Audio signal bandwidth extension in CELP-based speech coder

1. A method for decoding an audio signal having an audio bandwidth extending beyond an audio bandwidth of a CELP excitation signal in an audio decoder including a CELP-based decoder element, the method comprising:

obtaining a second excitation signal having an audio bandwidth extending beyond the audio bandwidth of the CELP excitation signal;

obtaining a set of signals by filtering the second excitation signal with a set of bandpass filters;

scaling the set of signals using a set of energy-based parameters;

obtaining a composite output signal by combining the scaled set of signals with a signal based on the audio signal decoded by the CELP-based decoder element; and

scaling the set of signals based on energy at an output of the set of bandpass filters in the audio decoder;

wherein the energy at the output of the set of bandpass filters in the audio decoder is determined by an energy measurement interval based on a pitch period of the CELP-based decoder element; and

wherein the energy measurement interval, given by I e , is related to the pitch period, T, of the CELP-based decoder element and is dependent upon a level of voicing, V, estimated in the decoder by the following equations:

I

e

=

{

LT

;

V

≥

0.7

S

;

V

<

0.7

where S is a fixed number of samples that correspond to a speech synthesis interval and L is an up-sampling factor.

2. The method of claim 1 further comprising decoding the audio signal with the CELP-based decoder element while obtaining the second excitation signal and while obtaining the set of signals.

3. The method of claim 2 wherein the composite output signal includes a bandwidth portion that extends beyond a bandwidth of the CELP excitation signal.

4. The method of claim 1 further comprising:

obtaining an up-sampled CELP excitation signal based on the CELP excitation signal; and

obtaining the second excitation signal from the up-sampled CELP excitation signal.

5. The method of claim 1 wherein the filtering performed by the set of bandpass filters in the audio decoder includes combining outputs of a set of complementary all-pass filters.

6. The method of claim 1 wherein the filtering performed by the set of bandpass filters includes filtering by a wide bandpass filter.

7. The method of claim 4 wherein the filtering performed by the set of bandpass filters includes filtering by a set of complementary all-pass filters.

8. The method of claim 1 wherein the filtering performed by the set of bandpass filters in the audio decoder corresponds to an equivalent process applied to a sub-band of an input audio signal at the encoder.

9. The method of claim 1 wherein the filtering performed by the set of bandpass filters in the audio decoder corresponds to an equivalent bandpass filtering process applied to the input audio signal at an encoder.

10. The method of claim 1 wherein the set of energy-based parameters used at the decoder are representative of a process of bandpass filtering an input audio signal at the encoder and wherein the bandpass filtering process performed at the encoder is equivalent to the bandpass filtering of the second excitation signal at the decoder.

11. The method of claim 1 further comprising extending the audio bandwidth of the second excitation signal beyond the audio bandwidth of the CELP excitation signal by applying a non-linear operation to a precursor of the second excitation signal.