Regeneration of wideband speech

Abstract

A method of regenerating wideband speech from narrowband speech, the method comprising: receiving samples of a narrowband speech signal in a first range of frequencies; modulating received samples of the narrowband speech signal with a modulation signal having a modulating frequency adapted to upshift each frequency in the first range of frequencies by an amount determined by the modulating frequency wherein the modulating frequency is selected to translate into a target band a selected frequency band within the first range of signals; filtering the modulated samples using a target band filter to form a regenerated speech signal in the target band; and combining the narrow band speech signal with the regenerated speech signal in the target band to regenerate a wideband speech signal, the method comprising the step of controlling the modulated samples to lie in a second range of frequencies identified by determining a signal characteristic of frequencies in the first range of frequencies.

Description

This application is a continuation-in-part of U.S. application Ser. No. 12/456,033, filed on Jun. 10, 2009, and claims priority under 35 U.S.C. § 119 or 365 to Great Britain Application No. 0822537.7, filed Dec. 10, 2008. The entire teachings of the above applications are incorporated herein by reference.

The present invention lies in the field of artificial bandwidth extension (ABE) of narrow band telephone speech, where the objective is to regenerate wideband speech from narrowband speech in order to improve speech naturalness.

In many current speech transmission systems (phone networks for example) the audio bandwidth is limited, at the moment to 0.3-3.4 kHz. Speech signals typically cover a wider band of frequencies, between 50 Hz and 8 kHz being normal. For transmission, a speech signal is encoded and sampled, and a sequence of samples is transmitted which defines speech but in the narrowband permitted by the available bandwidth. At the receiver, it is desired to regenerate the wideband speech, using an ABE method.

ABE algorithms are commonly based on a source-filter model of speech production, where the estimation of the wideband spectral envelope and the wideband excitation regeneration are treated as two independent sub-problems. Moreover, ABE algorithms typically aim at doubling the sampling frequency, for example from 7 to 14 kHz or from 8 to 16 kHz. Due to the lack of shared information between the narrowband and the missing wideband representations, ABE algorithms are prone to yield artefacts in the reconstructed speech signal. A pragmatic approach to alleviate some of these artefacts is to reduce the extension frequency band, for example to only increase the sampling frequency from 8 kHz-12 kHz. While this is helpful, it does not resolve the artefacts completely.

Known spectral-based excitation regeneration techniques either translate or fold the frequency band 0-4 kHz into the 4-8 kHz frequency band. In fact, in speech signals transmitted through current audio channels, the audio bandwidth is 0.3-3.4 kHz (that is, not precisely 0-4 kHz). Translation of the lower frequency band (0-4 kHz) into the upper frequency band (4-8 kHz) results in the frequency sub-band 0-2 kHz being translated (possibly pitch dependent) into the 4-6 kHz sub-band. Due to the commonly much stronger harmonics in the 0-2 kHz region, this typically yields metallic artefacts in the upper band region. Spectral folding produces a mirrored copy of the 2-4 kHz band into the 4-6 kHz band but without preserving the harmonic structure during voice speech. Another possibility is folding and translation around 3.5 kHz for the 7 to 14 kHz case.

A paper entitled “High Frequency Regeneration In Speech Coding Systems”, authored by Makhoul, et al, IEEE International Conference Acoustics, Speech and Signal Processing, April 1979, pages 428-431, discusses these techniques. FIG. 1 is a block diagram of a typical receiver for a baseband decoder in a radio transmission system. A decoder 2 receives a signal transmitted over a transmission channel and decodes the signal to recover speech samples v which were encoded and transmitted at the transmitter (not shown). The speech residual samples v are subject to interpolation at an interpolator 4 to generate a baseband speech signal b. This is in the narrowband 0.3-3.4 kHz. The signal is subject to high frequency regeneration 6 followed by high pass filtering 8. The resulting signal z represents the regenerated wideband part of the speech signal and is added to the narrowband part b at adder 10. The added signal is supplied to a filter 12 (typically an LPC based synthesis filter) which generates an output speech signal r. A number of different high frequency regeneration techniques are discussed in the paper. For a doubling of the sampling frequency spectral folding is obtained by inserting a zero between every speech signal sample. This creates a mirrored spectrum around the frequency corresponding to half the original sampling frequency. Such processing destroys the harmonic structure of the speech signal (unless the fundamental frequency is a multiple of the sampling frequency). Moreover, since speech harmonicity typically decreases as a function of frequency, the spectral folding show too strong spectral peaks in the highest frequencies resulting in strong metallic artefacts.

In a spectral translation approach discussed in the paper, the high band excitation is constructed by adding up-sampled low pass filtered narrowband excitation to a mirrored up-sampled and high pass filtered narrowband excitation.

The mirrored up-sampled narrowband excitation is obtained by first multiplying each sample with (−1)ⁿ, where n denotes the sample index, and then inserting a zero between every sample. Finally, the signal is high pass filtered. As for the spectral folding, the location of the spectral peaks in the high band are most likely not located at a multiple of the pitch frequency. Thus, the harmonic structure is not necessarily preserved in this approach.

It is an aim of the present invention to generate more natural speech from a narrowband speech signal.

According to an aspect of the present invention there is provided a method of regenerating wideband speech from narrowband speech, the method comprising: receiving samples of a narrowband speech signal in a first range of frequencies; modulating received samples of the narrowband speech signal with a modulation signal having a modulating frequency adapted to upshift each frequency in the first range of frequencies by an amount determined by the modulating frequency wherein the modulating frequency is selected to translate into a target band a selected frequency band within the first range of signals; filtering the modulated samples using a target band filter to form a regenerated speech signal in the target band; and combining the narrow band speech signal with the regenerated speech signal in the target band to regenerate a wideband speech signal, the method comprising the step of controlling the modulated samples to lie in a second range of frequencies identified by determining a signal characteristic of frequencies in the first range of frequencies.

The second range of frequencies can be selected by controlling the first range of frequencies and/or the modulating frequency. In that case, the target band filter is a high pass filter wherein the lower limit of the high pass filter defines the lowermost frequency in the target band. Alternatively, the second range of frequencies can be selected by controlling one or more such target band filter to cut as a band pass filter to filter bands determined by analysing the input samples.

It is advantageous to select the modulating frequency so as to upshift a frequency band in the narrowband that is more likely to have a harmonic structure closer to that of the missing (high) frequency band to which it is translated.

Another aspect of the invention provides a system for generating wideband speech from narrowband speech, the system comprising: means for receiving samples of a narrowband speech signal in a first range of frequencies; means for modulating received samples of the narrowband speech signal with a modulation signal having a modulating frequency adapted to upshift each frequency in the first range of frequencies by an amount determined by the modulating frequency wherein the modulating frequency is selected to translate into a target band a selected frequency band within the first range of signals; a target band filter for filtering the modulated samples to form a regenerated speech signal in a target band; means for combining the narrowband speech signal with the regenerated speech signal in the target band to regenerate a wideband speech signal; and means for controlling the modulated samples to lie in a second range of frequencies identified by determining a signal characteristic of frequencies in the first range of frequencies.

The signal characteristic which is determined for selecting frequencies can be chosen from a number of possibilities including frequencies having a minimum echo, minimum pre-processor distortion, degree of voicing and particular temporal structures such as temporal localisation or concentration.

As a particular example, the signal characteristic can be a good signal to noise ratio. Improvements can be gained by selecting a frequency band in the narrowband speech signal that has a good signal-to-noise ratio, and modulating that frequency band for regenerating the missing target band.

The target band filter can be a high pass filter wherein the lower limit of the high pass filter is above the uppermost frequency of the narrowband speech.

It is also possible to average a set of translated signals from overlapping or non-overlapping frequency bands in the narrowband speech signal.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of a prior art HFR approach;

FIG. 2 is a schematic block diagram illustrating the context of the invention;

FIG. 3 is a schematic block diagram of a system according to one embodiment;

FIGS. 4A and 4B are graphs illustrating a typical speech spectrum in the frequency domain;

FIG. 5 is a schematic block diagram of a system according to another embodiment; and

FIG. 6 is a schematic block diagram illustrating alternate embodiments.

Reference will first be made to FIG. 2 to describe the context of the invention.

FIG. 2 is a schematic block diagram illustrating an artificial bandwidth extension system in a receiver. A decoder 14 receives a speech signal over a transmission channel and decodes it to extract a baseband speech signal B. This is typically at a sampling frequency of 8 kHz. The baseband signal B is up-sampled in up-sampling block 16 to generate an up-sampled decoded narrowband speech signal x. The speech signal x is subject to a whitening filter 17 and then wideband excitation regeneration in excitation regeneration block 18 and an estimation of the wideband spectral envelope is then applied at block 20 The thus regenerated extension (high) frequency band of the speech signal is added to the incoming narrowband speech signal x at adder 21 to generate the wideband recovered speech signal r.

Embodiments of the present invention relate to excitation regeneration in the scenario illustrated in the schematic of FIG. 2. In the following described embodiments, a pitch dependent spectral translation translates a frequency band (a range of frequencies from the narrowband speech signal) into a target frequency band with properly preserved harmonics. In the embodiment discussed below, the range of the frequencies from 2-4 kHz is translated to the target frequency band of between 4 and 6 kHz. However, it will be clear from the following that these can be selected differently without diverging from the concepts of the invention. They are used here merely as exemplifying numbers.

FIG. 3 is a schematic block diagram illustrating an excitation regeneration system for use in a receiver receiving speech signals over a transmission channel. The decoder 14 and up-sampler 16 perform functions as described with reference to FIG. 2. That is, the incoming signal is decoded and up-sampled from 8 kHz to 12 kHz. A low pass filter 22 is provided for some embodiments to select a region of the narrowband speech signal x for modulation, but this is not required in all embodiments and will be described later.

A modulator 24 receives a modulation signal m which modulates a range of frequencies of the speech signal x to generate a modulated signal y. If the filter 22 is not present, this is all frequencies in the narrowband speech signal. In this embodiment, the modulation signal is at 2 kHz and so moves the frequencies 0-4 kHz into the 2-6 kHz range (that is, by an amount 2 kHz). The signal y is passed through a high pass filter 26 having a lower limit at 4 kHz, thereby discarding the 0-4 kHz translated signal. Thus a high band reconstructed speech signal z is generated, the high band being the target frequency band of 4-6 kHz. The regenerated high band signal is subject to a spectral envelope and the resulting signal is added back to the original speech signal x to generate a speech signal r as described with reference to FIG. 2.

The modulation signal m is of the form2πf_modn+φ, where f_mod denotes the modulating frequency, φ the phase and n a running index. The modulation signal is generated by block 28 which chooses the modulating frequency f mod and the phase φ. The modulation frequency f_mod is determined such as to preserve the harmonic structure in the regenerated excitation high band. In the present implementation, the modulating frequency is normalised by the sampling frequency.

Taking the specific example, consider the pitch frequency to be 180 Hz, then the closest frequency to 2 kHz that is an integer multiple of the pitch frequency is floor(200/180)*180 (1980 Hz). Normalised by 1200 Hz it becomes 0.165. For a sampling frequency (after upsampling) of 12 kHz and a value of 2 kHz of the frequency shift, the frequency f_mod can be expressed as f_mod=floor(p/6)/p, where p represents the fractional pitch-lag.

The speech signal x is in the form [x(n), . . . , x(n+T−1)] which denotes a speech block of length T of up-sampled decoded narrow band speech. To ensure signal continuity between adjacent speech blocks, the phase φ is updated every block as follows φ=_mod(φ+πf_modT,2π), where mod(.,.) denotes the modulo operator (remainder after division). Each signal block of length T is multiplied by the T-dim vector[cos(2*π*f_mod*1+φ), . . . cos(2*π*f_mod*T+φ]. Thus,y=[y(n), . . . y(n+T−1)]=[2x(n)cos(2πf_mod+φ), . . . 2x(n+T−1)cos(2πf_modT+φ].

The frequency band of the narrow band speech x which is translated can be selected to alleviate metallic artefacts by selection of a frequency band that is more likely to have harmonic structure closer to that of the missing (high) frequency band by selection of a frequency band that includes frequencies showing an identified signal characteristic, e.g. a good signal-to-noise ratio. The method can include averaging a set of translated signals with overlapping bands.

Reference will now be made to FIG. 4A to describe how the preceding described embodiment translates a frequency band which has a harmonic structure close to that of the missing high frequency band. FIG. 4A shows the spectrum of the speech signal in the frequency domain. “i” denotes the envelope of speech as originally recorded, and “ii” denotes the envelope for transmission in the 0.3-3.4 (approximated as 0-4) kHz range. By application of a modulation signal with a frequency of 2 kHz to all the frequencies in the transmitted narrowband speech (envelope ii), the spectrum is shifted upwards by 2 kHz, denoted by the arrow on FIG. 4A. This has the effect of moving the 0-2 kHz range up to 2-4 kHz, and the 2-4 kHz range up to 4-6 kHz. The high pass filter 26 filters out the signal below the 4 kHz level and thus regenerates the missing high band 4-6 kHz speech.

An alternative possibility is shown in FIG. 4B. If a modulating frequency of 3 kHz is applied, the spectrum shifts by 3 kHz, moving the 0-1 kHz range to 3-4 kHz, and the 1-3 kHz range to 4-6 kHz. The 0-1 kHz translation is filtered out with the high pass filter 26. In order to avoid aliasing, in this embodiment the low pass filter 22 filters out frequencies above 3 kHz so that these are not subject to modulation. It can be seen that by using this technique, it is possible to select frequency bands of the transmitted narrowband speech by controlling the modulating frequency. One possibility, as mentioned above, is to select the frequency bands by determining a signal characteristic of frequencies in the narrowband speech.

In FIG. 3, control block 30 is shown as having this function.

The control block 30 receives the speech signal x and has a process for evaluating a signal characteristic for the purpose of selecting the frequency band that is to be translated.

The signal characteristic can be chosen from a number of different possibilities. According to one example, the block 30 is a signal to noise ratio block which evaluates a signal to noise ratio in each frequency band in the narrow band speech signal, and selects the frequency band to be translated to include frequencies with the highest signal to noise ratio.

A further possibility is that the block 30 is an echo detection block, which evaluates the frequency bands with minimum echo.

A further possibility is that the block 30 determines the degree of voicing. According to one example, a measure of the degree of voicing can be the normalised correlation between the signal inside a frequency band and the same signal one pitch-cycle earlier. Smoothed versions of this measure can also be used to determine whether or not a frequency should be included in the first range of frequencies for translation.

As a further alternative, a measure of temporal structure can be provided, such as a measure of temporal localisation or temporal concentration. One measure of temporal localisation could be developed in accordance with the equation given below, although it will be appreciated that other measures of localisation could be utilised.

$\frac{\sum _{\mathrm{frame}}\left({x^2\left(t-t_{\mathrm{mean}}\right)}^2\right)}{\sum _{\mathrm{frame}}{x}^2}$ where

$\sum _{\mathrm{frame}}$ means the sum over a frame of samples, x denotes a sample index, t denotes a time index and t_mean=Σx²t/Σx².

FIG. 5 is a schematic block diagram of a high band regeneration system which allows for a set of translated signals with overlapping or non-overlapping bands to be averaged. For example, the band 1 to 3 kHz could be taken and averaged with the band 2 to 4 kHz for regeneration of excitation in the 4 to 6 kHz range. This allows simultaneous excitation regeneration and noise reduction by varying the modulation frequency. FIG. 5 shows the speech signal x from the up-sampler 16 being supplied to each of a plurality of paths, three of which are shown in FIG. 5. It will be appreciated that any number is possible. The signal is supplied to a low pass filter in each path 22a, 22b and 22c, each low pass filter being adapted to select the band which is to be translated by setting an upper frequency limit as described above. Not all paths need to have a filter.

The low pass filtered signal from each filter is supplied to respective modulator 24a, 24b, 24c, each modulator being controlled by a modulation signal ma, mb, mc at different frequencies. The resulting modulated signal is supplied to a high pass filter 26a, 26b, 26c in each path to produce a plurality of high band regenerated excitation signals. The high pass filters have their lower limits set appropriately, e.g. to 4 kHz lower limit of the missing (or desired target) high band, if different. The signals are weighted using weighting functions 34a, 34b, 34c by respective weights w1, w2, w3, and the weighted values are supplied to a summer 36. The output of the summer 36 is the desired regenerated excitation high band signal. This is subject to a spectral envelope 20 and added to the original narrow band speech signal x as in FIG. 2 to generate the speech signal r.

The described embodiments of the present invention have significant advantages when compared with the prior art approaches. The approach described herein combines the preservation of harmonic structure and allows for the selection of a frequency band that is more likely to have a harmonic structure closer to that of the missing (high) frequency band, thus alleviating some of the metallic artefacts. Furthermore, if the original narrow band speech signal contains noise (due to acoustic noise and/or coding) it is beneficial to spectrally translate a region of the narrow band speech signal that shows the highest signal-to-noise ratio or perform several different spectral translations and linearly combine these to achieve simultaneous excitation regeneration and noise reduction (as shown in FIG. 5). *In the extreme case of zero linear combination weight for some frequency regions, this becomes equivalent with combining frequency intervals of less than 2 kHz to form a band of for example 2 kHz width. Also, the same frequency component may be replicated more than once within the 2 kHz range. In the general case number frequency shifted versions would be filtered each through a specific weighting filter and then added to create the combined signal in the full frequency range of interest.

By using a set of overlap/non-overlap sub-bands, it is possible to regenerate a given frequency band with less artefacts than would otherwise be experienced.

Reference will now be made to FIG. 6 to describe a further embodiment of the present invention. In the embodiment described above with reference to FIG. 3, the purpose of the control block is to select a modulating frequency which will have the effect of translating a controlled range of input frequencies by a shift determined by the control block 30. The range of input frequencies is controlled by the low pass filter 22 in FIG. 3. The combination of control of the input frequencies by the low pass filter 22 and control of the up-shift by the modulating frequency as managed by control block 30 significantly improves the naturalness of the speech which is generated in the reconstructive speech signal.

FIG. 6 illustrates other possibilities for achieving this aim. In FIG. 6, the control block 30 is replaced by a signal analyser 60 and a control unit 62. The signal analyser 60 is responsible for determining the signal characteristics mentioned above which can be used to control the range of frequencies. This analysis is performed on the input samples x. The result of the analysis is supplied to the control unit 62 which can select to control one or more of the low pass filter 22, the modulating frequency f_m, a target band filter 26′ primed or weighting function w.

In some embodiments, the target band filter 26′ will be a high pass filter such as that denoted by 26 in FIG. 3. In other embodiments however it can be a filterbank which is capable of selecting individual bands from within a frequency range which can then be combined by weighting functions (for example as described with reference to FIG. 5).

The control unit 62 can control one or more of the above parameters depending on the implementation possibilities and the desired output. It will be appreciated that, for example, where the first range of frequencies is controlled using the low pass filter 22 so that the first range of frequencies satisfy certain identified signal characteristics, it may not be necessary to additionally alter or control the modulating frequency fm. Moreover, the target band filter 26′ could then be a high pass filter with its lower limits set at the lower most frequency in the target band.

In an alternative scenario, the modulating frequency fm can be controlled as described above with reference to FIG. 3, and in that case can operate on all input frequencies (without the low pass filter 22), or on a filtered range of frequencies.

A still further possibility is to control the output band using the target band filter 26′ such that only selected frequencies are combined to form a regenerated feature signal in the target band, these frequencies being based on frequencies analysed on the input side as having certain identified signal characteristics of the type mentioned above.

Claims

1. A system implemented in a receiver for generating wideband speech from narrowband speech, the system comprising: means for receiving samples of a narrowband speech signal in a first range of frequencies, the narrowband speech signal missing at least a portion of the wideband speech from which the narrowband speech signal was generated; means for controlling which frequencies in the first range of frequencies are to be translated into a target band, the controlling including determining a signal characteristic for selecting which of the frequencies in the first range of frequencies are to be translated into the target band, the signal characteristic comprising one of a minimum echo, a minimum pre-processor distortion, or a minimum degree of voicing, and the signal characteristic that is determined being identifiable in the frequencies in the first range that are more likely, when translated according to a pitch-dependent spectral translation, to result in a regenerated wideband speech signal having a harmonic structure that approximates a harmonic structure of the portion of the wideband speech that is missing; means for modulating the received samples of the narrowband speech signal with a modulation signal having a modulating frequency adapted to upshift each frequency in the first range of frequencies by the modulating frequency, the modulating frequency selected to translate into the target band a selected frequency band of the first range of frequencies that is selected according the determined signal characteristic; a target band filter implemented at least partially in hardware for filtering the modulated samples to form a regenerated speech signal in a target band; and means for combining the narrowband speech signal with the regenerated speech signal in the target band to regenerate a wideband speech signal.

2. A system according to claim 1, further comprising means for selecting said first range of frequencies from frequencies in the narrowband speech signal.

3. A system according to claim 1, further comprising means for generating the modulation signal controlling the modulating frequency and controlling a phase of the modulation signal.

4. A system according to claim 1, further comprising means for determining the signal characteristic at each frequency in the narrowband speech signal, said first range of frequencies being those with the determined signal characteristic.

5. A system according to claim 1 wherein the means for controlling is configured to selectively control at least one of the first range of frequencies, the modulating frequency, and or target band filter.

6. A system according to claim 1, further comprising a plurality of paths, each path configured to receive samples of a narrowband speech signal, there being a plurality of modulating means associated respectively with the paths and a plurality of high pass filters associated respectively with the paths, the system further comprising means for combining the modulated, filtered signals on each path to form the regenerated speech signal in the target band.

7. A system according to claim 6, wherein at least one of said paths comprises means for selecting the first range of frequencies from the narrowband speech signal.

8. A system according to claim 6, further comprising weighting means associated with each pith for weighting the modulated, filtered signals prior to the combining means.

9. A system according to claim 2, wherein the selecting means is a low pass filter.