Abstract:
A method, system and program for encoding and decoding speech according to a source-filter model whereby speech is modelled to comprise a source signal filtered by a time-varying filter. The method comprises: receiving a speech signal comprising successive frames, for each of a plurality of frames of the speech signal, deriving a first line spectral frequency vector for a first portion of the frame, and a second line spectral frequency vector for a second portion of the frame, and determining a transmit line spectral frequency vector and an interpolation factor based on the first and second line spectral frequency vectors, and on the transmit line spectral frequency vector for a preceding one of the frames.

Description:
FIELD OF THE INVENTION 
       [0001]    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 electro-magnetic signal over a wireless connection. 
       BACKGROUND 
       [0002]    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 . For “voiced” speech, 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. For “unvoiced” speech, the vocal chords are not utilized and the source becomes more of a noisy signal. 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. 
         [0003]    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. 
         [0004]    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 comprising a series of pulses 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 four pulses which may vary gradually in form and amplitude from one period to the next. 
         [0005]    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.    
         [0006]    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) a set of parameters representing the pulses of the source signal  202 . 
         [0007]    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 both the inter-period correlation and the spectral envelope removed. 
         [0008]    Temporal fluctuations of spectral envelopes can cause perceptual degradation and a loss in coding efficiency. One way to mitigate these negative effects is to shorten the frame size, or frame skip, of the spectral analysis thereby lowering the fluctuations between the spectra. This approach unfortunately leads to a considerably higher transmit bit rate. However, it is desirable to reduce the transmit bit rate. 
         [0009]    The coefficients generated by linear predictive coding are very sensitive to errors, and therefore a small error may distort the whole spectrum of the reconstructed signal, or may even result in the prediction filter becoming unstable. Therefore, the transmission of LPC coefficients is often avoided, and the LPC coefficients information is further encoded to provide a more robust parameter set. 
         [0010]    To avoid these problems, it is common to represent the LPC coefficients as Line Spectral Pairs (LSP) also known as Line Spectral Frequencies (LSF), which are more robust to small errors introduced during transmission. 
         [0011]    Due to the nature of LSFs, it is possible to interpolate between values for adjacent frames. This interpolation results in a smoothing of the signal, thereby reducing the effect of the temporal fluctuations of the spectral envelopes. Interpolation is performed using a fixed interpolation factor, typically having a value of 0.5. In the case for which the interpolation is taken fully into account in the estimation of which vector to transmit, the fixed interpolation factor may provide smoothing of the signal but may potentially lead to lower performance than without the interpolation. 
         [0012]    It is an aim of some embodiments of the present invention to address, or at least mitigate, some of the above identified problems of the prior art. 
       SUMMARY 
       [0013]    According to an aspect of the invention, there is provided a method of determining line spectral frequency vectors representing filter coefficients for a time-varying filter for encoding speech according to a source-filter model, whereby speech is modelled to comprise a source signal filtered by the time-varying filter, the method comprising: receiving a speech signal comprising successive frames, for each of a plurality of frames of the speech signal, deriving a first line spectral frequency vector for a first portion of the frame, and a second line spectral frequency vector for a second portion of the frame, and determining a transmit line spectral frequency vector and an interpolation factor based on the first and second line spectral frequency vectors, and on the transmit line spectral frequency vector for a preceding one of the frames. 
         [0014]    In embodiments, the first and second line spectral frequency vectors may comprise optimal line spectral frequency vectors for the first and second portions of the frame. 
         [0015]    The determining of the transmit line spectral frequency vector and the interpolation factor may comprise minimizing a difference between the second line spectral frequency vector and the transmit line spectral frequency vector and between the first line spectral frequency vector and an interpolated line spectral frequency vector based on the interpolation factor and the transmit line spectral frequency vector. Minimizing the difference may comprise minimizing a residual energy for the frame. 
         [0016]    The first portion of the frame may comprise a first half of the frame, and the second portion of the frame may comprise a second half of the frame. 
         [0017]    The determining of the transmit line spectral frequency vector and the interpolation factor may comprise alternately calculating the transmit line spectral frequency vector for a constant interpolation factor and then the interpolation factor for the calculated transmit line spectral frequency vector for a plurality of iterations. 
         [0018]    The determining of the transmit line spectral frequency vector and the interpolation factor may comprise alternately calculating the transmit line spectral frequency vector for a constant interpolation factor and then the interpolation factor for the calculated transmit line spectral frequency vector until the calculation converges on optimum values for the interpolation factor and the line spectral frequency vector. 
         [0019]    The plurality of iterations may comprise a pre-defined number of iterations. 
         [0020]    The method may further comprise arithmetically encoding the interpolation factor and the transmit line spectral frequency vector. 
         [0021]    The method may further comprise multiplexing the encoded interpolation factor and transmit line spectral frequency vector into a bit stream for transmission. 
         [0022]    According to a further aspect of the invention, there is provided a method of decoding line spectral frequency vectors representing filter coefficients for a time-varying filter for encoding speech according to a source-filter model, whereby speech is modelled to comprise a source signal filtered by the time-varying filter, the method comprising receiving an encoded bit stream, the encoded bit stream representing a plurality of successive frames of a speech signal, each frame having a first portion and a second portion, and for each frame of the speech signal: extracting an interpolation factor from the bit stream; extracting line spectral frequency indices from the bit stream and converting the line spectral frequency indices to a received line spectral frequency vector, the received line spectral frequency vector associated with a second portion of the frame; and determining an interpolated line spectral frequency vector associated with a first portion of the frame based on the interpolation factor, the received line spectral frequency vector for the frame, and the received line spectral frequency vector for the previous frame. 
         [0023]    A decoded speech signal may be generated based on the received line spectral frequency vector and the interpolated line spectral frequency vector. 
         [0024]    According to another aspect of the 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 comprising successive frames, a first signal-processing module configured to derive, for each of a plurality of frames of the speech signal, a first line spectral frequency vector for a first portion of the frame, and a second line spectral frequency vector for a second portion of the frame, and a second signal-processing module configured to determine a transmit line spectral frequency vector and an interpolation factor based on the first and second line spectral frequency vectors, and on the transmit line spectral frequency vector for a preceding one of the frames. 
         [0025]    According to another aspect of the 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, the encoded signal representing a plurality of successive frames of a speech signal, each frame having a first portion and a second portion, and a signal-processing module configured to extract, for each frame of the speech signal, an interpolation factor and line spectral frequency indices from the encoded signal, wherein the signal-processing module is further configured to convert the line spectral frequency indices to a received line spectral frequency vector, the received line spectral frequency vector associated with a second portion of the frame, and to determine an interpolated line spectral frequency vector associated with a first portion of the frame based on the interpolation factor, the received line spectral frequency vector for the frame, and the received line spectral frequency vector for the previous frame. 
         [0026]    According to another aspect of the present invention, there is provided a computer program product for determining line spectral frequency vectors representing filter coefficients for a time-varying filter 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 comprising successive frames;   for each of a plurality of frames of the speech signal, derive a first line spectral frequency vector for a first portion of the frame, and a second line spectral frequency vector for a second portion of the frame; and   determine a transmit line spectral frequency vector and an interpolation factor based on the first and second line spectral frequency vectors, and on the transmit line spectral frequency vector for a preceding one of the frames.       
 
         [0030]    According to another aspect of the present invention, there is provided a computer program product for decoding line spectral frequency vectors representing filter coefficients for a time-varying filter 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 an encoded bit stream, the encoded bit stream representing a plurality of successive frames of a speech signal, each frame having a first portion and a second portion; and   for each frame of the speech signal:   extract an interpolation factor from the bit stream;   extract line spectral frequency indices from the bit stream and convert the line spectral frequency indices to a received line spectral frequency vector, the received line spectral frequency vector associated with a second portion of the frame; and   determine an interpolated line spectral frequency vector associated with a first portion of the frame based on the interpolation factor, the received line spectral frequency vector for the frame, and the received line spectral frequency vector for the previous frame.       
 
         [0036]    According to further aspects of the present invention, there are provided corresponding computer program products such as client application products arranged so as when executed on a processor to perform the steps of the methods described above. 
         [0037]    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 
         [0038]    Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying figures, in which: 
           [0039]      FIG. 1   a  is a schematic representation of a source-filter model of speech, 
           [0040]      FIG. 1   b  is a schematic representation of a frame, 
           [0041]      FIG. 2   a  is a schematic representation of a source signal, 
           [0042]      FIG. 2   b  is a schematic representation of variations in a spectral envelope, 
           [0043]      FIG. 3  illustrates the initial LPC analyses, conversion to LSF vectors and calculation of LSF error weight matrices according to an embodiment of the invention, 
           [0044]      FIG. 4  illustrates an alternating optimization procedure for optimizing an interpolation value according to an embodiment of the invention, 
           [0045]      FIG. 5  shows an example speech signal, along with the coding gain increase and the optimum interpolation factors using an embodiment of the invention, 
           [0046]      FIG. 6  shows a histogram of the interpolation factors for the example shown in  FIG. 4 , 
           [0047]      FIG. 7  shows an encoder according to an embodiment of the invention, 
           [0048]      FIG. 8  shows a noise shaping quantizer according to an embodiment of the invention, 
           [0049]      FIG. 9  shows a decoder suitable for decoding a signal encoded using the encoder of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0050]    Embodiments of the invention are described herein by way of particular examples and specifically with reference to exemplary embodiments. It will be understood by one skilled in the art that the invention is not limited to the details of the specific embodiments given herein. 
         [0051]    Embodiments of the invention provide an LSF interpolation scheme which applies a parametric model with a single scalar variable fully describing an additional interpolated LSF vector such that just this single model parameter needs to be transmitted in addition to the already transmitted single LSF vector per frame. The transmitted LSF vector and interpolation parameter are estimated in a joint manner where also the interpolated LSF vector is taken into account. 
         [0052]    Embodiments of the present invention deal with high temporal fluctuations of all-pole speech spectral envelopes. At low bit rates, speech spectral envelope fluctuations are known to degrade the perceptual quality more than high absolute modelling error. 
         [0053]      FIG. 3  illustrates the initial LPC analyses, conversion to LSF vectors, and calculation of LSF error weight matrices. The full input frame is subjected to LPC analysis  302 . The LSF conversion of the full frame LPC coefficients  304  is calculated only when the interpolation factor is determined to be one, and no interpolation is applied. 
         [0054]    In addition to the full frame LPC vector for frame n, say, LPC n , LPC vectors are also calculated for the first half, LPC n,0  at  306 , and for the second half, LPC n,1  at  308 . The LPC coefficients do not quantize nor interpolate well, so prior to interpolation the LPC vectors are converted to LSF vectors at  310  and  312 , which are better suited for this purpose, thus providing LSFopt n,0  and LSFopt n,1 , respectively. The half frame coefficients are first used to find diagonal error weight matrices W n,0  and W n,1  at  314  and  316 . The error weight matrices map errors in the LSF domain to residual energy. 
         [0055]    Next, the optimum half frame LSF vectors LSFopt n,0  and LSFopt n,1  are used as targets for the estimation of the optimum vectors in the interpolation scheme. To keep the rate low, a parametric model is enforced on the LSF coefficients, 
         [0000]        LSF   n,0 =(1− i )· LSF   n-1,1   +i·LSF   n,1 , 
         [0000]    where the interpolated first half frame LSF vector, that is, LSF n,0  is a weighted average, described by the interpolation factor i, of the second half LSF vector from the previous frame LSF n-1,1  and the second half LSF vector LSF n,1  from the current frame. Given this parametric model, equations for the optimum model parameters are derived by minimizing the full frame residual energy, with the interpolation and the second half frame LSF vector as the unknown variables, i.e., 
         [0000]    
       
         
           
             
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         [0056]    In this equation we substitute the interpolated LSF n,0  by expressing it in terms of the interpolation factor and the second half LSF vectors for the previous and the current frame, that is, 
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         [0057]    This results in an optimization problem where a bi-convex objective function needs to be minimized.  FIG. 4  shows an iterative algorithm  400  for finding the optimized interpolation factor i and the LSF vector LSF n,1 . The stationary points of the objective function are found for LSF n,1  when i is treated as a constant in block  404 , and for i when LSF n,1  is treated as a vector of constants in block  402 . Each of these tasks results in a closed form equation for the optimum solution for one given the other being constant. Using these equations the optimization problem may be solved in real-time in an iterative manner by low-complexity alternating optimization, which means that given either one of the interpolation factor i and the last half frame LSF vector LSF n,1 , evaluating the obtained closed form equations provides a value for the LSF vector LSF n,1 , or the interpolation factor i respectively. 
         [0058]    In the second last iteration or when the alternating optimization has converged, the interpolation factor is quantized and the optimum second half LSF vector is estimated given this finally chosen value. 
         [0059]    Whenever it is determined in closed loop analysis that LSF interpolation does not lead to a lower residual energy for the given frame, an interpolation factor i equal to one is used, resulting in LSF n,1  of the parametric model describing the full frame. In this case, LSF conversion of the LPC analysis for the full input frame is performed. LSF n,1  is then set equal to the vector that was obtained from the full frame analysis, i.e., LSF n . 
         [0060]    An example where the interpolation scheme is applied is shown in  FIG. 5 , and  FIG. 6 . In this example,  FIG. 6  shows that the LSF interpolation factor is different from 1 in 65% of the frames, indicating that the described interpolation method results in lower residual energy per frame, and therefore improved coding efficiency for a majority of frames. As can be seen in  FIG. 5 , the largest improvements in coding gain are seen during speech transitions. 
         [0061]      FIG. 7  shows an encoder  700  that can be used to encode a speech signal. The encoder  700  of  FIG. 7  comprises a high-pass filter  702 , a linear predictive coding (LPC) analysis block  704 , a line spectral frequency (LSF) interpolation block  722 , a scalar quantizer  720 , a vector quantizer  706 , an open-loop pitch analysis block  708 , a long-term prediction (LTP) analysis block  710 , a second vector quantizer  712 , a noise shaping analysis block  714 , a noise shaping quantizer  716 , and an arithmetic encoding block  718 . 
         [0062]    The high pass filter  702  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  704 , noise shaping analysis block  714  and noise shaping quantizer  716 . The LPC analysis block  704  has an output coupled to an input of the LSF interpolation block  722 . The LSF interpolation block  722  has outputs coupled to inputs of the scalar quantizer  720 , the first vector quantizer  706  and the LTP analysis block  710 . The scalar quantizer  720 , and the first vector quantizer  706  each have outputs coupled to inputs of the arithmetic encoding block  718  and noise shaping quantizer  716 . 
         [0063]    The LPC analysis block  704  has outputs coupled to inputs of the open-loop pitch analysis block  708  and the LTP analysis block  710 . The LTP analysis block  710  has an output coupled to an input of the second vector quantizer  712 , and the second vector quantizer  712  has outputs coupled to inputs of the arithmetic encoding block  718  and noise shaping quantizer  716 . The open-loop pitch analysis block  708  has outputs coupled to inputs of the LTP analysis block  710  and the noise shaping analysis block  714 . The noise shaping analysis block  714  has outputs coupled to inputs of the arithmetic encoding block  718  and the noise shaping quantizer  716 . The noise shaping quantizer  716  has an output coupled to an input of the arithmetic encoding block  718 . The arithmetic encoding block  718  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. 
         [0064]    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, and has a bit rate that varies depending on a quality setting provided to the encoder and on the complexity and estimated perceptual importance of the input signal. 
         [0065]    The speech input signal is input to the high-pass filter  704  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  704  is preferably a second order auto-regressive moving average (ARMA) filter. 
         [0066]    The high-pass filtered input x HP  is input to the linear prediction coding (LPC) analysis block  704 , which calculates 16 LPC coefficients a i  using the covariance method which minimizes the energy of the LPC residual r LPC : 
         [0000]    
       
         
           
             
               
                 
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         [0000]    where n is the sample number. The LPC coefficients are used with an LPC analysis filter to create the LPC residual. 
         [0067]    LPC analysis is performed for the full frame, LPC n  and also for each half of the frame, LPC n,0  and LPC n,1 , as described above. 
         [0068]    The LPC coefficients vectors are input to the LSF interpolation block, which transforms the LPC coefficients to LSF vectors, and performs the interpolation optimization to generate an interpolation factor and a LSF vector representing the frame. 
         [0069]    The resulting LSF vector is quantized using the second vector quantizer  706 , 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 a Q  for each half of the frame using the estimated interpolation factor and the previously transmitted LSF vector, for use in the noise shaping quantizer  716 . 
         [0070]    The LSF interpolation factor is quantized using the first vector quantizer  720  and the quantized LSF interpolation factor is input to arithmetic encoding block  718 . 
         [0071]    The LPC residual is input to the open loop pitch analysis block  708 , 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  718  and noise shaping quantizer  716 . 
         [0072]    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  704  to the LTP analysis block  710 . For each subframe, the LTP analysis block  710  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: 
         [0000]    
       
         
           
             
               
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         [0000]    is minimized. 
         [0073]    The LTP coefficients for each frame are quantized using a vector quantizer (VQ). The resulting VQ codebook index is input to the arithmetic coder, and the quantized LTP coefficients b Q  are input to the noise shaping quantizer. 
         [0074]    The high-pass filtered input is analyzed by the noise shaping analysis block  714  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. 
         [0075]    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  718 . The quantized quantization gains are input to the noise shaping quantizer  716 . 
         [0076]    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: 
         [0000]      a shape, i =a autocorr, i  g i    
         [0000]    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. 
         [0077]    For voiced frames, the noise shaping quantizer also applies long-term noise shaping. It uses three filter taps, described by: 
         [0000]        b   shape =0.5 sqrt(PitchCorrelation) [0.25, 0.5, 0.25]. 
         [0078]    The short-term and long-term noise shaping coefficients are input to the noise shaping quantizer  716 . The high-pass filtered input is also input to the noise shaping quantizer  716 . 
         [0079]    An example of the noise shaping quantizer  716  is now discussed in relation to  FIG. 8 . 
         [0080]    The noise shaping quantizer  716  comprises a first addition stage  802 , a first subtraction stage  804 , a first amplifier  806 , a scalar quantizer  808 , a second amplifier  809 , a second addition stage  810 , a shaping filter  812 , a prediction filter  814  and a second subtraction stage  816 . The shaping filter  812  comprises a third addition stage  818 , a long-term shaping block  820 , a third subtraction stage  822 , and a short-term shaping block  824 . The prediction filter  814  comprises a fourth addition stage  826 , a long-term prediction block  828 , a fourth subtraction stage  830 , and a short-term prediction block  832 . 
         [0081]    The first addition stage  802  has an input arranged to receive the high-pass filtered input from the high-pass filter  702 , and another input coupled to an output of the third addition stage  818 . The first subtraction stage has inputs coupled to outputs of the first addition stage  802  and fourth addition stage  826 . 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  808 . The first amplifier  806  also has a control input coupled to the output of the noise shaping analysis block  714 . The scalar quantiser  808  has outputs coupled to inputs of the second amplifier  809  and the arithmetic encoding block  718 . The second amplifier  809  also has a control input coupled to the output of the noise shaping analysis block  714 , and an output coupled to the an input of the second addition stage  810 . The other input of the second addition stage  810  is coupled to an output of the fourth addition stage  826 . An output of the second addition stage is coupled back to the input of the first addition stage  802 , and to an input of the short-term prediction block  832  and the fourth subtraction stage  830 . An output of the short-tem prediction block  832  is coupled to the other input of the fourth subtraction stage  830 . The fourth addition stage  826  has inputs coupled to outputs of the long-term prediction block  828  and short-term prediction block  832 . The output of the second addition stage  810  is further coupled to an input of the second subtraction stage  816 , and the other input of the second subtraction stage  816  is coupled to the input from the high-pass filter  702 . An output of the second subtraction stage  816  is coupled to inputs of the short-term shaping block  824  and the third subtraction stage  822 . An output of the short-tem shaping block  824  is coupled to the other input of the third subtraction stage  822 . The third addition stage  818  has inputs coupled to outputs of the long-term shaping block  820  and short-term prediction block  824 . 
         [0082]    The purpose of the noise shaping quantizer  716  is to quantize the LTP residual signal in a manner that weights the distortion noise created by the quantisation into parts of the frequency spectrum where the human ear is more tolerant to noise. 
         [0083]    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  716  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  812 , described in detail later. The output of the shaping filter  812  is added to the input signal at the first addition stage  802  in order to effect the spectral shaping of the quantization noise. From the resulting signal, the output of the prediction filter  814 , described in detail below, is subtracted at the first subtraction stage  804  to create a residual signal. The residual signal is multiplied at the first amplifier  806  by the inverse quantized quantization gain from the noise shaping analysis block  714 , and input to the scalar quantizer  808 . The quantization indices of the scalar quantizer  808  represent an excitation signal that is input to the arithmetically encoder  718 . The scalar quantizer  808  also outputs a quantization signal, which is multiplied at the second amplifier  809  by the quantized quantization gain from the noise shaping analysis block  714  to create an excitation signal. The output of the prediction filter  814  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  814 . 
         [0084]    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 the output. 
         [0085]    The shaping filter  812  inputs the quantization error signal d(n) to a short-term shaping filter  824 , 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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         [0086]    The short-term shaping signal is subtracted at the third addition stage  822  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  820  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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         [0087]    The short-term and long-term shaping signals are added together at the third addition stage  818  to create the shaping filter output signal. 
         [0088]    The prediction filter  814  inputs the quantized output signal y(n) to a short-term prediction filter  832 , which uses the quantized LPC coefficients a Q  to create a short-term prediction signal p short (n), according to the formula: 
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         [0089]    The short-term prediction signal is subtracted at the fourth subtraction stage  830  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  828  which uses the quantized long-term prediction coefficients b Q  to create a long-term prediction signal p long (n), according to the formula: 
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         [0090]    The short-term and long-term prediction signals are added together at the fourth addition stage  826  to create the prediction filter output signal. 
         [0091]    The LSF indices, LSF interpolation factor, LTP indices, quantization gains indices, pitch lags and the excitation quantization indices are each arithmetically encoded and multiplexed by the arithmetic encoder  718  to create the payload bitstream. The arithmetic encoder  718  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. 
         [0092]    An example decoder  900  for use in decoding a signal encoded according to embodiments of the present invention is now described in relation to  FIG. 9 . 
         [0093]    The decoder  900  comprises an arithmetic decoding and dequantizing block  902 , an excitation generation block  904 , an LTP synthesis filter  906 , and an LPC synthesis filter  908 . The arithmetic decoding and dequantizing block  902  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  904 , LTP synthesis filter  906  and LPC synthesis filter  908 . The excitation generation block  904  has an output coupled to an input of the LTP synthesis filter  906 , and the LTP synthesis block  906  has an output connected to an input of the LPC synthesis filter  908 . 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. 
         [0094]    At the arithmetic decoding and dequantizing block  902 , the arithmetically encoded bitstream is demultiplexed and decoded to create LSF indices, LSF interpolation factor, LTP codebook index and 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, one from each of the ten stages of the MSVQ. Using the interpolation factor and the transmitted LSF vector for the previous frame, the quantized LSFs are obtained for each frame half. The two sets of quantized LSFs are then transformed to quantized LPC coefficients. 
         [0095]    The LTP codebook index 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. The LTP indices and gains indices are converted to quantized LTP coefficients and quantization gains, through look ups in the quantization codebooks. 
         [0096]    At the excitation generation block, the excitation quantization indices signal is multiplied by the quantization gain to create an excitation signal e(n). 
         [0097]    The excitation signal is input to the LTP synthesis filter  906  to create the LPC excitation signal e ltp (n) according to: 
         [0000]    
       
         
           
             
               
                 
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         [0000]    using the pitch lag and quantized LTP coefficients b Q . 
         [0098]    The long term excitation signal is input to the LPC synthesis filter to create the decoded speech signal y(n) according to: 
         [0000]    
       
         
           
             
               
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         [0000]    using the quantized LPC coefficients a Q . 
         [0099]    For the first half of the frame synthesis is performed using the coefficients obtained from the interpolated LSF n,0  and for the second half we use the coefficients obtained from LSF n,1 . 
         [0100]    The encoder  700  and decoder  900  are preferably implemented in software, such that each of the components  702  to  832  and  902  to  908  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  700  and decoder  900  are preferably implemented in client application software executed on end-user terminals of two users communicating over the P2P system. 
         [0101]    An advantage of some embodiments of the invention over the prior art is that the spectral fluctuations are reduced by interpolation only when there is an actual gain from doing it. Embodiments of the invention are generalizations of the regular method of having a single spectral model for each frame, and have a very low cost in terms of bit-rate. A further advantage is that the decoded spectral envelope matches that of the input better, over time. This provides better sound quality of the decoded signal, and reduces the energy of the residual signal, which consequently can be coded more efficiently, reducing the bit-rate. 
         [0102]    The improvement is generally biggest during a transition. If the transition happens around the middle of the frame it is advantageous to use LSFs close to those of the previous frame for the first half of the frame, and new ones for the second half. On the contrary, if the transition happens around the start of the frame, it is better to use the same LSFs for the entire frame and have no interpolation at all. Having a variable interpolation factor enables this form of adaptation. 
         [0103]    According to embodiments of the invention, a closed loop interpolation scheme is used that will deviate from the regular approach only when it leads to better performance to do so. The model is always applied, but as it generalizes the regular approach, there is a mode with the interpolation factor equal to 1 where it performs exactly as the regular approach except for the small bit-rate increase from transmitting the scalar interpolation factor. In this context, “the regular approach” is where one constant LPC vector is used per frame, or alternatively, a transmitted LPC vector is used for the second half of the frame, and a LPC vector is interpolated with a constant interpolation factor from the transmitted LPC vector and the LPC vector from the previous frame. 
         [0104]    As embodiments of the invention generalize the regular approach, the performance for each frame is guaranteed to be no worse than the regular approach, except for the increase in bit-rate from sending an additional scalar value for each frame. The transmitted LSF vector can be optimized given the applied model and the estimated interpolation factor. 
         [0105]    The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. 
         [0106]    According to the invention in certain embodiments there is provided an encoder as herein described having the following features. 
         [0107]    The first signal-processing module may be further configured to derive optimal line spectral frequency vectors for the first and second portions of the frame. 
         [0108]    The second signal-processing module may be further configured to determine the transmit line spectral frequency vector and the interpolation factor based on minimizing a difference between the second line spectral frequency vector and the transmit line spectral frequency vector and between the first line spectral frequency vector and an interpolated line spectral frequency vector based on the interpolation factor and the transmit line spectral frequency vector. 
         [0109]    The minimizing of a difference may comprise minimizing a residual energy for the frame. 
         [0110]    The second signal-processing module may be further configured to alternately calculate the transmit line spectral frequency vector for a constant interpolation factor and then the interpolation factor for the calculated transmit line spectral frequency vector for a plurality of iterations. 
         [0111]    The second signal-processing module may be configured to alternately calculate the transmit line spectral frequency vector for a constant interpolation factor and then the interpolation factor for the calculated transmit line spectral frequency vector until the calculation converges on optimum values for the interpolation factor and the line spectral frequency vector. 
         [0112]    The plurality of iterations may comprise a pre-defined number of iterations. 
         [0113]    The encoder may comprise an arithmetic encoder configured to arithmetically encode the interpolation factor and the transmit line spectral frequency vector. 
         [0114]    The encoder may comprise a multiplexer configured to multiplex the encoded interpolation factor and transmit line spectral frequency vector into a bit stream for transmission. 
         [0115]    According to the invention in certain embodiments there is provided a decoder as herein described having the feature that the signal-processing module is further configured to generate a decoded speech signal based on the received line spectral frequency vector and the interpolated line spectral frequency vector.