Sinusoidal model based coding of audio signals

• US 20030009332A1
• Filed: 07/01/2002
• Published: 01/09/2003
• Est. Priority Date: 11/03/2000
• Status: Active Grant

• Current Assignee: Pendragon Wireless LLC (Pendrell Corporation)
• Original Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Inventors: Heusdens, Richard, Kleijn, Willem Bastiaan, Vafin, Renat
• Granted Patent: US 7,120,587 B2
• Time in Patent Office: Days
• US Class Current: 704/240
• CPC Class Codes: G10L 19/02 using spectral analysis, e.g. transform vocoders or subband vocoders
• CPC Class Codes: G10L 2019/0014 Selection criteria for distances
• CPC Class Codes: G10L 21/0364 for improving intelligibility

Abstract

The invention relates to an apparatus and method of signal coding. An analysis-by-synthesis algorithm for sinusoidal modelling is used. An input signal to be modelled is divided in time to produce a plurality of frames. Functions from a dictionary are selected to form an approximation of the section of the input signal contained in each frame, with the selection carried out based on a psychoacoustic norm. In an embodiment of the method, the function dictionary is made up of complex exponentials and these are selected iteratively to make up the section of the input signal contained in each frame. The psychoacoustic norm adapts after each iteration according to the changing masking threshold of the residual signal to be modelled in the next step.

Claims

1. A method of signal coding, the method comprising the steps of:

(a) receiving an input signal;

(b) dividing the input signal in time to produce a plurality of frames each containing a section of the input signal; and

(c) selecting functions from a function dictionary to form an approximation of the signal in each frame;

wherein the selection process of step (c) is carried out on the basis of a norm which is based on a combination, such as a product, of a weighting function expressed as a function of frequency and a product of a window function defining each frame in the plurality of frames and the section of the input signal to be modelled, the product of the window function and the section of the input signal to be modelled being expressed as a function of frequency.

2. A method of signal coding according to claim 1, wherein the norm is defined by

∥

Rx∥

={square root}{square root over (∫

)}{overscore (a)}(ƒ

)|({overscore (wRx)})(ƒ

)|²dƒ

in which Rx represents a section of the input signal to be modelled, {overscore (a)}(ƒ

) represents the weighting function expressed as a function of frequency and ({overscore (wRx)})(ƒ

) represents the transform, such as a Fourier transform, of the product of the window function defining each frame in the plurality of frames, w, and Rx.

3. The method of signal coding according to claim 1 or 2, wherein the weighting function incorporates knowledge of the psychoacoustics of human hearing to aid the selection process of step (c).

4. The method of signal coding according to claim 3, wherein the knowledge of the psychoacoustics of human hearing is incorporated into the norm through the function {overscore (a)}(ƒ

).

5. The method of signal coding according to claim 4, wherein {overscore (a)}(ƒ

) is based on the masking threshold of the human auditory system and is the inverse of the masking threshold.

6. The method of signal coding according to claim 5, wherein {overscore (a)}(ƒ

) is computed using a known model of the masking threshold.

7. The method of signal coding according to any of the claims 1 to 6, wherein the selection process of step (c) is carried out in a plurality of substeps, in each substep a single function from a function dictionary being identified.

8. The method of signal coding according to claim 7, wherein the function identified at the first substep is subtracted from the input signal in the frame to form a residual signal and at each subsequent substep a function is identified and subtracted from the residual signal to form a further residual signal, with the sum of the functions identified at each substep forming an approximation of the signal in each frame.

9. The method of signal coding according to any preceding claim, where the norm adapts at each substep of the selection process of step (c).

10. The method of signal coding according to claim 9, wherein a new norm is induced at each substep of the selection process of step (c) based on a current residual signal, with {overscore (a)}(ƒ

) also updated to take into account the masking characteristics of the residual signal.

11. The method of signal coding according to claim 1 or 2, wherein the weighting function is held independent of iteration number.

12. The method of signal coding according to claim 11, wherein the function {overscore (a)}(ƒ

) is based on the masking threshold of the human auditory system, is the inverse of the masking threshold for the section of an input signal in a frame being coded and is calculated using a known model of the masking threshold.

13. The method of any preceding claim, wherein the norm is induced according to the inner product:

<

x,y>

=∫

₀¹{overscore (a)}(ƒ

)({overscore (wx)})*(ƒ

)dƒ

 

 

(4)

14. The method of audio coding according to claim 13, wherein denoting the residual at iteration m as R^mx and the weighting function from the previous iteration {overscore (a)}_m−

1 the function identified from the function dictionary minimises ∥

R^mx∥

<sub>{overscore (a)}m−

1</sub>, with ∥

•

∥

<sub>{overscore (a)}m−

1</sub>representing the norm calculated using {overscore (a)}<sub>m−

1.</sub>

15. The method of signal coding according to claim 14, wherein the convergence of the method of audio coding is guaranteed by the validity of the theorem that for all m >

0 there exists a λ

>

0 such that ∥

R^mx∥

_{{overscore (a)}m}≦

2<sup>−

λ

m∥

x∥</sup>

_{{overscore (a)}0}, where x represents an initial section of the input signal to be modelled.

16. The method of signal coding according to claim 13, wherein the convergence of the method of audio coding is guaranteed by the increase or invariance in each frame of the masking threshold at each substep, such that {overscore (a)}_m(ƒ

)≦

{overscore (a)}<sub>m−

1</sub>(ƒ

) over the entire frequency range ƒ

ε

[0,1).

17. The method of signal coding according to any preceding claim, wherein the window function is any one of a Hanning window, a Hamming window, a rectangular window or another suitable window.

18. Coding apparatus (10) operating in accordance with the method of any of the preceding claims.

19. A transmitting apparatus (1) comprising;

a source (11) for providing an input signal;

a coding apparatus (10) according to claim 18 for coding the input signal to obtain a coded signal, and

an output unit for outputting the coded signal.