Abstract:
A method, system and program for encoding and decoding speech according to a source-filter model whereby speech is modeled to comprise a source signal filtered by a time-varying filter. The method comprises: receiving a speech signal; and from the speech signal, deriving a spectral envelope signal representing the modeled filter and a remaining signal representing the modeled source. At intervals during the encoding, the method further comprises determining a period between portions of the remaining signal having a degree of repetition and determining a correlation between said portions based on that period, thus producing a respective vector of the correlation for each interval. Once every number of said intervals, the method further comprises selecting a codebook from a plurality of codebooks for quantizing the vectors, quantizing the vectors of that number of intervals according to the selected codebook, and transmitting the quantized vectors along with an indication of the selected codebook.

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119 or 365 to Great Britain Application No. 0900144.7, filed Jan. 6, 2009. The entire teachings of the above application are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the encoding of speech for transmission over a transmission medium, such as by means of an electronic signal over a wired connection or electromagnetic signal over a wireless connection. 
     BACKGROUND 
     A source-filter model of speech is illustrated schematically in  FIG. 1   a . As shown, speech can be modelled as comprising a signal from a source  102  passed through a time-varying filter  104 . The source signal represents the immediate vibration of the vocal chords, and the filter represents the acoustic effect of the vocal tract formed by the shape of the throat, mouth and tongue. The effect of the filter is to alter the frequency profile of the source signal so as to emphasise or diminish certain frequencies. Instead of trying to directly represent an actual waveform, speech encoding works by representing the speech using parameters of a source-filter model. 
     As illustrated schematically in  FIG. 1   b , the encoded signal will be divided into a plurality of frames  106 , with each frame comprising a plurality of subframes  108 . For example, speech may be sampled at 16 kHz and processed in frames of 20 ms, with some of the processing done in subframes of 5 ms (four subframes per frame). Each frame comprises a flag  107  by which it is classed according to its respective type. Each frame is thus classed at least as either “voiced” or “unvoiced”, and unvoiced frames are encoded differently than voiced frames. Each subframe  108  then comprises a set of parameters of the source-filter model representative of the sound of the speech in that subframe. 
     For voiced sounds (e.g. vowel sounds), the source signal has a degree of long-term periodicity corresponding to the perceived pitch of the voice. In that case, the source signal can be modelled as comprising a quasi-periodic signal, with each period corresponding to a respective “pitch pulse” comprising a series of peaks of differing amplitudes. The source signal is said to be “quasi” periodic in that on a timescale of at least one subframe it can be taken to have a single, meaningful period which is approximately constant; but over many subframes or frames then the period and form of the signal may change. The approximated period at any given point may be referred to as the pitch lag. An example of a modelled source signal  202  is shown schematically in  FIG. 2   a  with a gradually varying period P 1 , P 2 , P 3 , etc., each comprising a pitch pulse of four peaks which may vary gradually in form and amplitude from one period to the next. 
     According to many speech coding algorithms such as those using Linear Predictive Coding (LPC), a short-term filter is used to separate out the speech signal into two separate components: (i) a signal representative of the effect of the time-varying filter  104 ; and (ii) the remaining signal with the effect of the filter  104  removed, which is representative of the source signal. The signal representative of the effect of the filter  104  may be referred to as the spectral envelope signal, and typically comprises a series of sets of LPC parameters describing the spectral envelope at each stage.  FIG. 2   b  shows a schematic example of a sequence of spectral envelopes  204   1 ,  204   2 ,  204   3 , etc. varying over time. Once the varying spectral envelope is removed, the remaining signal representative of the source alone may be referred to as the LPC residual signal, as shown schematically in  FIG. 2   a . The short-term filter works by removing short-term correlations (i.e. short term compared to the pitch period), leading to an LPC residual with less energy than the speech signal. 
     The spectral envelope signal and the source signal are each encoded separately for transmission. In the illustrated example, each subframe  106  would contain: (i) a set of parameters representing the spectral envelope  204 ; and (ii) an LPC residual signal representing the source signal  202  with the effect of the short-term correlations removed. 
     To improve the encoding of the source signal, its periodicity may be exploited. To do this, a long-term prediction (LTP) analysis is used to determine the correlation of the LPC residual signal with itself from one period to the next, i.e. the correlation between the LPC residual signal at the current time and the LPC residual signal after one period at the current pitch lag (correlation being a statistical measure of a degree of relationship between groups of data, in this case the degree of repetition between portions of a signal). In this context the source signal can be said to be “quasi” periodic in that on a timescale of at least one correlation calculation it can be taken to have a meaningful period which is approximately (but not exactly) constant; but over many such calculations then the period and form of the source signal may change more significantly. A set of parameters derived from this correlation are determined to at least partially represent the source signal for each subframe. The set of parameters for each subframe is typically a set of coefficients C of a series, which form a respective vector C LTP =(C 1 , C 2 , . . . C i ). 
     The effect of this inter-period correlation is then removed from the LPC residual, leaving an LTP residual signal representing the source signal with the effect of the correlation between pitch periods removed. To represent the source signal, the LTP vectors and LTP residual signal are encoded separately for transmission. 
     The sets of LPC parameters, the LTP vectors and the LTP residual signal are each quantized prior to transmission (quantization being the process of converting a continuous range of values into a set of discrete values, or a larger approximately continuous set of discrete values into a smaller set of discrete values). The advantage of separating out the LPC residual signal into the LTP vectors and LTP residual signal is that the LTP residual typically has a lower energy than the LPC residual, and so requires fewer bits to quantize. 
     So in the illustrated example, each subframe  106  would comprise: (i) a quantised set of LPC parameters representing the spectral envelope, (ii)(a) a quantised LTP vector related to the correlation between pitch periods in the source signal, and (ii)(b) a quantised LTP residual signal representative of the source signal with the effects of this inter-period correlation removed. 
     To compress the LTP vectors for transmission, they are quantized according to a vector quantization. This is done using a predetermined codebook comprising a plurality of discrete, predetermined vectors each being allocated a corresponding index. The vector quantization process then involves determining which of the predetermined vectors the vector being quantized is most similar to, and then representing that vector using the corresponding index from the codebook. An example codebook  302  having M entries each with a vector of i parameters is shown schematically in  FIG. 3 . The codebook is known to both the encoder and decoder. Thus only a single codebook index is needed to encode a vector, rather than the actual values of the parameters making up the vector. This therefore requires fewer bits to encode, and so reduces transmission overhead. 
     However, it would be desirable to further improve the quantization of encoding schemes such as LTP which encode speech using a correlation between approximately periodic portions of a source signal of a source-filter model. 
     SUMMARY 
     According to one aspect of the present invention, there is provided a method of encoding speech according to a source-filter model whereby speech is modelled to comprise a source signal filtered by a time-varying filter, the method comprising: receiving a speech signal; from the speech signal, deriving a spectral envelope signal representative of the modelled filter and a first remaining signal representative of the modelled source signal; at each of a plurality of intervals during the encoding, determining a period between portions of the first remaining signal having a degree of repetition and determining a correlation between said portions based on said period, thus producing a respective vector of the correlation for each interval, each vector comprising a plurality of parameters derived from the respective correlation; once every number of said intervals, selecting a codebook from a plurality of codebooks for quantizing said vectors, quantizing the vectors of that number of intervals according to the selected codebook, and transmitting the quantized vectors along with an indication of the selected codebook over a transmission medium as part of an encoded signal representative of said speech signal. 
     In embodiments, the selection may comprise quantizing at least one of the vectors of said number of intervals according to each of said plurality of codebooks, and selecting a codebook based on comparison of said quantizations. 
     The selection may comprise quantizing all of the vectors of said number of intervals according to each of said plurality of codebooks, and selecting a codebook based on comparison of said quantizations. 
     The selection may be based on comparison of a distortion measure evaluated for the vectors of said number of intervals as quantized according to each of said codebooks. 
     The comparison may be based on the distortion measure weighed against a bitrate required to encode the vectors of said number of intervals according to each codebook. 
     The encoding may be performed over a plurality of frames, each frame comprising a plurality of subframes; each of said intervals may be a subframe; and said number may be the number of subframes per frame such that said selection is performed once per frame. Alternatively, said number may be one. 
     The method may further comprise: extracting a signal comprising said vectors from the first remaining signal, thus leaving a second remaining signal; and transmitting parameters of the second remaining signal over the communication medium as part of said encoded signal 
     The extraction of said second remaining signal from the first remaining signal may be by long term prediction. 
     The derivation of said first remaining signal from the speech signal may be by linear predictive coding. 
     According to another aspect of the present invention, there is provided a method of decoding an encoded signal comprising speech encoded according to a source-filter model whereby the speech is modelled to comprise a source signal filtered by a time-varying filter, the method comprising: receiving a encoded signal over a communication medium; at intervals during the decoding of said encoded signal, determining an index of a respective quantized vector from the encoded signal, each vector relating to a correlation between portions of the modelled source signal having a degree of repetition; once every number of said intervals, determining an indicator of a codebook from the encoded signal, selecting the indicated codebook from a plurality of codebooks said vectors, and using the selected codebook to determine the vectors of said number of intervals from their respective indices; generating a decoded speech signal based on the determined vectors, and outputting the decoded speech signal to an output device. 
     According to another aspect of the present invention, there is provided an encoder for encoding speech according to a source-filter model whereby speech is modelled to comprise a source signal filtered by a time-varying filter, the encoder comprising: an input arranged to receive a speech signal; a first signal-processing module configured to derive, from the speech signal, a spectral envelope signal representative of the modelled filter and a first remaining signal representative of the modelled source signal; a second signal-processing module configured to determine, at each of a plurality of intervals during the encoding, a period between portions of the first remaining signal having a degree of repetition and determine a correlation between said portions based on said period, thus producing a respective vector of the correlation for each interval, each vector comprising a plurality of parameters derived from the respective correlation; wherein the second signal-processing module is further configured to select, once every number of said intervals, a codebook from a plurality of codebooks for quantizing said vectors, to quantize the vectors of that number of intervals according to the selected codebook, and to transmit the quantized vectors along with an indication of the selected codebook over a transmission medium as part of an encoded signal representative of said speech signal. 
     According to another aspect of the present invention, there is provided a decoder for decoding an encoded signal comprising speech encoded according to a source-filter model whereby the speech is modelled to comprise a source signal filtered by a time-varying filter, the decoder comprising: an input module for receiving an encoded signal over a communication medium; and a signal-processing module configured to determine, at intervals during the decoding of said encoded signal, an index of a respective quantized vector from the encoded signal, each vector relating to a correlation between portions of the modelled source signal having a degree of repetition; wherein the signal-processing module is further configured to determine, once every number of said intervals, an indicator of a codebook from the encoded signal, to select the indicated codebook from a plurality of codebooks said vectors, and to use the selected codebook to determine the vectors of said number of intervals from their respective indices; and the decoder further comprises an output module configured to generate a decoded speech signal based on the determined vectors, and output the decoded speech signal to an output device. 
     According to another aspect of the present invention, there is provided a computer program product for encoding speech according to a source-filter model whereby the speech is modelled to comprise a source signal filtered by a time-varying filter, the program comprising code arranged so as when executed on a processor to:
         receive a speech signal;   from the speech signal, derive a spectral envelope signal representative of the modelled filter and a first remaining signal representative of the modelled source signal;   at each of a plurality of intervals during the encoding, determine a period between portions of the first remaining signal having a degree of repetition and determine a correlation between said portions based on said period, thus producing a respective vector of the correlation for each interval, each vector comprising a plurality of parameters derived from the respective correlation;   once every number of said intervals, select a codebook from a plurality of codebooks for quantizing said vectors, quantize the vectors of that number of intervals according to the selected codebook, and transmit the quantized vectors along with an indication of the selected codebook over a transmission medium as part of an encoded signal representative of said speech signal.       

     According to another aspect of the present invention, there is provided a computer program product for decoding an encoded signal comprising speech encoded according to a source-filter model whereby the speech is modelled to comprise a source signal filtered by a time-varying filter, the program comprising code arranged so as when executed on a processor to:
         receive an encoded signal over a communication medium;   at intervals during the decoding of said encoded signal, determine an index of a respective quantized vector from the encoded signal, each vector relating to a correlation between portions of the modelled source signal having a degree of repetition;   once every number of said intervals, determine an indicator of a codebook from the encoded signal, select the indicated codebook from a plurality of codebooks said vectors, and use the selected codebook to determine the vectors of said number of intervals from their respective indices; and   generate a decoded speech signal based on the determined vectors, and outputting the decoded speech signal to an output device.       

     According to further aspects of the present invention, there are provided corresponding computer program products such as client application products. 
     According to another aspect of the present invention, there is provided a communication system comprising a plurality of end-user terminals each comprising a corresponding encoder and/or decoder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show how it may be carried into effect, reference will now be made by way of example to the accompanying drawings in which: 
         FIG. 1   a  is a schematic representation of a source-filter model of speech, 
         FIG. 1   b  is a schematic representation of a frame 
         FIG. 2   a  is a schematic representation of a source signal 
         FIG. 2   b  is a schematic representation of variations in a spectral envelope, 
         FIG. 3  is a schematic representation of a codebook for quantising vectors, 
         FIG. 4  is another schematic representation of a frame, 
         FIG. 5  is a schematic block diagram of an encoder, 
         FIG. 6  is a schematic block diagram of a noise shaping quantizer, and 
         FIG. 7  is a schematic block diagram of a decoder. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Long-term prediction (LTP) is a common technique in speech coding, whereby correlations between pitch pulses are exploited to improve coding efficiency. In the encoder, an LTP analysis filter uses one or more pitch lags and one or more LTP coefficients to compute an LTP residual signal from an LPC residual. The LTP residual has smaller variance and can thus be encoded more efficiently than the LPC residual. The pitch lags and LTP coefficients are sent to the decoder together with the coded LTP residual, and used to construct the speech output signal. 
     In order to minimize the LTP residual, it is advantageous to update the LTP coefficients frequently. Typically, new coefficients are defined for every subframe of 5 or 10 milliseconds. However, transmitting quantized LTP coefficients comes at a cost in bitrate, as it typically takes 4 to 6 bits to encode one LTP vector. 
     One approach to reducing the bitrate is to jointly quantize the LTP coefficients for all subframes with a single vector quantizer. However, such a vector quantizer uses a large codebook of thousands of codebook vectors, requiring a large amount of ROM storage and incurring a high cost in computation complexity. 
     In preferred embodiments, the present invention provides a method of encoding a speech signal using multiple vector quantization codebooks for quantizing long-term prediction coefficients, and selecting an LTP quantization codebook out of multiple LTP quantization codebooks to quantize multiple LTP vectors. 
     For frames classified as voiced, a long-term prediction (LTP) filter reduces the energy of the linear prediction coding (LPC) residual. The resulting LTP residual can be quantized and coded more efficiently than the LPC residual. The LTP filter is preferably a five-tap filter for which the coefficients are found in an LTP analysis. Since the decoder needs to apply an inverse LTP filtering to construct the decoded speech signal, the LTP filter coefficients are quantized and transmitted to the decoder. The LTP coefficients are updated every subframe, where four subframes are contained in a frame, and in each subframe five LTP coefficients are specified. 
     The LTP coefficients for each subframe are quantized using Entropy Constrained Vector Quantization. A total of three vector codebooks are available for quantization, with difference rate-distortion trade-offs. The three codebooks have 10, 20 and 40 vectors and average rates of about 3, 4, and 5 bits per vector, respectively. The codebook search for the subframe LTP vectors is constrained to only allow codebook vectors that are chosen from the same codebook. 
     To find the best codebook, each of the three vector codebooks is used to quantize each subframe LTP vector and produce a weighted rate-distortion measure, and the vector codebook with the lowest combined rate-distortion over all subframes is chosen. The quantized LTP vectors are used in the noise shaping quantizer, and the index of the codebook plus the four indices for the four subframe codebook vectors are entropy coded and sent to the decoder. 
     Selecting and indicating one of several smaller codebooks to quantize multiple LTP vectors leads to a lower bitrate than using one large codebook. If the large codebook were to be constructed from the several smaller codebooks, then a method to encode the quantization index for an LTP vector would be to first indicate one of the smaller codebooks and subsequently index a vector in the indicated smaller codebook. This encoding method uses a codebook indicator for every LTP vector. The preferred method of the present invention, however, uses only one codebook indicator for all LTP vectors in a frame. This results in a lower bitrate. 
     Using the same codebook for quantizing multiple LTP vectors in a frame puts a constraint on the codebook vectors that can be used to represent different LTP vectors. However, this has little impact on quantization performance because which codebook is most efficient for quantizing an LTP vector depends on the periodicity of the speech signal and the change in pitch pulse amplitude. Both these aspects are typically almost constant during a frame for speech. Consequently, one codebook can usually efficiently encode all LTP vectors in a frame. 
       FIG. 4  is a schematic representation of a frame according to a preferred embodiment of the present invention. In addition to the classification flag  107  and subframes  108  as discussed in relation to  FIG. 1   b , the frame additionally comprises an indicator  109  of the codebook selected to quantize the vectors of that frame. 
     An example of an encoder  500  for implementing the present invention is now described in relation to  FIG. 5 . 
     The encoder  500  comprises a high-pass filter  502 , a linear predictive coding (LPC) analysis block  504 , a first vector quantizer  506 , an open-loop pitch analysis block  508 , a long-term prediction (LTP) analysis block  510 , a second vector quantizer  512 , a noise shaping analysis block  514 , a noise shaping quantizer  516 , and an arithmetic encoding block  518 . The high pass filter  502  has an input arranged to receive an input speech signal from an input device such as a microphone, and an output coupled to inputs of the LPC analysis block  504 , noise shaping analysis block  514  and noise shaping quantizer  516 . The LPC analysis block has an output coupled to an input of the first vector quantizer  506 , and the first vector quantizer  506  has outputs coupled to inputs of the arithmetic encoding block  518  and noise shaping quantizer  516 . The LPC analysis block  504  has outputs coupled to inputs of the open-loop pitch analysis block  508  and the LTP analysis block  510 . The LTP analysis block  510  has an output coupled to an input of the second vector quantizer  512 , and the second vector quantizer  512  has outputs coupled to inputs of the arithmetic encoding block  518  and noise shaping quantizer  516 . The open-loop pitch analysis block  508  has outputs coupled to inputs of the LTP  510  analysis block  510  and the noise shaping analysis block  514 . The noise shaping analysis block  514  has outputs coupled to inputs of the arithmetic encoding block  518  and the noise shaping quantizer  516 . The noise shaping quantizer  516  has an output coupled to an input of the arithmetic encoding block  518 . The arithmetic encoding block  518  is arranged to produce an output bitstream based on its inputs, for transmission from an output device such as a wired modem or wireless transceiver. 
     In operation, the encoder processes a speech input signal sampled at 16 kHz in frames of 20 milliseconds, with some of the processing done in subframes of 5 milliseconds. The output bitsream payload contains arithmetically encoded parameters, and has a bitrate that varies depending on a quality setting provided to the encoder and on the complexity and perceptual importance of the input signal. 
     The speech input signal is input to the high-pass filter  504  to remove frequencies below 80 Hz which contain almost no speech energy and may contain noise that can be detrimental to the coding efficiency and cause artifacts in the decoded output signal. The high-pass filter  504  is preferably a second order auto-regressive moving average (ARMA) filter. 
     The high-pass filtered input x HP  is input to the linear prediction coding (LPC) analysis block  504 , which calculates 16 LPC coefficients a i  using the covariance method which minimizes the energy of the LPC residual r LPC : 
                   r     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (   n   )       =         x   HP     ⁡     (   n   )       -       ∑     i   =   1     16     ⁢         x   HP     ⁡     (     n   -   i     )       ⁢     a   i             ,         
where n is the sample number. The LPC coefficients are used with an LPC analysis filter to create the LPC residual.
 
     The LPC coefficients are transformed to a line spectral frequency (LSF) vector. The LSFs are quantized using the first vector quantizer  506 , a multi-stage vector quantizer (MSVQ) with 10 stages, producing 10 LSF indices that together represent the quantized LSFs. The quantized LSFs are transformed back to produce the quantized LPC coefficients for use in the noise shaping quantizer  516 . 
     The LPC residual is input to the open loop pitch analysis block  508 , producing one pitch lag for every 5 millisecond subframe, i.e., four pitch lags per frame. The pitch lags are chosen between 32 and 288 samples, corresponding to pitch frequencies from 56 to 500 Hz, which covers the range found in typical speech signals. Also, the pitch analysis produces a pitch correlation value which is the normalized correlation of the signal in the current frame and the signal delayed by the pitch lag values. Frames for which the correlation value is below a threshold of 0.5 are classified as unvoiced, i.e., containing no periodic signal, whereas all other frames are classified as voiced. The pitch lags are input to the arithmetic coder  518  and noise shaping quantizer  516 . 
     For voiced frames, a long-term prediction analysis is performed on the LPC residual. The LPC residual r LPC  is supplied from the LPC analysis block  504  to the LTP analysis block  510 . For each subframe, the LTP analysis block  510  solves normal equations to find 5 linear prediction filter coefficients b i  such that the energy in the LTP residual r LTP  for that subframe: 
                 r     L   ⁢           ⁢   T   ⁢           ⁢   P       ⁡     (   n   )       =         r     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (   n   )       -       ∑     i   =     -   2       2     ⁢         r     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (     n   -   lag   -   i     )       ⁢     b   i                 
is minimized. The normal equations are solved as:
 
 b=W   LTP   −1   C   LTP ,
 
where W LTP  is a weighting matrix containing correlation values
 
                   W     L   ⁢           ⁢   T   ⁢           ⁢   P       ⁡     (     i   ,   j     )       =       ∑     n   =   0     79     ⁢         r     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (     n   +   2   -   lag   -   i     )       ⁢       r     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (     n   +   2   -   lag   -   j     )             ,         
and C LTP  is a correlation vector:
 
     
       
         
           
             
               
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     For voiced frames, the prediction analysis described above results in four sets (one set per subframe) of five LTP coefficients, plus four weighting matrices. The LTP coefficients for each subframe are quantized using Entropy Constrained Vector Quantization. A total of three vector codebooks are available for quantization, with different rate-distortion trade-offs. The three codebooks have 10, 20 and 40 vectors and average rates of about 3, 4, and 5 bits per vector, respectively. Consequently, the first codebook has larger average quantization distortion at a lower rate, whereas the last codebook has smaller average quantization distortion at a higher rate. 
     The energy of the LTP residual is computed as 
                 E     L   ⁢           ⁢   T   ⁢           ⁢   P       =       ∑     n   =   0     79     ⁢         r     L   ⁢           ⁢   T   ⁢           ⁢   P       ⁡     (   n   )       2         ,         
and used to create the normalized weighting matrix W LTP,norm  
 
     
       
         
           
             
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     Given the weighting matrix W LTP,norm , LTP residual energy E LTP  and LTP vector b, the weighted rate-distortion measure for a codebook vector cb i  with rate r i  is give by:
 
 RD=u ( b−cb   j ) T   W   LTP,norm ( b−cb   j )+ r   j ,
 
where u is a fixed, heuristically determined parameter balancing the distortion and rate. Which codebook gives the best performance for a given LTP vector depends on the normalized weighting matrix for that LTP vector. For example, for a small W LTP,norm , it is advantageous to use the codebook with 10 vectors as it has a lower average rate. For a large W LTP,norm , on the other hand, it is often better to use the codebook with 40 vectors, as it is more likely to contain a codebook vector resulting in a small distortion.
 
     The normalized weighting matrix W LTP,norm  depends mostly on two aspects of the input signal. The first is the periodicity of the signal; the more periodic the larger W LTP,norm . The second is the change in signal energy in the current subframe, relative to the signal one pitch lag earlier. A decaying energy leads to a larger W LTP,norm  than an increasing energy. Both aspects do not fluctuate very fast which causes the W LTP,norm  matrices for different subframes of one frame often to be similar. As a result, typically one of the three codebooks gives good performance for all subframes. Therefore the codebook search for the subframe LTP vectors is constrained to only allow codebook vectors that are chosen from the same codebook, which results in a rate reduction. 
     To find the best codebook, each of the three vector codebooks is used to quantize each subframe LTP vector and produce a weighted rate-distortion measure, and the vector codebook with the lowest combined rate-distortion over all subframes is chosen. The quantized LTP vectors are used in the noise shaping quantizer  516 , and the index of the codebook plus the four indices for the four subframe codebook vectors are entropy coded and sent to the decoder. 
     The high-pass filtered input is analyzed by the noise shaping analysis block  514  to find filter coefficients and quantization gains used in the noise shaping quantizer. The filter coefficients determine the distribution over the quantization noise over the spectrum, and are chose such that the quantization is least audible. The quantization gains determine the step size of the residual quantizer and as such govern the balance between bitrate and quantization noise level. 
     All noise shaping parameters are computed and applied per subframe of 5 milliseconds. First, a 16 th  order noise shaping LPC analysis is performed on a windowed signal block of 16 milliseconds. The signal block has a look-ahead of 5 milliseconds relative to the current subframe, and the window is an asymmetric sine window. The noise shaping LPC analysis is done with the autocorrelation method. The quantization gain is found as the square-root of the residual energy from the noise shaping LPC analysis, multiplied by a constant to set the average bitrate to the desired level. For voiced frames, the quantization gain is further multiplied by 0.5 times the inverse of the pitch correlation determined by the pitch analyses, to reduce the level of quantization noise which is more easily audible for voiced signals. The quantization gain for each subframe is quantized, and the quantization indices are input to the arithmetically encoder  518 . The quantized quantization gains are input to the noise shaping quantizer  516 . 
     Next a set of short-term noise shaping coefficients a shape, i  are found by applying bandwidth expansion to the coefficients found in the noise shaping LPC analysis. This bandwidth expansion moves the roots of the noise shaping LPC polynomial towards the origin, according to the formula:
 
a shape, i =a autocorr, i  g i  
 
where a autocorr, i  is the ith coefficient from the noise shaping LPC analysis and for the bandwidth expansion factor g a value of 0.94 was found to give good results.
 
     For voiced frames, the noise shaping quantizer also applies long-term noise shaping. It uses three filter taps, described by:
 
 b   shape =0.5sqrt(PitchCorrelation) [0.25, 0.5, 0.25].
 
     The short-term and long-term noise shaping coefficients are input to the noise shaping quantizer  516 . The high-pass filtered input is also input to the noise shaping quantizer  516 . 
     An example of the noise shaping quantizer  516  is now discussed in relation to  FIG. 6 . 
     The noise shaping quantizer  516  comprises a first addition stage  602 , a first subtraction stage  604 , a first amplifier  606 , a scalar quantizer  608 , a second amplifier  609 , a second addition stage  610 , a shaping filter  612 , a prediction filter  614  and a second subtraction stage  616 . The shaping filter  612  comprises a third addition stage  618 , a long-term shaping block  620 , a third subtraction stage  622 , and a short-term shaping block  624 . The prediction filter  614  comprises a fourth addition stage  626 , a long-term prediction block  628 , a fourth subtraction stage  630 , and a short-term prediction block  632 . 
     The first addition stage  602  has an input arranged to receive the high-pass filtered input from the high-pass filter  502 , and another input coupled to an output of the third addition stage  618 . The first subtraction stage has inputs coupled to outputs of the first addition stage  602  and fourth addition stage  626 . The first amplifier has a signal input coupled to an output of the first subtraction stage and an output coupled to an input of the scalar quantizer  608 . The first amplifier  606  also has a control input coupled to the output of the noise shaping analysis block  514 . The scalar quantizer  608  has outputs coupled to inputs of the second amplifier  609  and the arithmetic encoding block  518 . The second amplifier  609  also has a control input coupled to the output of the noise shaping analysis block  514 , and an output coupled to the an input of the second addition stage  610 . The other input of the second addition stage  610  is coupled to an output of the fourth addition stage  626 . An output of the second addition stage is coupled back to the input of the first addition stage  602 , and to an input of the short-term prediction block  632  and the fourth subtraction stage  630 . An output of the short-term prediction block  632  is coupled to the other input of the fourth subtraction stage  630 . The output of the fourth subtraction stage  630  is coupled to the input of the long-term prediction block  628 . The fourth addition stage  626  has inputs coupled to outputs of the long-term prediction block  628  and short-term prediction block  632 . The output of the second addition stage  610  is further coupled to an input of the second subtraction stage  616 , and the other input of the second subtraction stage  616  is coupled to the input from the high-pass filter  502 . An output of the second subtraction stage  616  is coupled to inputs of the short-term shaping block  624  and the third subtraction stage  622 . An output of the short-term shaping block  624  is coupled to the other input of the third subtraction stage  622 . The output of third subtraction stage  622  is coupled to the input of the long-term shaping block  620 . The third addition stage  618  has inputs coupled to outputs of the long-term shaping block  620  and short-term shaping block  624 . The short-term and long-term shaping blocks  624  and  620  are each also coupled to the noise shaping analysis block  514 , and the long-term shaping block  620  is also coupled to the open-loop pitch analysis block  508  (connections not shown). Further, the short-term prediction block  632  is coupled to the LPC analysis block  504  via the first vector quantizer  506 , and the long-term prediction block  628  is coupled to the LTP analysis block  510  via the second vector quantizer  512  (connections also not shown). 
     The purpose of the noise shaping quantizer  516  is to quantize the LTP residual signal in a manner that weights the distortion noise created by the quantisation into less noticeable parts of the frequency spectrum, e.g. where the human ear is more tolerant to noise and/or where the speech energy is high so that the relative effect of the noise is less. 
     In operation, all gains and filter coefficients and gains are updated for every subframe, except for the LPC coefficients, which are updated once per frame. The noise shaping quantizer  516  generates a quantized output signal that is identical to the output signal ultimately generated in the decoder. The input signal is subtracted from this quantized output signal at the second subtraction stage  616  to obtain the quantization error signal d(n). The quantization error signal is input to a shaping filter  612 , described in detail later. The output of the shaping filter  612  is added to the input signal at the first addition stage  602  in order to effect the spectral shaping of the quantization noise. From the resulting signal, the output of the prediction filter  614 , described in detail below, is subtracted at the first subtraction stage  604  to create a residual signal. The residual signal is multiplied at the first amplifier  606  by the inverse quantized quantization gain from the noise shaping analysis block  514 , and input to the scalar quantizer  608 . The quantization indices of the scalar quantizer  608  represent an excitation signal that is input to the arithmetically encoder  518 . The scalar quantizer  608  also outputs a quantization signal, which is multiplied at the second amplifier  609  by the quantized quantization gain from the noise shaping analysis block  514  to create an excitation signal. The output of the prediction filter  614  is added at the second addition stage to the excitation signal to form the quantized output signal. The quantized output signal is input to the prediction filter  614 . 
     On a point of terminology, note that there is a small difference between the terms “residual” and “excitation”. A residual is obtained by subtracting a prediction from the input speech signal. An excitation is based on only the quantizer output. Often, the residual is simply the quantizer input and the excitation is its output. 
     The shaping filter  612  inputs the quantization error signal d(n) to a short-term shaping filter  624 , which uses the short-term shaping coefficients a shape,i  to create a short-term shaping signal s short (n), according to the formula: 
     
       
         
           
             
               
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     The short-term shaping signal is subtracted at the third addition stage  622  from the quantization error signal to create a shaping residual signal f(n). The shaping residual signal is input to a long-term shaping filter  620  which uses the long-term shaping coefficients b shape,i  to create a long-term shaping signal s long (n), according to the formula: 
     
       
         
           
             
               
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     The short-term and long-term shaping signals are added together at the third addition stage  618  to create the shaping filter output signal. 
     The prediction filter  614  inputs the quantized output signal y(n) to a short-term prediction filter  632 , which uses the quantized LPC coefficients a i  to create a short-term prediction signal p short (n), according to the formula: 
     
       
         
           
             
               
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     The short-term prediction signal is subtracted at the fourth subtraction stage  630  from the quantized output signal to create an LPC excitation signal e LPC (n). The LPC excitation signal is input to a long-term prediction filter  628  which uses the quantized long-term prediction coefficients b i  to create a long-term prediction signal p long (n), according to the formula: 
     
       
         
           
             
               
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     The short-term and long-term prediction signals are added together at the fourth addition stage  626  to create the prediction filter output signal. 
     The LSF indices, LTP indices, quantization gains indices, pitch lags and excitation quantization indices are each arithmetically encoded and multiplexed by the arithmetic encoder  518  to create the payload bitstream. The arithmetic encoder  518  uses a look-up table with probability values for each index. The look-up tables are created by running a database of speech training signals and measuring frequencies of each of the index values. The frequencies are translated into probabilities through a normalization step. 
     An example decoder  700  for use in decoding a signal encoded according to embodiments of the present invention is now described in relation to  FIG. 7 . 
     The decoder  700  comprises an arithmetic decoding and dequantizing block  702 , an excitation generation block  704 , an LTP synthesis filter  706 , and an LPC synthesis filter  708 . The arithmetic decoding and dequantizing block  702  has an input arranged to receive an encoded bitstream from an input device such as a wired modem or wireless transceiver, and has outputs coupled to inputs of each of the excitation generation block  704 , LTP synthesis filter  706  and LPC synthesis filter  708 . The excitation generation block  704  has an output coupled to an input of the LTP synthesis filter  706 , and the LTP synthesis block  706  has an output connected to an input of the LPC synthesis filter  708 . The LPC synthesis filter has an output arranged to provide a decoded output for supply to an output device such as a speaker or headphones. 
     At the arithmetic decoding and dequantizing block  702 , the arithmetically encoded bitstream is demultiplexed and decoded to determine the LTP codebook indicator  109  for each frame, and to create LSF indices, LTP indices, quantization gains indices, pitch lags and a signal of excitation quantization indices. The LSF indices are converted to quantized LSFs by adding the codebook vectors of the ten stages of the MSVQ. The quantized LSFs are transformed to quantized LPC coefficients. The LTP codebook indicator  109  is used to select an LTP codebook, which is then used to convert the LTP indices to quantized LTP coefficients. The gains indices are converted to quantization gains, through look ups in the gain quantization codebook. 
     At the excitation generation block, the excitation quantization indices signal is multiplied by the quantization gain to create an excitation signal e(n). 
     The excitation signal is input to the LTP synthesis filter  706  to create the LPC excitation signal e LPC (n) according to: 
                   e     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (   n   )       =       e   ⁡     (   n   )       +       ∑     i   =     -   2       2     ⁢       e   ⁡     (     n   -   lag   -   i     )       ⁢     b   i             ,         
using the pitch lag and quantized LTP coefficients b i .
 
     The LPC excitation signal is input to the LPC synthesis filter to create the decoded speech signal y(n) according to: 
                 y   ⁡     (   n   )       =         e     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (   n   )       +       ∑     i   =   1     16     ⁢         e     L   ⁢           ⁢   P   ⁢           ⁢   C       ⁡     (     n   -   i     )       ⁢     a   i             ,         
using the quantized LPC coefficients a i .
 
     The encoder  500  and decoder  700  are preferably implemented in software, such that each of the components  502  to  632  and  702  to  708  comprise modules of software stored on one or more memory devices and executed on a processor. A preferred application of the present invention is to encode speech for transmission over a packet-based network such as the Internet, preferably using a peer-to-peer (P2P) system implemented over the Internet, for example as part of a live call such as a Voice over IP (VoIP) call. In this case, the encoder  500  and decoder  700  are preferably implemented in client application software executed on end-user terminals of two users communicating over the P2P system. 
     It will be appreciated that the above embodiments are described only by way of example. For instance, some or all of the modules of the encoder and/or decoder could be implemented in dedicated hardware units. Further, the invention is not limited to use in a client application, but could be used for any other speech-related purpose such as cellular mobile telephony. Further, instead of only selecting the codebook once per frame, in other embodiments a codebook could be selected less or more frequently, even up to once for each vector. Further, instead of a user input device like a microphone, the input speech signal could be received by the encoder from some other source such as a storage device and potentially be transcoded from some other form by the encoder; and/or instead of a user output device such as a speaker or headphones, the output signal from the decoder could be sent to another source such as a storage device and potentially be transcoded into some other form by the decoder. Other applications and configurations may be apparent to the person skilled in the art given the disclosure herein. The scope of the invention is not limited by the described embodiments, but only by the appended claims. 
     According to the invention in certain embodiments there is provided an encoder as therein described having the following features: 
     The second signal-processing module may be configured to quantize at least one of the vectors of said number of intervals according to each of said plurality of codebooks, and select the codebook based on comparison of said quantizations. 
     The second signal-processing module may be configured to quantize all of the vectors of said number of intervals according to each of said plurality of codebooks, and selecting the codebook based on comparison of said quantizations. 
     The second signal-processing module may be configured to perform said selection based on comparison of a distortion measure evaluated for the vectors of said number of intervals as quantized according to each of said codebooks. 
     The second signal-processing module may be configured to perform said comparison based on the distortion measure weighed against a bitrate required to encode the vectors of said number of intervals according to each codebook. 
     The second signal processing means may be configured to operate over a plurality of frames, each frame comprising a plurality of subframes; each of said intervals is a subframe; and said number may be the number of subframes per frame such that said selection is performed once per frame. 
     The number of intervals may be one. 
     The second signal-processing means may be configured to extract a signal comprising said vectors from the first remaining signal, thus leaving a second remaining signal, and to transmit parameters of the second remaining signal over the communication medium as part of said encoded signal. 
     The second signal-processing module may comprise a long-term prediction module. 
     The first signal-processing module may comprise a linear predictive coding module. 
     According to the invention in certain embodiments there is provided a decoder as described above heaving the feature of a signal processing means comprises a long-term prediction synthesis filter.