Method and system for encoding left and right channels of a stereo sound signal selecting between two and four sub-frames models depending on the bit budget

1. A stereo sound encoding method for encoding left and right channels of a stereo sound signal, comprising:

down mixing the left and right channels of the stereo sound signal to produce primary and secondary channels;

encoding the primary channel and encoding the secondary channel, wherein encoding the primary channel and encoding the secondary channel comprise determining a first bit budget to encode the primary channel and a second bit budget to encode the secondary channel;

wherein:

if the second bit budget is sufficient, the secondary channel is encoded using a four sub-frames model; and

if the second bit budget is insufficient for using the four sub-frames model, the secondary channel is encoded using a two sub-frames model;

wherein encoding the primary channel comprises producing primary channel coding parameters, and encoding the secondary channel comprises:

producing secondary channel coding parameters;

determining a bit budget required to encode, in a current frame, secondary channel coding parameters including (a) LP filter coefficients and/or (b) pitch information, and gains, that are not re-used from the primary channel encoding;

determining if a remaining bit budget allows to quantize, in the current frame, four algebraic codebooks or only two algebraic codebooks;

doubling a sub-frame length when the two sub-frames model is used; and

interpolating the LP filter coefficients of the primary channel, when re-used, to adapt said primary channel LP filter coefficients by taking into account the two sub-frames model.

2. The method as defined in claim 1 , wherein down mixing the left and right channels of the stereo sound signal comprises time domain down mixing the left and right channels of the stereo sound signal to produce the primary and secondary channels.

3. The method as defined in claim 1 , comprising selecting between time domain down mixing and frequency domain down mixing.

4. The method as defined in claim 1 , comprising:

converting the left and right channels from time domain to frequency domain; and

frequency domain down mixing the frequency domain left and right channels to produce frequency domain primary and secondary channels.

5. The method as defined in claim 4 , comprising:

converting the frequency domain primary and secondary channels back to time domain for encoding by a time domain encoder.

6. A stereo sound encoding system for encoding left and right channels of a stereo sound signal, comprising:

at least one processor; and

a memory coupled to the processor and comprising non-transitory instructions that when executed cause the processor to implement:

a down mixer of the left and right channels of the stereo sound signal to produce primary and secondary channels;

an encoder of the primary channel and an encoder of the secondary channel;

a bit allocation estimator of a first bit budget to encode the primary channel and a second bit budget to encode the secondary channel; and

a decision module to select, if the second bit budget is sufficient, encoding of the secondary channel using a four sub-frames model, and, if the second bit budget is insufficient for using the four sub-frames model, encoding of the secondary channel using a two sub-frames model;

wherein the primary channel encoder produces primary channel coding parameters, and

wherein the secondary channel encoder:

produces secondary channel coding parameters;

determines a bit budget required to encode, in a current frame, secondary channel coding parameters including (a) LP filter coefficients and/or (b) pitch information, and gains, that are not re-used from the primary channel encoding;

determines if a remaining bit budget allows to quantize, in the current frame, four algebraic codebooks or only two algebraic codebooks;

doubles a sub-frame length when the two sub-frames model is used; and

interpolates the LP filter coefficients of the primary channel, when re-used, to adapt said primary channel LP filter coefficients by taking into account the two sub-frames model.

7. The system as defined in claim 6 , wherein the down mixer is a time domain down mixer of the left and right channels of the stereo sound signal to produce the primary and secondary channels.

8. The system as defined in claim 6 , wherein the down channel mixer selects between time domain down mixing and frequency domain down mixing.

9. The system as defined in claim 6 , comprising:

a converter of the left and right channels from time domain to frequency domain;

wherein the down channel mixer mixes the frequency domain left and right channels to produce frequency domain primary and secondary channels.

10. The system as defined in claim 9 , comprising:

a converter of the frequency domain primary and secondary channels back to time domain for encoding by a time domain encoder.

11. A stereo sound encoding system for encoding left and right channels of a stereo sound signal, comprising:

a down mixer of the left and right channels of the stereo sound signal to produce primary and secondary channels;

an encoder of the primary channel and an encoder of the secondary channel;

a bit allocation estimator of a first bit budget to encode the primary channel and a second bit budget to encode the secondary channel; and

a decision module to select, if the second bit budget is sufficient, encoding of the secondary channel using a four sub-frames model, and, if the second bit budget is insufficient for using the four sub-frames model, encoding of the secondary channel using a two sub-frames model;

wherein the primary channel encoder produces primary channel coding parameters, and

wherein the secondary channel encoder:

produces secondary channel coding parameters;

determines a bit budget required to encode, in a current frame, secondary channel coding parameters including (a) LP filter coefficients and/or (b) pitch information, and gains, that are not re-used from the primary channel encoding;

determines if a remaining bit budget allows to quantize, in the current frame, four algebraic codebooks or only two algebraic codebooks;

doubles a sub-frame length when the two sub-frames model is used; and

interpolates the LP filter coefficients of the primary channel, when re-used, to adapt said primary channel LP filter coefficients by taking into account the two sub-frames model.

12. A stereo sound encoding system for encoding left and right channels of a stereo sound signal, comprising:

at least one processor; and

a memory coupled to the processor and comprising non-transitory instructions that when executed cause the processor to:

down mix the left and right channels of the stereo sound signal to produce primary and secondary channels;

encode the primary channel and encode the secondary channel;

estimate a first bit budget to encode the primary channel and a second bit budget to encode the secondary channel; and

select, if the second bit budget is sufficient, encoding of the secondary channel using a four sub-frames model, and, if the second bit budget is insufficient for using the four sub-frames model, encoding of the secondary channel using a two sub-frames model;

wherein a primary channel encoder produces primary channel coding parameters, and

wherein a secondary channel encoder:

produces secondary channel coding parameters;

determines a bit budget required to encode, in a current frame, secondary channel coding parameters including (a) LP filter coefficients and/or (b) pitch information, and gains, that are not re-used from the primary channel encoding;

determines if a remaining bit budget allows to quantize, in the current frame, four algebraic codebooks or only two algebraic codebooks;

doubles a sub-frame length when the two sub-frames model is used; and

interpolates the LP filter coefficients of the primary channel, when re-used, to adapt said primary channel LP filter coefficients by taking into account the two sub-frames model.

13. A processor-readable memory comprising non-transitory instructions that, when executed, cause a processor to implement the operations of the method as recited in claim 1 .