Hierarchical coding of digital audio signals

1. A method for scalar quantization-based coding of the samples of a digital audio signal, comprising:

coding the samples over a pre-determined number of bits in order to obtain a binary frame of quantization indices, the coding being carried out according to an amplitude compression law, where a pre-determined number of least significant bits are not taken into account in the compression operation for forming the binary frame of quantization indices;

storing at least a part of the least significant bits that are not taken into account in the compression operation for forming the quantization index binary frame; and

determining an enhancement bit stream comprising at least one of the stored bits.

2. The method as claimed in claim 1 , wherein the stored bits are the most significant bits amongst the bits that are not taken into account in the compression operation for forming the binary frame of quantization indices.

3. The method as claimed in claim 1 , wherein the number of bits taken into account for determining the enhancement bit stream is a function of the bit rate available during a transmission to a decoder.

4. The method as claimed in claim 1 , wherein the scalar quantization step is a quantization of the PCM type according to a logarithmic amplitude compression coding law of the A type or of the mu type in accordance with the ITU-T G.711 standard.

5. A method for decoding said method comprising:

decoding a binary frame of quantization indices comprising a pre-determined number of bits by an inverse quantization step and according to an amplitude compression law;

receiving an enhancement bit stream comprising one or more extension bits determined according to the method as claimed in claim 1 ; and

concatenating the extension bits behind the bits coming from the binary frame in order to obtain a decoded audio signal.

6. The decoding method as claimed in claim 5 , further comprising a step for adapting a rounding value according to the number of extension bits received in order to obtain the decoded audio signal.

7. An audio coder comprising:

a module for scalar quantization of the samples of a digital audio signal, the samples being coded over a pre-determined number of bits in order to obtain a binary frame of quantization indices, the coding being applied according to an amplitude compression law, a pre-determined number of least significant bits not being taken into account in the compression operation for forming the binary frame of quantization indices;

memory capable of storing at least a part of the least significant bits that are not taken into account in the compression operation for forming the quantization index binary frame; and

means for determining an enhancement bit stream comprising at least one bit thus stored.

8. An audio decoder:

said audio decoder being capable of decoding a binary frame of quantization indices comprising a pre-determined number of bits by an inverse quantization module and according to an amplitude compression law; said audio decoder comprising:

means for receiving an enhancement bit stream comprising one or more extension bits determined by a coder as claimed in claim 7 ; and

means for concatenating the extension bits behind the bits coming from the binary frame in order to obtain a decoded audio signal.

9. A non-transitory computer readable storage medium storing a computer program comprising code instructions for the implementation of the steps of the coding method as claimed in claim 1 when it is executed by a processor of a coder.

10. A non-transitory computer readable storage medium storing a computer program comprising code instructions for the implementation of the steps of the coding method as claimed in claim 5 when it is executed by a processor of a decoder.