers, video/audio settop boxes, mobile devices, audio players, video players, etc.
The methods and systems for transposition and/or high frequency reconstruction described in the present document may be implemented as software, firmware and/or hardware. Certain components may e.g. be implemented as software running on a digital signal processor or microprocessor. Other components may e.g. be implemented as hardware and or as application specific integrated circuits. The signals encountered in the described methods and systems may be stored on media such as random access memory or optical storage media. They may be transferred via networks, such as radio networks, satellite networks, wireless networks or wireline networks, e.g. the internet. Typical devices making use of the methods and systems described in the present document are portable electronic devices or other consumer equipment which are used to store and/or render audio signals. The methods and system may also be used on computer systems, e.g. internet web servers, which store and provide audio signals, e.g. music signals, for download.
This application is a continuation of, and claims the benefit of priority to, U.S. patent application Ser. No. 14/614,172 filed on Feb. 4, 2015 and U.S. patent application Ser. No. 13/321,910 filed on Nov. 22, 2011, now issued U.S. Pat. No. 8,983,852 on Mar. 17, 2015, which is a 371 national application of International Patent Application No. PCT/EP2010/057176, filed May 25, 2010, which claims priority and the benefit of U.S. Provisional Patent Application Ser. No. 61/312,107 filed on Mar. 9, 2010 and U.S. Provisional Patent Application Ser. No. 61/181,364 filed on May 27, 2009, the contents of all of which are incorporated by reference herein in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 61312107 | Mar 2010 | US | |
| 61181364 | May 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14614172 | Feb 2015 | US |
| Child | 14882559 | US | |
| Parent | 13321910 | Nov 2011 | US |
| Child | 14614172 | US |
