Methods and devices for low-frequency emphasis during audio compression based on ACELP/TCX

• US 20070225971A1
• Filed: 02/15/2007
• Published: 09/27/2007
• Est. Priority Date: 02/18/2004
• Status: Active Grant

• Current Assignee: VoiceAge Corporation
• Original Assignee: VoiceAge Corporation
• Inventors: Bessette, Bruno
• Granted Patent: US 7,933,769 B2
• Time in Patent Office: Days
• US Class Current: 704/203
• CPC Class Codes: G10L 19/005 Correction of errors induced by the transmission channel, if related to the coding algorithm
• CPC Class Codes: G10L 19/0208 Subband vocoders
• CPC Class Codes: G10L 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding
• CPC Class Codes: G10L 19/265 Pre-filtering, e.g. high frequency emphasis prior to encoding
• CPC Class Codes: G10L 21/0232 Processing in the frequency domain

Abstract

A first aspect of the present invention relates to a method for low-frequency emphasizing the spectrum of a sound signal transformed in a frequency domain and comprising transform coefficients grouped in a number of blocks, in which a maximum energy for one block is calculated and a position index of the block with maximum energy is determined, a factor is calculated for each block having a position index smaller than the position index of the block with maximum energy the calculated maximum energy and the energy of the block, and, for each block, a gain determining from the factor is applied to the transform coefficients of the block. Another aspect of the invention is concerned with an HF coding method for coding, through a bandwidth extension scheme, an HF signal obtained from separation of a full-bandwidth sound signal into the HF signal and a LF signal, in which an estimation of the an HF gain is calculated from LPC coefficients, the energy of the HF signal is calculated, the LF signal is processed to produce a synthesized version of the HF signal, the energy of the synthesized version of the HF signal is calculated, a ratio between the energy of the HF signal and the energy of the synthesized version of the HF signal is calculated and expressing as an HF gain, and a difference between the estimation of the HF gain and the HF gain is calculated to obtain a gain correction. A third aspect of the invention is concerned with a method for producing from a decoded target signal an overlap-add target signal in a current frame coded according to a first coding mode. According to this method, the decoded target signal of the current frame is windowed and a left portion of the window is skipped. A zero-input response of a weighting filter of the previous frame coded according to a second coding mode is calculated and windowed so that the zero-input response has an amplitude monotonically decreasing to zero after a predetermined time period. Finally, the calculated zero-input response is added to the decoded target signal to reconstruct the overlap-add target signal.

Claims

1. A method for low-frequency emphasizing the spectrum of a sound signal transformed in a frequency domain and comprising transform coefficients grouped in a number of blocks, comprising:

calculating a maximum energy for one block having a position index;

calculating a factor for each block having a position index smaller than the position index of the block with maximum energy, the calculation of a factor comprising, for each block;

computing an energy of the block; and

computing the factor from the calculated maximum energy and the computed energy of the block; and

for each block, determining from the factor a gain applied to the transform coefficients of the block.

2. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, wherein the transform coefficients are Fast Fourier Transform coefficients.

3. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, comprising applying an adaptive low-frequency emphasis to the spectrum of the sound signal to minimize a perceived distortion in lower frequencies of the spectrum.

4. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, comprising grouping the transform coefficients in blocks of a predetermined number of consecutive transform coefficients.

5. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, wherein:

calculating a maximum energy for one block comprises;

computing the energy of each block up to a given position in the spectrum; and

storing the energy of the block with maximum energy; and

determining a position index comprises;

storing the position index of the block with maximum energy.

6. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 5, wherein computing the energy of each block up to a given position in the spectrum comprises:

computing the energy of each block up to the first quarter of the spectrum.

7. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, wherein computing the factor for each block comprises:

computing a ratio R_m for each block with a position index m smaller than the position index of the block with maximum energy, using the relation

R_m=E_max/E_m

 

where E_max is the calculated maximum energy and E_m the computed energy for block corresponding to position index m.

8. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 7, comprising setting the ratio R_m to a predetermined value when R_m is larger than said predetermined value.

9. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 7, comprising setting the ratio R_m=R_(m−

1) when R_m>

R<sub>(m−

1)</sub>.

10. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, wherein computing the factor comprises setting the factor to a predetermined value when the factor is larger than said predetermined value.

11. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 1, wherein computing the factor comprises setting the factor for one block to the factor of the preceding block when the factor of said one block is larger than the factor of the preceding block.

12. A method for low-frequency emphasizing the spectrum of a sound signal as defined in claim 7, wherein computing the factor further comprises calculating a value (R_m)^1/4, and applying the value (R_m)^1/4 as a gain for the transform coefficient of the corresponding block.

13. A device for low-frequency emphasizing the spectrum of a sound signal transformed in a frequency domain and comprising transform coefficients grouped in a number of blocks, comprising:

means for calculating a maximum energy for one block having a position index;

means for calculating a factor for each block having a position index smaller than the position index of the block with maximum energy, the factor calculating means comprising, for each block;

means for computing an energy of the block; and

means for computing the factor from the calculated maximum energy and the computed energy of the block; and

means for determining, for each block and from the factor, a gain applied to the transform coefficients of the block.

14. A device for low-frequency emphasizing the spectrum of a sound signal transformed in a frequency domain and comprising transform coefficients grouped in a number of blocks, comprising:

a calculator of a maximum energy for one block having a position index;

a calculator of a factor for each block having a position index smaller than the position index of the block with maximum energy, wherein the factor calculator, for each block;

computes an energy of the block; and

computes the factor from the calculated maximum energy and the computed energy of the block; and

a calculator of a gain, for each block and in response to the factor, the gain being applied to the transform coefficients of the block.

15. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the transform coefficients are Fast Fourier Transform coefficients.

16. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the transform coefficients are grouped in blocks of a predetermined number of consecutive transform coefficients.

17. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the maximum energy calculator:

computes the energy of each block up to a predetermined position in the spectrum; and

comprises a store for the maximum energy; and

comprises a store for the position index of the block with maximum energy.

18. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 17, wherein the maximum energy calculator computes the energy of each block up to the first quarter of the spectrum.

19. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the factor calculator:

computes a ratio R_m for each block with a position index m smaller than the position index of the block with maximum energy, using the relation

R_m=E_max/E_m

where E_max is the calculated maximum energy and E_m the computed energy for the block corresponding to the position index m.

20. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 19, wherein the factor calculator sets the ratio R_m to a predetermined value when R_m is larger than said predetermined value.

21. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 19, wherein the factor calculator sets the ratio the ratio R_m=R_(m−

1) when R_m>

R<sub>(m−

1)</sub>.

22. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the factor calculator sets the factor to a predetermined value when the factor is larger than said predetermined value.

23. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 14, wherein the factor calculator sets the factor for one block to the factor of the preceding block when the factor of said one block is larger than the factor of the preceding block.

24. A device for low-frequency emphasizing the spectrum of a sound signal as defined in claim 19, wherein:

the factor calculator computes a value (R_m)^1/4; and

the gain calculator applies the value (R_m)^1/4 as a gain for the transform coefficient of the corresponding block.

25. A method for processing a received, coded sound signal, comprising:

extracting coding parameters from the received, coded sound signal, the extracted coding parameters including transform coefficients of a frequency transform of said sound signal, wherein the transform coefficients are grouped in a number of blocks and are low-frequency emphasized using following steps;

(i) calculating a maximum energy for one block having a position index;

(ii) calculating a factor for each block having a position index smaller than the position index of the block with maximum energy, the calculation of a factor comprising, for each block;

computing an energy of the block; and

computing the factor from the calculated maximum energy and the computed energy of the block; and

(iii) for each block, determining from the factor a gain applied to the transform coefficients of the block; and

processing the extracted coding parameters to synthesize the sound signal; and

processing the extracted coding parameters comprising low-frequency de-emphasizing the low-frequency emphasized transform coefficients.

26. A method for processing a received, coded sound signal as defined in claim 25, wherein:

extracting coding parameters comprises dividing the low-frequency emphasized transform coefficients into a number K of blocks of transform coefficients; and

low-frequency de-emphasizing the low-frequency emphasized transform coefficients comprises scaling the transform coefficients of at least a portion of the K blocks to cancel the low-frequency emphasis of the transform coefficients.

27. A method for processing a received, coded sound signal as defined in claim 26, wherein:

low-frequency de-emphasizing the low-frequency emphasized transform coefficients comprises scaling the transform coefficients of the first K/s blocks of said K blocks of transform coefficients, s being an integer.

28. A method for processing a received, coded sound signal as defined in claim 27, wherein scaling the transform coefficients comprises:

computing the energy ε

_k of each of the K blocks of transform coefficients;

computing the maximum energy ε

_max of one block amongst the first K/s blocks; and

computing for each of the first K/s blocks a factor fac_k; and

scaling the transform coefficients of each of the first K/s blocks using the factor fac_k of the corresponding block.

29. A method for processing a received, coded sound signal as defined in claim 28, wherein computing for each of the first K/s blocks, up to a position index of the block with maximum energy, a factor fac_k comprises using the following expressions:

fac₀=max((ε

₀/ε

_max)^0.5, 0.1)

fac_k=max((ε

_k/ε

_max)^0.5, fac<sub>k−

1</sub>) for k=1, . . . , K/s−

1, where ε

_k is the energy of the block with index k.

30. A decoder for processing a received, coded sound signal, comprising:

an input decoder portion supplied with the received, coded sound signal and implementing an extractor of coding parameters from the received, coded sound signal, the extracted coding parameters including transform coefficients of a frequency transform of said sound signal, wherein the transform coefficients are low-frequency emphasized using a device for low-frequency emphasizing the spectrum of the sound signal transformed in a frequency domain and comprising transform coefficients grouped in a number of blocks, the device including (i) a calculator of a maximum energy for one block having a position index;

(ii) a calculator of a factor for each block having a position index smaller than the position index of the block with maximum energy, wherein the factor calculator, for each block;

(a) computes an energy of the block; and

(b) computes the factor from the calculated maximum energy and the computed energy of the block; and

(iii) a calculator of a gain, for each block and in response to the factor, the gain being applied to the transform coefficients of the block; and

a processor of the extracted coding parameters to synthesize the sound signal, said processor comprising a low-frequency de-emphasis module supplied with the low-frequency emphasized transform coefficients.

31. A decoder as defined in claim 30, wherein:

the extractor divides the low-frequency emphasized transform coefficients into a number K of blocks of transform coefficients; and

the low-frequency de-emphasis module scales the transform coefficients of at least a portion of the K blocks to cancel the low-frequency emphasis of the transform coefficients.

32. A decoder as defined in claim 31, wherein:

the low-frequency de-emphasis module scales the transform coefficients of the first K/s blocks of said K blocks of transform coefficients, s being an integer.

33. A decoder as defined in claim 32, wherein the low-frequency de-emphasis module:

computes the energy ε

_k of each of the K/s blocks of transform coefficients;

computes the maximum energy ε

_max of one block amongst the first K/s blocks; and

computes for each of the first K/s blocks a factor fac_k; and

scales the transform coefficients of each of the first K/s blocks using the factor fac_k of the corresponding block.

34. A decoder as defined in claim 33, wherein the low-frequency de-emphasis module calculates the factor fac_k using the following expressions:

fac₀=max((ε

₀/ε

_max)^0.5, 0.1)
fac_k=max((ε

_k/ε

_max)^0.5, fac<sub>k−

1</sub>) for k=1, . . . , K/s−

1,

where ε

_k is the energy of the block with index k.

35-92. -92. (canceled)