Speech post-processing using MDCT coefficients

1. A method of post-processing a speech signal having a high-band frequency range and a low-band frequency range to generate a post-processed speech signal, the method comprising:

applying a time-domain post-processing to the speech signal, using LPC (Linear Prediction Coding) coefficients, for the low-band frequency range of the speech signal;

applying a frequency-domain post-processing to the speech signal, using MDCT (Modified Discrete Cosine Transform) coefficients, for the high-band frequency range of the speech signal;

wherein applying the frequency-domain post-processing includes:

decoding an encoded speech signal to obtain MDCT coefficients representative of the speech signal divided into a plurality of sub-bands;

generating an envelope for each sub-band of the plurality of sub-bands as an average magnitude of the MDCT coefficients of the sub-band;

generating an envelope modification factor for each sub-band of the plurality of sub-bands using the MDCT coefficients of the sub-band;

determining a gain based on the envelope and the envelope modification factor of the sub-bands;

generating a fine structure modification factor for each MDCT coefficient in each sub-band of the plurality of sub-band using the MDCT coefficients of the sub-band;

modifying the MDCT coefficients in each sub-band by multiplying by the gain, the envelope modification factor of the sub-band and the fine structure modification factor of the MDCT coefficient of the sub-band to provide post-processed MDCT coefficients;

generating the post-processed speech signal using the post-processed MDCT coefficients; and

converting the post-processed speech signal from a digital form into an analog form using an digital-to-analog converter.

2. The method of claim 1 , wherein the envelope is defined by:

ENV

⁡

(

k

)

=

∑

i

=

0

15

⁢

⁢

Y

k

⁡

(

i

)

,

⁢

k

=

0

,

1

,

…

⁢

,

9

;

where magnitudes of the MDCT coefficients in each of the plurality of sub-bands is represented by:

Y k ( i )=| Ŷ k ( i )| k= 0, 1, . . . , 9 ;i= 0, 1, . . . , 15;

where the high-band frequency range is divided into 10 sub-bands, where each of the plurality of sub-bands includes 16 MDCT coefficients, and where the 160 MDCT coefficients are expressed as follows:

Ŷ k ( i )= Ŷ (160+ k* 16+ i ), k= 0, 1, . . . , 9; i= 0, 1, . . . , 15;

where k is a sub-band index, and i is a coefficient index within each of the plurality of sub-bands, and Ŷ(j), j=0, 1, . . . , 159 are the MDCT coefficients.

3. The method of claim 1 , wherein each sub-band of the plurality of sub-bands includes at least one harmonic peak.

4. The method of claim 1 , wherein the generating of the envelope modification factor further uses the envelope.

5. The method of claim 1 , wherein the generating of the envelope modification factor further uses the maximum value of the envelope of each the sub-band of the plurality of sub-bands.

6. A speech post-processor for post-processing a speech signal having a high-band frequency range and a low-band frequency range to generate a post-processed speech signal, the speech post-processor comprising:

software and circuitry for:

applying a time-domain post-processing to the speech signal, using LPC (Linear Prediction Coding) coefficients, for the low-band frequency range of the speech signal;

applying a frequency-domain post-processing to the speech signal, using MDCT (Modified Discrete Cosine Transform) coefficients, for the high-band frequency range of the speech signal;

wherein applying the frequency-domain post-processing includes:

decoding an encoded speech signal to obtain MDCT coefficients representative of the speech signal divided into a plurality of sub-bands;

generating an envelope for each sub-band of the plurality of sub-bands as an average magnitude of the MDCT coefficients of the sub-band;

generating an envelope modification factor for each sub-band of the plurality of sub-bands using the MDCT coefficients of the sub-band;

determining a gain based on the envelope and the envelope modification factor of the sub-bands;

generating a fine structure modification factor for each MDCT coefficient in each sub-band of the plurality of sub-band using the MDCT coefficients of the sub-band;

modifying the MDCT coefficients in each sub-band by multiplying by the gain, the envelope modification factor of the sub-band and the fine structure modification factor of the MDCT coefficient of the sub-band to provide post-processed MDCT coefficients;

generating the post-processed speech signal using the post-processed MDCT coefficients; and

converting the post-processed speech signal from a digital form into an analog form using an digital-to-analog converter.

7. The speech post-processor of claim 6 , wherein the envelope is defined by:

ENV

⁡

(

k

)

=

∑

i

=

0

15

⁢

⁢

Y

k

⁡

(

i

)

,

⁢

k

=

0

,

1

,

…

⁢

,

9

;

where magnitudes of the MDCT coefficients in each of the plurality of sub-bands is represented by:

Y k ( i )=| Ŷ k ( i )|k=0, 1, . . . , 9 ;i= 0, 1, . . . , 15;

where the high-band frequency range is divided into 10 sub-bands, where each of the plurality of sub-bands includes 16 MDCT coefficients, and where the 160 MDCT coefficients are expressed as follows:

Ŷ k ( i )={circumflex over ( Y )}(160 +k* 16 +i ),k=0, 1, . . . , 9 ;i= 0, 1, . . . , 15;

where k is a sub-band index, and i is a coefficient index within each of the plurality of sub-bands, and Ŷ(j), j=0, 1, . . . , 159 are the MDCT coefficients.

8. The speech post-processor of claim 6 , wherein each sub-band of the plurality of sub-bands includes at least one harmonic peak.

9. The speech post-processor of claim 6 , wherein the generating of the envelope modification factor further uses the envelope.

10. The speech post-processor of claim 6 , wherein the generating of the envelope modification factor further uses the maximum value of the envelope of each the sub-band of the plurality of sub-bands.