Robust composite quantization with sub-quantizers and inverse sub-quantizers using illegal space

1. In a composite quantizer including first and second sub-quantizers, a method of sub-quantizing a residual vector representative of a portion of a signal using the first sub-quantizer, comprising:

(a) receiving the residual vector that represents the difference between a Line Spectral Frequency (LSF) vector and a prediction of the LSF vector;

(b) transforming each sub-codevector of a set of sub-codevectors into a corresponding candidate codevector by combining each sub-codevector with a transformation vector, thereby producing a set of candidate codevectors;

(c) determining legal candidate codevectors among the set of candidate codevectors;

(d) determining a best sub-codevector corresponding to a legal candidate codevector among the legal candidate codevectors, wherein the best sub-codevector corresponds to a quantization of the residual vector; and

(e) outputting at least one of:

the best sub-codevector, and

an index identifying the best sub-codevector.

2. The method of claim 1 , wherein step (e) further comprises:

outputting at least one of:

a best legal candidate codevector among the legal candidate codevectors, and

an index identifying the best legal candidate codevector.

3. The method of claim 1 , wherein the first sub-quantizer is a composite quantizer.

4. The method of claim 1 , wherein step (c) comprises:

(c)(i) determining whether each candidate codevector belongs to an illegal space representing illegal vectors; and

(c)(ii) declaring as a legal candidate codevector each candidate codevector not belonging to the illegal space.

5. The method of claim 4 , wherein:

the illegal space is represented as an illegal vector criterion; and

step (c)(i) includes determining whether the candidate codevector satisfies the illegal vector criterion.

6. The method of claim 1 , the method further comprising:

prior to step (d), deriving an error term corresponding to each legal candidate codevector, each error term being a function of the residual vector and the sub-codevector corresponding to the legal candidate codevector,

wherein step (d) comprises determining the best sub-codevector based on the error terms.

7. The method of claim 1 , wherein step (b) further comprises deriving the transformation vector based on one or more past best sub-codevectors.

8. The method of claim 1 , wherein step (c) comprises:

determining whether each candidate codevector belongs to an illegal space representing illegal LSF vectors; and

declaring as a legal candidate codevector each candidate codevector that does not belong to the illegal space.

9. The method of claim 8 , further comprising:

deriving a transformation vector including an approximation of LSFs, wherein step (b) comprises separately combining each sub-codevector with the transformation vector to produce the corresponding plurality of candidate codevectors.

10. The method of claim 8 , wherein the illegal space is represented as an illegal criterion for LSF vectors, and the illegal criterion includes first and second successive LSFs in a pair of LSFs being out-of-order.

11. The method of claim 8 , wherein the illegal space is represented as an illegal criterion for LSF vectors, and the illegal criterion for LSF vectors includes first and second successive LSFs in a pair of LSFs being closer to each other than a minimum separation distance.

12. The method of claim 1 , wherein the residual vector relates to a speech and/or audio signal.

13. A computer program product (CPP) comprising a computer usable medium having computer readable program code (CRPC) means embodied in the medium for causing an application program to execute on a computer processor, in a composite quantizer, including first and second sub-quantizers, to perform sub-quantization of a residual vector representative of a portion of a signal using the first sub-quantizer, the CRPC means comprising:

first CRPC means for receiving the residual vector that represents the difference between a Line Spectral Frequency (LSF) vector and a prediction of the LSF vector;

second CRPC means for causing the processor to transform each sub-codevector of a set of sub-codevectors into a corresponding candidate codevector by combining each sub-codevector with a transformation vector, thereby producing a set of candidate codevectors;

third CRPC means for causing the processor to determine legal candidate codevectors among the set of candidate codevectors;

fourth CRPC means for causing the processor to determine a best sub-codevector corresponding to a legal candidate codevector among the legal candidate codevectors, wherein the best sub-codevector corresponds to a quantization of the residual vector; and

fifth CRPC means for causing the processor to output at least one of

the best sub-codevector, and

an index identifying the best sub-codevector.

14. The CPP of claim 13 , wherein the fifth CRPC means further comprises means for causing the processor to output at least one of:

a best legal candidate codevector among the legal candidate codevectors, and

an index identifying the best legal candidate codevector.

15. The CPP of claim 13 , wherein the first sub-quantizer is a composite quantizer.

16. The CPP of claim 13 , wherein the third CRPC means comprises:

sixth CRPC means for causing the computer to determine whether each candidate codevector belongs to an illegal space representing illegal vectors; and

seventh CRPC means for causing the computer to declare as a legal candidate codevector each candidate codevector not belonging to the illegal space.

17. The CPP of claim 16 , wherein:

the illegal space is represented as an illegal vector criterion; and

the sixth CRPC means includes CRPC means for causing the computer to determine whether the candidate codevector satisfies the illegal vector criterion.

18. The CPP of claim 13 , further comprising:

sixth CRPC means for causing the computer to derive an error term corresponding to each legal candidate codevector, each error term being a function of the residual vector and the sub-codevector corresponding to the legal candidate codevector,

wherein the fourth CRPC means includes CRPC means for causing the computer to determine the best sub-codevector based on the error terms.

19. The CPP of claim 13 , further comprising sixth CRPC means for causing the computer to derive the transformation vector based on one or more past best sub-codevectors.

20. The CPP of claim 13 , wherein the third CRPC means comprises:

CRPC means for causing the processor to determine whether each candidate codevector belongs to an illegal space representing illegal LSF vectors; and

CRPC means for causing the processor to declare as a legal candidate codevector each candidate codevector that does not belong to the illegal space.

21. The CPP of claim 20 , further comprising:

sixth CRPC means for causing the processor to derive a transformation vector, wherein the second CRPC means includes CRPC means for causing the processor to separately combine each sub-codevector with the transformation vector to produce the corresponding plurality of candidate codevectors.

22. The CPP of claim 20 , wherein the illegal space is represented as an illegal criterion for LSF vectors, and the illegal criterion includes first and second successive LSFs in a pair of LSFs being out-of-order.

23. The CPP of claim 20 , wherein the illegal space is represented as an illegal criterion for LSF vectors, and the illegal criterion for LSF vectors includes first and second successive LSFs in a pair of LSFs being closer to each other than a minimum separation distance.

24. The CPP of claim 13 , wherein the residual vector relates to a speech and/or audio signal.

25. A sub-quantizer in a composite quantizer including a plurality of sub-quantizers, the sub-quantizer for quantizing a residual vector representative of a portion of a signal, comprising:

first means for receiving the residual vector that represents the difference between a Line Spectral Frequency (LSF) vector and a prediction of the LSF vector;

second means for transforming each sub-codevector of a set of sub-codevectors into a corresponding candidate codevector by combining each sub-codevector with a transformation vector, thereby producing a set of candidate codevectors,

third means for determining legal candidate codevectors among the set of candidate codevectors;

fourth means for determining a best sub-codevector corresponding to a legal candidate codevector among the legal candidate codevectors, wherein the best sub-codevector corresponds to a quantization of the residual vector; and

fifth means for outputting at least one of

the best sub-codevector, and

an index identifying the best sub-codevector.

26. The sub-quantizer of claim 25 , further comprising:

sixth means for storing a definition of an illegal space representing illegal vectors, wherein the third means includes

means for determining whether each candidate codevector belongs to the illegal space; and

means for declaring as a legal candidate codevector each candidate codevector not belonging to the illegal space.

27. The sub-quantizer of claim 25 , further comprising:

means for deriving an error term corresponding to each legal candidate codevector, each error term being a function of the residual vector and the sub-codevector corresponding to the legal candidate codevector,

wherein the fourth means includes means for determining the best sub-codevector based on the error terms.