Device and Method for Quanitizing and Inverse Quanitizing LPC Filters in a Super-Frame

• US 20100023324A1
• Filed: 07/10/2009
• Published: 01/28/2010
• Est. Priority Date: 07/10/2008
• Status: Active Grant

• Current Assignee: VoiceAge Corporation
• Original Assignee: VoiceAge Corporation
• Inventors: Salami, Redwan, Gournay, Philippe, BESSETTE, Bruno
• Granted Patent: US 8,712,764 B2
• Time in Patent Office: Days
• US Class Current: 704/219
• CPC Class Codes: G10L 19/06 Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
• CPC Class Codes: G10L 19/18 Vocoders using multiple modes
• CPC Class Codes: H03M 7/3082 Vector coding for television signals, see H04N19/94

Abstract

A device and a method for quantizing, in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence. The LPC filter quantizing device and method comprises: an absolute quantizer for first quantizing one of the LPC filters using absolute quantization; and at least one quantizer of the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters. For inverse quantizing, at least the first quantized LPC filter is received and an inverse quantizer inverse quantizes the first quantized LPC filter using absolute inverse quantization. If any quantized LPC filter other than the first quantized LPC filter is received, an inverse quantizer inverse quantizes this quantized LPC filter using one of absolute inverse quantization and differential inverse quantization relative to at least one previously received quantized LPC filter.

Claims

1. A method for quantizing, in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein the LPC filter quantizing method comprises:

first quantizing one of the LPC filters using absolute quantization; and

quantizing the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

2. A method for quantizing in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein the LPC filter quantizing method comprises:

first quantizing one of the LPC filters that is always transmitted to a decoder using absolute quantization; and

quantizing the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

3. A method for quantizing in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein one of the LPC filters is calculated during each frame of the sequence including a filter LPC_last calculated during a last frame of the sequence, and wherein the LPC filter quantizing method comprises:

first quantizing the filter LPC_last using absolute quantization; and

quantizing the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

4. A method for quantizing LPC filters in a super-frame including a sequence of a first frame during which a filter LPC1 is calculated, a second frame during which a filter LPC2 is calculated, a third frame during which a filter LPC3 is calculated, and a fourth frame during which a filter LPC4 is calculated, the LPC filter quantizing method comprising:

quantizing the filter LPC4 using absolute quantization;

quantizing the filter LPC2 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4;

quantizing the filter LPC1 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2; and

quantizing the filter LPC3 using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4.

5. A method for quantizing LPC filters as defined in claim 4, comprising using a set of coding modes for coding the super-frame, including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames.

6. A method for quantizing LPC filters as defined in claim 5, wherein the at least one first coding mode comprises ACELP and TCX256, the second coding mode is TCX512, and the fourth coding mode is TCX1024.

7. A method for quantizing LPC filters as defined in claim 5, comprising:

transmitting the quantized filter LPC4 to a decoder;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, transmitting the quantized filter LPC2 to the decoder;

if the first and second frames of the super-frame are coded using the at least one first coding mode, transmitting the quantized filter LPC1 to the decoder; and

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, transmitting the quantized filter LPC3 to the decoder.

8. A method for quantizing LPC filters as defined in claim 4, wherein quantizing the filter LPC3 using differential quantization relative to both the quantized filters LPC2 and LPC4 comprises differentially quantizing the filter LPC3 relative to (quantized filter LPC2+quantized filter LPC4)/2.

10. A method for quantizing LPC filters as defined in claim 5, comprising selecting at least one coding mode amongst the at least one first coding mode, the second coding mode and the third coding mode for coding the first, second, third and fourth frames of the super-frame after all the filters LPC1, LPC2, LPC3 and LPC4 have been quantized.

9. A method for quantizing LPC filters in a super-frame including a sequence of a first frame during which a filter LPC1 is calculated, a second frame during which a filter LPC2 is calculated, a third frame during which a filter LPC3 is calculated, and a fourth frame during which a filter LPC4 is calculated, the LPC filter quantizing method comprising:

quantizing the filter LPC4 using absolute quantization;

quantizing the filter LPC2 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4;

quantizing the filter LPC1 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2; and

quantizing the filter LPC3 using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the filter quantized LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4;

wherein the method further comprises quantizing a filter LPC0 corresponding to a last LPC filter calculated during a previous super-frame, wherein quantizing the filter LPC1 comprises using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, and differential quantization relative to both the quantized filters LPC0 and LPC2.

11. A device for quantizing, in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein the LPC filter quantizing device comprises:

an absolute quantizer for first quantizing one of the LPC filters using absolute quantization; and

at least one quantizer of the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

12. A device for quantizing in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein the LPC filter quantizing device comprises:

an absolute quantizer for first quantizing one of the LPC filters that is always transmitted to a decoder using absolute quantization; and

a quantizer of the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

13. A device for quantizing in a super-frame including a sequence of frames, LPC filters calculated during the frames of the sequence, wherein one of the LPC filters is calculated during each frame of the sequence including a filter LPC_last calculated during a last frame of the sequence, and wherein the LPC filter quantizing device comprises:

an absolute quantizer for first quantizing the filter LPC_last using absolute quantization; and

a quantizer of the other LPC filters using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters.

14. A device for quantizing LPC filters in a super-frame including a sequence of a first frame during which a filter LPC1 is calculated, a second frame during which a filter LPC2 is calculated, a third frame during which a filter LPC3 is calculated, and a fourth frame during which a filter LPC4 is calculated, the LPC filter quantizing device comprising:

a quantizer of the filter LPC4 using absolute quantization;

a quantizer of the filter LPC2 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4;

a quantizer of the filter LPC1 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2; and

a quantizer of the filter LPC3 using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4.

15. A device for quantizing LPC filters as defined in claim 14, comprising a set of coding modes for coding the super-frame, including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames.

16. A device for quantizing LPC filters as defined in claim 15, wherein the at least one first coding mode comprises ACELP and TCX256, the second coding mode is TCX512, and the fourth coding mode is TCX1024.

17. A device for quantizing LPC filters as defined in claim 15, comprising:

a transmitter of the quantized filter LPC4 to a decoder;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, the transmitter transmits the quantized filter LPC2 to the decoder;

if the first and second frames of the super-frame are coded using the at least one first coding mode, the transmitter transmits the quantized filter LPC1 to the decoder; and

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, the transmitter transmits the quantized filter LPC3 to the decoder.

18. A device for quantizing LPC filters as defined in claim 14, wherein the quantizer of the filter LPC3 using differential quantization relative to both the quantized filters LPC2 and LPC4 differentially quantizes the filter LPC3 relative to (quantized filter LPC2+quantized filter LPC4)/2.

20. A device for quantizing LPC filters as defined in claim 15, comprising a selector of at least one coding mode amongst the at least one first coding mode, the second coding mode and the third coding mode for coding the first, second, third and fourth frames of the super-frame after all the filters LPC1, LPC2, LPC3 and LPC4 have been quantized.

19. A device for quantizing LPC filters in a super-frame including a sequence of a first frame during which a filter LPC1 is calculated, a second frame during which a filter LPC2 is calculated, a third frame during which a filter LPC3 is calculated, and a fourth frame during which a filter LPC4 is calculated, the LPC filter quantizing device comprising:

a quantizer of the filter LPC4 using absolute quantization;

a quantizer of the filter LPC2 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4;

a quantizer of the filter LPC1 using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2; and

a quantizer of the filter LPC3 using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4;

wherein the device further comprises a quantizer of a filter LPC0 corresponding to a last LPC filter calculated during a previous super-frame, wherein the quantizer of the filter LPC1 uses a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, and differential quantization relative to both the quantized filters LPC0 and LPC2.

21. A method for inverse quantizing at least one LPC filter in a super-frame including a sequence of frames each associated to a LPC filter, wherein one of the LPC filters is first quantized using absolute quantization and the other LPC filters are subsequently quantized using a quantization process selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters, and wherein the method for inverse quantizing the at least one LPC filter comprises:

receiving at least the first quantized LPC filter;

inverse quantizing the first quantized LPC filter using absolute inverse quantization; and

if any quantized LPC filter other than the first quantized LPC filter is received, inverse quantizing said quantized LPC filter using one of absolute inverse quantization and differential inverse quantization relative to at least one previously received quantized LPC filter.

22. A method for inverse quantizing at least one LPC filter as defined in claim 21, wherein:

the super-frame includes a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4;

the filter LPC4 is the first quantized LPC filter using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4.

23. A method for inverse quantizing at least one LPC filter in a super-frame including a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4, wherein the filter LPC4 is first quantized using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4, wherein the super-frame is coded using a set of coding modes including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames, and wherein the method for inverse quantizing the at least one LPC filter comprises:

receiving the first quantized filter LPC4;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, receiving the quantized filter LPC2 and a first index indicative of one of the absolute quantization mode and the differential quantization mode;

if the first and second frames of the super-frame are coded using the at least one first coding mode, receiving the quantized filter LPC1 and a second index indicative of one of the absolute quantization mode and the differential quantization mode;

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, receiving the quantized filter LPC3 and a third index indicative of one of the absolute quantization mode and the differential quantization mode;

inverse quantizing the first quantized LPC filter using absolute inverse quantization;

if the quantized filter LPC2 is received, inverse quantizing the filter LPC2 using, as indicated by the first index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC4;

if the quantized filter LPC1 is received, inverse quantizing the filter LPC1 using, as indicated by the second index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC2; and

if the quantized filter LPC3 is received, inverse quantizing the filter LPC3 using, as indicated by the third index, one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, differential inverse quantization relative to the quantized filter LPC4, and differential inverse quantization relative to both the quantized filters LPC2 and LPC4.

24. A method for inverse quantizing at least one LPC filter as defined in claim 23, wherein the at least one first coding mode covering a duration of one frame comprises ACELP and TCX256, the second coding mode covering a duration of two frames is TCX512, and the third coding mode covering a duration of four frames is TCX1024.

25. A method for inverse quantizing at least one LPC filter as defined in claim 23, wherein inverse quantizing the filter LPC3 using differential inverse quantization relative to both the quantized filters LPC2 and LPC4 comprises inverse quantizing the filter LPC3 relative to (quantized filter LPC2+quantized filter LPC4)/2.

26. A method for inverse quantizing at least one LPC filter in a super-frame including a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4, wherein the filter LPC4 is first quantized using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4, wherein the super-frame is coded using a set of coding modes including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames, and wherein the method for inverse quantizing the at least one LPC filter comprises:

receiving the first quantized filter LPC4;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, receiving the quantized filter LPC2 and a first index indicative of one of the absolute quantization mode and the differential quantization mode;

if the first and second frames of the super-frame are coded using the at least one first coding mode, receiving the quantized filter LPC1 and a second index indicative of one of the absolute quantization mode and the differential quantization mode;

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, receiving the quantized filter LPC3 and a third index indicative of one of the absolute quantization mode and the differential quantization mode;

inverse quantizing the first quantized LPC filter using absolute inverse quantization;

if the quantized filter LPC2 is received, inverse quantizing the filter LPC2 using, as indicated by the first index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC4;

if the quantized filter LPC1 is received, inverse quantizing the filter LPC1 using, as indicated by the second index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC2; and

if the quantized filter LPC3 is received, inverse quantizing the filter LPC3 using, as indicated by the third index, one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, differential inverse quantization relative to the quantized filter LPC4, and differential inverse quantization relative to both the quantized filters LPC2 and LPC4;

wherein the method further comprises receiving a quantized filter LPC0 corresponding to a last LPC filter calculated during a previous super-frame, wherein inverse quantizing the filter LPC1 comprises using one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, and differential inverse quantization relative to both quantized filters LPC2 and LPC0.

27. A method for inverse quantizing at least one LPC filter as defined in claim 26, wherein inverse quantizing the filter LPC1 comprises using one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, and differential inverse quantization relative to both quantized filters LPC2 and LPC0, the method comprising estimating the quantized filter LPC0 at a decoder when switching from a non-LPC-based coding mode to an LPC-based coding mode.

28. A device for inverse quantizing at least one LPC filter in a super-frame including a sequence of frames each associated to a LPC filter, wherein one of the LPC filters is first quantized using absolute quantization and the other LPC filters are subsequently quantized using a quantization process selected from the group consisting of absolute quantization and differential quantization relative to at least one previously quantized filter amongst the LPC filters, and wherein the device for inverse quantizing the at least one LPC filter comprises:

means for receiving at least the first quantized LPC filter;

an inverse quantizer of the first quantized LPC filter using absolute inverse quantization; and

if any quantized LPC filter other than the first quantized LPC filter is received, an inverse quantizer of said quantized LPC filter using one of absolute inverse quantization and differential inverse quantization relative to at least one previously received quantized LPC filter.

29. A device for inverse quantizing at least one LPC filter as defined in claim 28, wherein:

the super-frame includes a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4;

the filter LPC4 is the first quantized LPC filter using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4.

30. A device for inverse quantizing at least one LPC filter in a super-frame including a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4, wherein the filter LPC4 is first quantized using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4, wherein the super-frame is coded using a set of coding modes including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames, and wherein the device for inverse quantizing the at least one LPC filter comprises:

means receiving the first quantized filter LPC4;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, means for receiving the quantized filter LPC2 and a first index indicative of one of the absolute quantization mode and the differential quantization mode;

if the first and second frames of the super-frame are coded using the at least one first coding mode, means for receiving the quantized filter LPC1 and a second index indicative of one of the absolute quantization mode and the differential quantization mode;

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, means for receiving the quantized filter LPC3 and a third index indicative of one of the absolute quantization mode and the differential quantization mode;

an inverse quantizer of the first quantized LPC filter using absolute inverse quantization;

if the quantized filter LPC2 is received, an inverse quantizer of the filter LPC2 using, as indicated by the first index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC4;

if the quantized filter LPC1 is received, an inverse quantizer of the filter LPC1 using, as indicated by the second index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC2; and

if the quantized filter LPC3 is received, an inverse quantizer of the filter LPC3 using, as indicated by the third index, one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, differential inverse quantization relative to the quantized filter LPC4, and differential inverse quantization relative to both the quantized filters LPC2 and LPC4.

31. A device for inverse quantizing at least one LPC filter as defined in claim 30, wherein the at least one first coding mode covering a duration of one frame comprises ACELP and TCX256, the second coding mode covering a duration of two frames is TCX512, and the third coding mode covering a duration of four frames is TCX1024.

32. A device for inverse quantizing at least one LPC filter as defined in claim 30, wherein the inverse quantizer of the filter LPC3 using differential inverse quantization relative to both the quantized filters LPC2 and LPC4 inverse quantizes the filter LPC3 relative to (quantized filter LPC2+quantized filter LPC4)/2.

33. A device for inverse quantizing at least one LPC filter in a super-frame including a sequence of a first frame associated to a filter LPC1, a second frame associated to a filter LPC2, a third frame associated to a filter LPC3, and a fourth frame associated to a filter LPC4, wherein the filter LPC4 is first quantized using absolute quantization, the filter LPC2 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC4, the filter LPC1 is quantized using a quantization mode selected from the group consisting of absolute quantization and differential quantization relative to the quantized filter LPC2, and the filter LPC3 is quantized using a quantization mode selected from the group consisting of absolute quantization, differential quantization relative to the quantized filter LPC2, differential quantization relative to the quantized filter LPC4, and differential quantization relative to both the quantized filters LPC2 and LPC4, wherein the super-frame is coded using a set of coding modes including at least one first coding mode covering a duration of one frame, a second coding mode covering a duration of two frames, and a third coding mode covering a duration of four frames, and wherein the device for inverse quantizing the at least one LPC filter comprises:

means receiving the first quantized filter LPC4;

if the first, second, third and fourth frames of the super-frame are not coded using the third coding mode, means for receiving the quantized filter LPC2 and a first index indicative of one of the absolute quantization mode and the differential quantization mode;

if the first and second frames of the super-frame are coded using the at least one first coding mode, means for receiving the quantized filter LPC1 and a second index indicative of one of the absolute quantization mode and the differential quantization mode;

if the third and fourth frames of the super-frame are coded using the at least one first coding mode, means for receiving the quantized filter LPC3 and a third index indicative of one of the absolute quantization mode and the differential quantization mode;

an inverse quantizer of the first quantized LPC filter using absolute inverse quantization;

if the quantized filter LPC2 is received, an inverse quantizer of the filter LPC2 using, as indicated by the first index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC4;

if the quantized filter LPC1 is received, an inverse quantizer of the filter LPC1 using, as indicated by the second index, one of absolute inverse quantization and differential inverse quantization relative to the quantized filter LPC2; and

if the quantized filter LPC3 is received, an inverse quantizer of the filter LPC3 using, as indicated by the third index, one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, differential inverse quantization relative to the quantized filter LPC4, and differential inverse quantization relative to both the quantized filters LPC2 and LPC4;

wherein the device further comprises means for receiving a quantized filter LPC0 corresponding to a last LPC filter calculated during a previous super-frame, wherein the inverse quantizer of the filter LPC1 uses one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, and differential inverse quantization relative to both quantized filters LPC2 and LPC0.

34. A device for inverse quantizing at least one LPC filter as defined in claim 33, wherein the inverse quantizer of the filter LPC1 uses one of absolute inverse quantization, differential inverse quantization relative to the quantized filter LPC2, and differential inverse quantization relative to both quantized filters LPC2 and LPC0, the device comprising an estimator of the quantized filter LPC0 at a decoder when switching from a non-LPC-based coding mode to an LPC-based coding mode.