Method and device for coding an audio signal by “forward” and “backward” LPC analysis

• US 6,327,562 B1
• Filed: 05/24/1999
• Issued: 12/04/2001
• Est. Priority Date: 04/16/1997
• Status: Expired due to Term

• Current Assignee: FRANCE TELECOM 6
• Original Assignee: Orange S.A.
• Inventors: Proust, Stéphane
• Primary Examiner(s): Tsang, Fan
• Assistant Examiner(s): Opsasnick, Michael N.
• Application Number:
  US09/202,753
• Time in Patent Office: 925 Days
• Field of Search: 704/219, 704/220, 704/221, 704/201
• 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

Abstract

A method and device for encoding a digital audio-signal divided into a succession of blocks according to a LPC “forward” and “backward” analysis respectively under a choice criterion. For coding each current block, the choice criterion is established by defining the degree of stationarity of the digital audio-signal according to a stationarity parameter belonging to a maximum and a minimum stationarity range value. An analysis choice value is established from a decision function and this stationarity parameter and thus applied to the digital audio-signal to have this audio digital signal encoded by “backward” LPC filtering for stationary zones. The “forward” and “backward” filtering mode are thus performed in relation to the degree of stationarity of the audio digital signal, the amount of switching from one to the other filtering modes being thus limited.

Claims

1. A method for encoding a digital audio signal by dual analysis according to a choice criterion of LPC “

forward” and

“

backward”

analysis respectively into a transmitted encoded signal consisting of LPC filtering parameters accompanied by analysis decision information, and into a residue encoding signal, not transmitted, said digital audio signal being subdivided into frames, a succession of blocks of a specified number of samples, the encoding of said digital audio signal being carried out on this signal through a “

forward”

LPC filtering for non-stationary zones respectively on a synthesis signal, obtained from said residue encoding signal, through a “

backward”

LPC filtering for stationary zones, wherein said choice criterion consists, on each current block of said succession of current blocks constituting a current frame;

in defining the degree of stationarity of the digital audio signal according to a stationarity parameter, the value of which lies between a maximum stationarity value and a minimum stationarity value;

in establishing, from said stationarity parameter, an analysis choice value, from a decision function;

in applying said analysis choice value to the “

forward”

LPC filtering so as to carry out the encoding of said digital audio signal by “

forward”

LPC filtering for non-stationary zones on said digital audio signal, and by “

backward”

LPC filtering respectively for stationary zones on said synthesis signal, which makes it possible to favor the maintenance of the digital audio signal in one of the “

forward” and

“

backward”

filtering modes respectively in relation to the degree of stationarity and to limit the amount of switching from one to the other and vice versa of the filtering modes.

2. The method according to claim 1, wherein said decision function is an adaptive function, actualized for each current block from the stationarity parameter, said actualization of said adaptive function making it possible to favor the maintenance of the digital audio signal filtering in one of the “

forward” and

“

backward”

filtering modes respectively as a function of the degree of stationarity of said digital audio signal and thus to limit the amount of switching from one to the other and vice versa of the filtering modes.

3. The method according to claim 1, wherein said analysis choice value established from said decision function corresponds to a “

forward”

LPC filtering mode priority value and a “

backward”

LPC filtering mode priority value respectively.

4. The method according to claim 1, wherein the stage consisting in specifying the degree of stationarity of each current block of said digital audio signal consists, starting from an arbitrary starting value of said stationarity parameter:

in calculating for said current block an intermediate stationarity parameter value, as a function of a specified number of analysis choice values, obtained for different successive blocks prior to said current block of said succession of blocks, and of the stationarity parameter value of the block preceding the said current block;

in tuning said intermediate stationarity parameter value as a function of the value of prediction gains of the “

forward” and

“

backward”

LPC filtering of the frame preceding said current frame.

5. The method according to claim 4, wherein the stage consisting, for each current block, in calculating an intermediate stationarity parameter value consists:

in discriminating between the “

forward”

LPC or “

backward”

LPC analysis mode of the block preceding said current block; and

for any previous block analyzed by “

backward”

LPC analysis mode;

in specifying the number of previous frames consecutively analyzed in “

backward”

LPC analysis mode,

in comparing, according to a superiority comparison criterion, said number of previous frames with a first arbitrary value representative of a number of successive frames analyzed in “

backward”

LPC mode, and on positive response to this superiority comparison,

attributing to said intermediate stationarity parameter value the stationarity parameter value of the block preceding said current block, augmented by a specified value function of said first arbitrary value, and on negative response to this superiority comparison,

attributing to said intermediate stationarity parameter value the stationarity parameter value of the block preceding said current block; and

for any previous block analyzed in “

forward”

LPC analysis mode,

in specifying according to a test criterion the occurrence of a transition from “

backward”

LPC analysis mode to “

forward”

LPC analysis mode between the block prior to said preceding block and said preceding block, and on positive response to said test of occurrence,

in comparing, according to an inferiority comparison criterion, said number of previous frames with a second arbitrary value representative of a number of successive frames analyzed in “

backward”

LPC mode preceding said preceding block, and on positive response to said inferiority comparison,

attributing to said intermediate stationarity parameter value the stationarity parameter value of said block preceding the current block, reduced by a specified value which is a function of said second arbitrary value, and on negative response to said inferiority comparison,

attributing to said intermediate stationarity parameter value the stationarity parameter value of said preceding block.

6. The method according to claim 4, wherein the stage consisting for each current block in tuning said intermediate stationarity parameter value consists:

in distinguishing between prediction gains of the “

forward”

LPC filtering and “

backward”

LPC filtering;

in modifying the intermediate stationarity parameter value of a refining value function of the relative value of prediction gains of “

forward” and

“

backward”

LPC filtering, the modification, increase or reduction, of the intermediate stationarity parameter value being proportional to said refining value.

7. The method according to claim 6, wherein the stage of increase proportional to said refining value of the intermediate stationarity parameter value is moreover subject to a condition of superiority of said value of “

backward”

LPC filtering gain relative to a first specified positive value and to a condition of inferiority of the value of said intermediate stationarity parameter value relative to a second specified positive value.

8. The method according to claim 6, wherein the stage of reduction proportional to said refining value of the intermediate stationarity parameter value is moreover subject to a condition of inferiority of said value of “

backward”

LPC filtering gain relative to a third specified positive value and to a condition of superiority of the value of said intermediate stationarity parameter value relative to a fourth specified positive value.

9. The method according to claim 6, wherein said relative value of the prediction gains of “

forward” and

“

backward”

LPC filtering consists in the ratio or the difference between prediction gains of “

forward” and

“

backward”

LPC filtering.

10. The method according to claim 1, wherein said method consists in addition, for each successive current block:

in establishing the average energy of said digital audio signal,

in comparing, according to an inferiority comparison criterion, said average energy with a specified threshold value representative of a silence frame, and on positive response to said inferiority comparison,

in attributing to said stationarity parameter of the current block the stationarity parameter value of the preceding block.

11. The method according to claim 2, wherein, for a degree of stationarity represented by a stationarity parameter between a minimum value and a maximum value, said minimum value representing the degree of stationarity of a substantially non-stationary digital signal and said maximum value representing the degree of stationarity of a substantially stationary signal, said adaptive function constituting the decision function is an increasing function of the priority value of the “

backward”

LPC filtering mode according to the increasing degree of stationarity of said digital signal.

12. An encoding device for a digital audio signal by dual analysis according to a choice criterion of “

forward” and

“

backward”

LPC analysis respectively into a transmitted encoded signal, said digital signal being subdivided into frames constituted by successive blocks comprising a specified number of samples, said encoding device comprising a “

forward”

LPC analysis filter and a “

backward”

LPC filter enabling delivery of a transmitted encoded signal consisting of LPC filtering parameters accompanied by an analysis decision indication and a means of encoding an encoding residue signal, not transmitted, enabling generation of a synthesis residue signal, the encoding of said digital audio signal being carried out on this digital audio signal from the “

forward”

LPC filter for non-stationary zones and on this synthesis signal, from the “

backward”

LPC filter respectively for stationary zones, wherein said encoding device comprises in addition, for each current LPC block;

calculation means of the degree of stationarity of said digital audio signal, according to a stationarity parameter the value of which is between a minimum stationarity value and a maximum stationarity value;

setting means, from a stationarity parameter, of a decision function enabling an LPC analysis choice value to be set;

discrimination means of LPC analysis receiving said analysis choice value and enabling delivery, for said LPC current block, of the value of the “

backward” and

“

forward”

LPC filtering parameters respectively as a function of said analysis choice value;

adaptive filtering means as a function of the degree of stationarity receiving said digital audio signal and the value of the “

forward” and

“

backward”

LPC filtering parameters respectively as a function of said analysis choice value and delivering the encoding residue signal to said encoding means of the encoding residue signal, which makes it possible to encode said digital audio signal and to favor the maintenance of said digital audio signal in one of the “

forward” and

“

backward”

filtering modes respectively in relation to the degree of stationarity of said digital signal and to limit the amount of switching from one to the other and vice versa of the filtering modes.

13. The encoding device according to claim 12, wherein said transmitted encoded signal consists, for each LPC analysis block, of:

said analysis value,

and in the case where the analysis choice value corresponds for LPC analysis block considered, to a “

forward”

LPC analysis;

said “

forward”

PC filtering parameters.

14. A decoding device of a digital audio signal encoded by dual analysis according to a choice criterion of “

forward” and

“

backward”

LPC analysis respectively, into a transmitted encoded signal consisting of LPC filtering parameters accompanied by an analysis decision indication, wherein that said transmitted encoded signal, consisting for each LPC analysis block of said analysis choice value and corresponding for the LPC analysis block considered to “

forward”

LPC analysis in “

forward”

LPC filtering parameters, said decoding device comprises at least;

synthesis means for the filtering residue signal receiving said encoding parameters of the LPC residue and delivering a synthesis residue signal,

reverse filtering adaptive means as a function of the degree of stationarity, receiving the synthesis residue signal and enabling generation of a synthesis signal representative of the digital audio signal and constituting the decoded signal,

“

backward”

LPC analysis means receiving said synthesis signal and enabling generation of “

backward”

LPC filtering parameters,

discriminating means between “

forward”

LPC analysis and “

backward”

LPC analysis respectively receiving, on the one hand, for discrimination control said analysis choice value and, on the other hand, the “

forward”

LPC filtering parameters and the “

backward”

LPC filtering parameters and enabling delivery as a function of said analysis choice value, of either “

forward”

LPC filtering parameters, or “

backward”

LPC filtering parameters to said reverse filtering adaptive means as a function of the degree of stationarity.