Method for linear predictive analysis of an audiofrequency signal, and method for coding and decoding an audiofrequency signal including application thereof

• US 5,787,390 A
• Filed: 12/11/1996
• Issued: 07/28/1998
• Est. Priority Date: 12/15/1995
• Status: Expired due to Term

• Current Assignee: 3G Licensing S.A. (Fineur International S.A.)
• Original Assignee: Orange S.A.
• Inventors: Quinquis, Catherine, Le Guyader, Alain
• Primary Examiner(s): Hudspeth, David R.
• Assistant Examiner(s): ABEBE, DANIEL DEMELASH
• Application Number:
  US08/763,457
• Time in Patent Office: 594 Days
• Field of Search: 704/219, 704/262, 704/205, 704/222, 704/223
• 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

Abstract

The linear predictive analysis method is used in order to determine the spectral parameters representing the spectral envelope of the audiofrequency signal. This method comprises q successive prediction stages, q being an integer greater than 1. At each prediction stage p(1≦p≦q), parameters are determined representing a predefined number Mp of linear prediction coefficients a₁^p, . . . , a_Mp^p of an input signal of the said stage. The audiofrequency signal to be analysed constitutes the input signal of the first stage. The input signal of a stage p+1 consists of the input signal of the stage p filtered with a filter with transfer function ##EQU1##

Claims

1. Method for linear predictive analysis of an audiofrequency signal, in order to determine spectral parameters dependent on a short-term spectrum of the audiofrequency signal, the method comprising q successive prediction stages, q being an integer greater than 1, wherein each prediction stage p(1≦

p≦

q) includes determining parameters representing a number Mp, predefined for each stage p, of linear prediction coefficients a₁^p, . . . , a_Mp^p of an input signal of said stage, wherein the audiofrequency signal to be analysed constitutes the input signal of stage 1, and wherein, for any integer p such that 1≦

p≦

q, the input signal of stage p+1 consists of the input signal of stage p filtered by a filter with transfer function ##EQU33##

2. Analysis method according to claim 1, wherein the number Mp of linear prediction coefficients increases from one stage to the next.

3. Method for coding an audiofrequency signal, comprising the following steps:

linear predictive analysis of the audiofrequency signal digitized in successive frames in order to determine parameters defining a short-term synthesis filter;

determination of excitation parameters defining an excitation signal to be applied to the short-term synthesis filter in order to produce a synthetic signal representing the audiofrequency signal; and

production of quantization values of the parameters defining the short-term synthesis filter and of the excitation parameters,

wherein the linear predictive analysis is a process with q successive stages, q being an integer greater than 1, wherein each prediction stage p(1≦

p≦

q) includes determining parameters representing a number Mp, predefined for each stage p, of linear prediction coefficients a₁^p, . . . , a_Mp^p of an input signal of said stage, wherein the audiofrequency signal to be coded constitutes the input signal of stage 1, wherein, for any integer p such that 1≦

p≦

q, the input signal of stage p+1 consists of the input signal of stage p filtered by a filter with transfer function ##EQU34## and wherein the short-term synthesis filter has a transfer function of the form 1/A(z) with ##EQU35##

4. Coding method according to claim 3, wherein the number Mp of linear prediction coefficients increases from one stage to the next.

5. Coding method according to claim 3, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form W(z)=A(z/γ

₁)/A(z/γ

₂) where γ

₁ and γ

₂ denote spectral expansion coefficients such that 0≦

γ

₂ ≦

γ

₁ ≦

1.

6. Coding method according to claim 3, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form ##EQU36## where γ

₁^p, γ

₁^p denote pairs of spectral expansion coefficients such that 0≦

γ

₂^p ≦

γ

₁^p ≦

1 for 1≦

p≦

q.

7. Method for decoding a bit stream in order to construct an audiofrequency signal coded by said bit stream, comprising the steps of:

receiving quantization values of parameters defining a short-term synthesis filter and of excitation parameters, wherein the parameters defining the synthesis filter represent a number q greater than 1 of sets of linear prediction coefficients, each set p(1≦

p≦

q) including a predefined number Mp of coefficients;

producing an excitation signal on the basis of the quantization values of the excitation parameters; and

producing a synthetic audiofrequency signal by filtering the excitation filter with a synthesis filter having a transfer function of the form 1/A(z) with ##EQU37## where the coefficients a₁^p, . . . , a_Mp^p correspond to the p-th set of linear prediction coefficients for 1≦

p≦

q.

8. Decoding method according to claim 7, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form A(z/β

₁)/A(z/β

₂), where β

₁ and β

₁ denote coefficients such that 0≦

β

₁ ≦

β

₂ ≦

1.

9. Decoding method according to claim 7, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form ##EQU38## where β

₁^p, β

₂^p denote pairs of coefficients such that 0≦

β

₁^p ≦

β

₂^p ≦

1 for 1≦

p≦

q, and A^p (z) represents, for the p-th set of linear prediction coefficients, the function ##EQU39##

10. Method for coding a first audiofrequency signal digitized in successive frames, comprising the following steps:

linear predictive analysis of a second audiofrequency signal in order to determine parameters defining a short-term synthesis filter;

determination of excitation parameters defining an excitation signal to be applied to the short-term synthesis filter in order to produce a synthetic signal representing the first audiofrequency signal, said synthetic signal constituting said second audiofrequency signal for at least one subsequent frame; and

production of quantization values of the excitation parameters,

wherein the linear predictive analysis is a process with q successive stages, q being an integer greater than 1, wherein each prediction stage p(1≦

p≦

q) includes determining parameters representing a number Mp, predefined for each stage p, of linear prediction coefficients a₁^p, . . . , a_Mp^p of an input signal of said stage, wherein the second audiofrequency signal constitutes the input signal of stage 1, wherein, for any integer p such that 1≦

p≦

q, the input signal of stage p+1 consists of the input signal of stage p filtered by a filter with transfer function ##EQU40## and wherein the short-term synthesis filter has a transfer function of the form 1/A(z) with ##EQU41##

11. Coding method according to claim 10, wherein the number Mp of linear prediction coefficients increases from one stage to the next.

12. Coding method according to claim 10, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the first audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form W(z)=A(z/γ

₁)/A(z/γ

₂) where γ

₁ and γ

₂ denote spectral expansion coefficients such that 0≦

γ

₂ ≦

γ

₁ ≦

1.

13. Coding method according to claim 10, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the first audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form ##EQU42## where γ

₁^p, γ

₂^p denote pairs of spectral expansion coefficients such that 0≦

γ

₂^p≦

γ

₁^p ≦

1 for 1≦

p≦

q.

14. Method for decoding a bit stream in order to construct in successive frames an audiofrequency signal coded by said bit stream, comprising the steps of:

receiving quantization values of excitation parameters;

producing an excitation signal on the basis of the quantization values of the excitation parameters;

producing a synthetic audiofrequency signal by filtering the excitation signal with a short-term synthesis filter; and

performing a linear predictive analysis of the synthetic signal in order to obtain coefficients of the short-term synthesis filter for at least one subsequent frame,

wherein the linear predictive analysis is a process with q successive stages, q being an integer greater than 1, wherein each prediction stage p(1≦

p≦

q) includes determining parameters representing a number Mp, predefined for each stage p, of linear prediction coefficients a₁^p, . . . , a_Mp^p of an input signal of said stage, wherein the synthetic signal constitutes the input signal of stage 1, wherein, for any integer p such that 1≦

p≦

q, the input signal of stage p+1 consists of the input signal of stage p filtered by a filter with transfer function ##EQU43## and wherein the short-term synthesis filter has a transfer function of the form 1/A(z) with ##EQU44##

15. Decoding method according to claim 14, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form A(z/β

₁ /A(z/β

₂), where β

₁ and β

₂ denote coefficients such that 0≦

β

₁ ≦

β

₂ ≦

1.

16. Decoding method according to claim 14, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form ##EQU45## where β

₁^p, β

₂^p denote pairs of coefficients such that 0≦

β

₁^p ≦

β

₂^p ≦

1 for 1≦

p≦

q.

17. Method for coding a first audiofrequency signal digitized in successive frames, comprising the following steps:

linear predictive analysis of the first audiofrequency signal in order to determine parameters defining a first component of a short-term synthesis filter;

determination of excitation parameters defining an excitation signal to be applied to the short-term synthesis filter in order to produce a synthetic signal representing the first audiofrequency signal;

production of quantization values of the parameters defining the first component of the short-term synthesis filter and of the excitation parameters;

filtering of the synthetic signal with a filter with transfer function corresponding to the inverse of the transfer function of the first component of the short-term synthesis filter; and

linear predictive analysis of the filtered synthetic signal in order to obtain coefficients of a second component of the short-term synthesis filter for at least one subsequent frame,

wherein the linear predictive analysis of the first audiofrequency signal is a process with q_F successive stages, q_F being an integer at least equal to 1, wherein each prediction stage p(1≦

p≦

q_F) of said process with q_F stages includes determining parameters representing a number MFp, predefined for each stage p, of linear prediction coefficients A₁^F,p, . . . , a_MFp^F,p of an input signal of said stage, wherein the first audiofrequency signal constitutes the input signal of stage 1 of the process with q_F stages, wherein, for any integer p such that 1≦

p<

q_F, the input signal of stage p+1 of the process with q_F stages consists of the input signal of stage p of the process with q_F stages filtered by a filter with transfer function ##EQU46## wherein the first component of the short-term synthesis filter has a transfer function of the form 1/A^F (z) with ##EQU47## wherein the linear predictive analysis of the filtered synthetic signal is a process with q_B successive stages, q_B being an integer at least equal to 1, wherein each prediction stage p(1≦

p≦

q_B) of said process with q_B stages includes determining parameters representing a number MBp, predefined for each stage p, of linear prediction coefficients a₁^b,p, . . . , a_MBp^B,p of an input signal of said stage, wherein the filtered synthetic signal constitutes the input signal of stage 1 of the process with q_B stages, wherein, for any integer p such that 1≦

p<

q_B, the input signal of stage p+1 of the process with q_B stages consists of the input signal of stage p of the process with q_B stages filtered by a filter with transfer function ##EQU48## wherein the second component of the short-term synthesis filter has a transfer function of the form 1/A^B (z) with ##EQU49## and wherein the short-term synthesis filter has a transfer function of the form 1/A(z) with A(z)=A^F (z).A^B (z).

18. Coding method according to claim 17, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the first audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form W(z)=A(z/γ

₁)/A(z/γ

₂) where γ

₁ and γ

₂ denote spectral expansion coefficients such that 0≦

γ

₂ ≦

γ

₁ ≦

1.

19. Coding method according to claim 17, wherein at least some of the excitation parameters are determined by minimizing an energy of an error signal resulting from a filtering of a difference between the first audiofrequency signal and the synthetic signal by at least one perceptual weighting filter having a transfer function of the form ##EQU50## where γ

₁^F,p, γ

₂^F,p denote pairs of spectral expansion coefficients such that 0≦

γ

₂^F,p ≦

γ

₁^F,p ≦

1 for 1≦

p≦

q_F, and γ

₁^B,p, γ

₂^B,p denote pairs of spectral expansion coefficients such that 0≦

γ

₂^B,p ≦

γ

₁^B,p ≦

1 for 1≦

p≦

q_B.

20. Method for decoding a bit stream in order to construct in successive frames an audiofrequency signal coded by said bit stream, comprising the steps of:

receiving quantization values of parameters defining a first component of a short-term synthesis filter and of excitation parameters, wherein the parameters defining the first component of the short-term synthesis filter represent a number q_F at least equal to 1 of sets of linear prediction coefficients a₁^F,p, . . . a_MFp^F,p for 1≦

p≦

q_F, each set p including a predefined number MFp of coefficients, wherein the first component of the short-term synthesis filter has a transfer function of the form 1/A^F (z) with ##EQU51## producing an excitation signal on the basis of the quantization values of the excitation parameters;

producing a synthetic audiofrequency signal by filtering the excitation signal with a short-term synthesis filter having a transfer function 1/A(z) with A(z)=A^F (z).A^B (z), where 1/A^B (z) represents a transfer function of a second component of the short-term synthesis filter;

filtering the synthetic signal with a filter with transfer function A^F (z); and

performing a linear predictive analysis of the filtered synthetic signal in order to obtain coefficients of the second component of the short-term synthesis filter for at least one subsequent frame,

wherein the linear predictive analysis of the filtered synthetic signal is a process with q_B successive stages, q_B being an integer at least equal to 1, wherein each prediction stage p(1≦

p≦

q_B) includes determining parameters representing a number MBp, predefined for each stage p, of linear prediction coefficients a₁^B,p, . . . , a_MBp^B,p of an input signal of the said stage, wherein the filtered synthetic signal constitutes the input signal of stage 1, wherein, for any integer p such that 1≦

p<

q_B, the input signal of stage p+1 consists of the input signal of stage p filtered by a filter with transfer function ##EQU52## and wherein the second component of the short-term synthesis filter has a transfer function of the form 1/A^B (z) with ##EQU53##

21. Decoding method according to claim 20, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form A(z/β

₁)/A(z/β

₂), where β

₁ and β

₂ denote coefficients such that 0≦

β

₁ ≦

β

₂ ≦

1.

22. Decoding method according to claim 20, further comprising the step of applying said synthetic audiofrequency signal to a postfilter whose transfer function includes a term of the form ##EQU54## where β

₁^F,P, β

₂^F,P denote pairs of coefficients such that 0≦

β

₁^F,p ≦

β

₂^F,p ≦

1 for 1≦

p≦

q_F, and β

₁^B,p, β

₂^B,p denote pairs of coefficients such that 0≦

β

₁^B,p ≦

β

₂^B,p ≦

1 for 1≦

p≦

q_B.