Pitch lag vectors for speech encoding

1. A method of encoding speech, the method comprising receiving a signal representative of speech to be encoded;

at each of a plurality of intervals during encoding of the speech, determining a pitch lag between portions of the signal having a degree of repetition;

selecting for a set of said intervals a pitch lag vector from a pitch lag codebook of such vectors, each pitch lag vector comprising a set of offsets corresponding to the offset between the pitch lag determined for each said interval and an average pitch lag for said set of intervals, and transmitting an indication of the selected vector and said average over a transmission medium as part of the encoded signal representative of said speech.

2. The method of claim 1 , wherein the encoding is performed over a plurality of frames, each frame comprising a plurality of subframes, each of said intervals is a subframe, and the set comprises the number of subframes per frame such that said selection and transmission are performed once per frame.

3. A method according to claim 2 , wherein there are four subframes per frame, and each pitch lag vector comprises four offsets.

4. A method according to claim 1 , wherein the pitch lag codebook comprises 32 pitch lag vectors.

5. A method according to claim 1 , wherein the step of determining a pitch lag comprises determining a correlation between portions of the signal having a degree of repetition, and determining a maximum correlation value for a plurality of pitch lags.

6. A method according to claim 2 , comprising the step of determining for each frame whether the frame is voiced or unvoiced, and transmitting an indication of the selected pitch lag vector and said pitch lag average only for voiced frames.

7. The method of claim 1 , wherein the speech is encoded according to a source filter model whereby speech is modelled to comprise a source signal filtered by a time varying filter.

8. The method of claim 7 , comprising deriving from a received speech signal a spectral envelope signal representative of the time varying filter and a first remaining signal representative of the modelled source signal, wherein the signal representative of speech is the first remaining signal.

9. A method according to claim 8 , wherein prior to determining the maximum correlation value the first remaining signal is downsampled.

10. The method of claim 8 , comprising extracting a signal from the first remaining signal, thus leaving a second remaining signal and the method comprises transmitting parameters of the second remaining signal over the communication medium as part of said encoded signal.

11. The method of claim 10 , wherein the extraction of said second remaining signal from the first remaining signal is by long term prediction filtering.

12. The method of claim 8 , wherein the derivation of said first remaining signal from the speech signal is by linear predictive coding.

13. An encoder for encoding speech, the encoder comprising:

means for determining at each of a plurality of intervals during encoding of a received signal representative of speech, a pitch lag between portions of said signal having a degree of repetition;

means for selecting for a set of said intervals a pitch lag vector from a pitch lag code book of such vectors, each pitch lag vector comprising a set of offsets corresponding to the offsets between the pitch lag determined for each said interval and an average pitch lag for said set of intervals; and

means for transmitting an indication of the selected vector and said average over a transmission medium as part of the encoded signal representative of said speech.

14. An encoder according to claim 13 , comprising a memory storing said pitch lag codebook of pitch lag vectors.

15. An encoder according to claim 13 , comprising means 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: means for deriving from the received signal a spectral envelope signal representative of the time varying filter and a first remaining signal representative of the modelled source signal.

16. A method of decoding an encoded signal representative of speech, the encoded signal comprising an indication of a pitch lag vector comprising a set of offsets corresponding to an offset between a pitch lag determined for each interval in said set and an average pitch lag for said set of intervals;

determining for each interval a pitch lag based on the average pitch lag for said set of intervals and each corresponding offset in the pitch lag vector identified by the indication; and

using the determined pitch lags to encode other portions of a received signal representative of said speech.

17. A decoder for decoding an encoded signal representative of speech, the decoder comprising:

means for identifying from a received indication in the encoded signal a pitch lag vector from a pitch lag codebook of such vectors; and

means for determining a pitch lag for each of a set of intervals from a corresponding offset in the pitch lag vector and an average pitch lag for said set of intervals, said average pitch lag being part of the encoded signal.

18. A computer program product for encoding speech, the program comprising code which when executed implements the coding method of:

receiving a signal representative of speech to be encoded;

at each of a plurality of intervals during the encoding, determining a pitch lag between portions of the signal having a degree of repetition;

selecting for a set of said intervals a pitch lag vector from a pitch lag codebook of such vectors, each pitch lag vector comprising a set of offsets corresponding to the offset between the pitch lag determined for each said interval and an average pitch lag for said set of intervals, and transmitting an indication of the selected vector and said average over a transmission medium as part of the encoded signal representative of said speech.

19. A computer program product for decoding an encoded signal representative of speech, then encoded signal comprising an indication of a pitch lag vector comprising a set of offsets corresponding to an offset between a pitch lag determined for each interval in said set and an average pitch lag for said set of intervals, the program comprising code which when executed implements the decoding method of:

determining for each interval a pitch lag based on the average pitch lag for said set of intervals and each corresponding offset in the pitch lag vector identified by the indication; and

using the determined pitch lags to encode other portions of a received signal representative of said speech.