Method and apparatus for diversity control in multiple description voice communication
A method and apparatus for performing multiple descriptive source coding in which a plurality of homogeneous encoders are advantageously employed in combination with a corresponding plurality of advantageously substantially identical decoders. In particular, diversity is provided to the multiple encoders by modifying the quantization process in at least one of the encoders such that the modified quantization process is based at least on a quantization error resulting from the quantization process of another one of the encoders. In this manner, diversity among the multiple bit streams is obtained, and in particular, the quality of a reconstructed signal based on a combination of multiple decoded bit streams at the receiver is advantageously superior to that based on any one of the decoded bit streams. In accordance with a first illustrative embodiment of the present invention, two Pulse Code Modulation (PCM) coders are employed. In accordance with a second illustrative embodiment of the present invention, two Adaptive Differential Pulse Code Modulation (ADPCM) coders are employed. And in accordance with a third illustrative embodiment of the present invention, two Low-Delay Code Excited Linear Prediction (LD-CELP) coders are employed. In each case, diversity is ensured by an appropriate modification to the quantization process of at least one of the encoders, and the total error may be advantageously reduced when decoded bit streams from both coders are combined at the receiver.
1. A multi-descriptive encoder for generating a plurality of multi-descriptive bit streams from a single source signal, the encoder comprising:
a source signal input port for supplying the source signal;
a first coder applied to the source signal input port, the first coder for generating a first multi-descriptive bit stream from the source signal, the first coder employing a first coding algorithm which includes a first quantization process in which a first data value based on the source signal is coded with use of a corresponding first quantized data value, thereby resulting in a corresponding first quantization error representative of a difference between said first data value and said first quantized data value; and
a second coder applied to the source signal input port, the second coder for generating a second multi-descriptive bit stream from the source signal, the second coder employing a second coding algorithm which includes a second quantization process in which a second data value, based on the source signal and corresponding to said first data value, is coded with use of a corresponding second quantized data value, thereby resulting in a corresponding second quantization error representative of a difference between said second data value and said second quantized data value,
wherein said second quantized data value as produced by said second quantization process is based at least in part on said first quantization error resulting from said first quantization process,
wherein said first quantization process and said second quantization process each select said corresponding quantized data values from a single predetermined set of quantization values, and
wherein said first and second coding algorithms each comprise a code excited linear predictive coding scheme, wherein said first and second quantization processes select said corresponding quantized data values from a single predetermined set of excitation vectors, and wherein said second quantization process selects a different excitation vector than said first quantization process.
2. The encoder of claim 1 wherein said first coding algorithm and said second coding algorithm differ in at most the corresponding quantization processes included therein.
3. The encoder of claim 1 wherein said source signal comprises a speech signal.
4. The encoder of claim 1 wherein said second quantized data value as produced by said second quantization process is determined so that said second quantization error resulting from said second quantization process is such that if combined with said first quantization error resulting from said first quantization process to produce a net quantization error, said net quantization error will be less than said first quantization error.
5. The encoder of claim 1 further comprising means for modifying said first and second quantization processes in a periodic fashion, such that after a first predetermined amount of time,
(i) said first quantization process produces a subsequent first quantized data value based at least in part on a corresponding subsequent second quantization error resulting from said second quantization process, and
(ii) said second quantization process produces a subsequent second quantized data value not based on a corresponding subsequent first quantization error resulting from said first quantization process; and
after a second predetermined amount of time,
(iii) said second quantization process produces a further subsequent second quantized data value based at least in part on a corresponding further subsequent first quantization error resulting from said first quantization, process, and
(iv) said first quantization process produces a further subsequent first quantized data value not based on a corresponding further subsequent second quantization error resulting from said second quantization process.
6. The encoder of claim 5 wherein said first predetermined amount of time and said second predetermined amount of time are equal.
7. The encoder of claim 5 wherein said first and second predetermined amounts of time are based on quality levels associated with a first communications channel used for transmitting said first multi-descriptive bit stream and a second communications channel used for transmitting said second multi-descriptive bit stream, respectively.
8. The encoder of claim 7 wherein a ratio of said first predetermined amount of time divided by said second predetermined amount of time is approximately equal to a ratio of an estimated probability of a frame erasure occurring in a transmission across said second communications channel divided by an estimated probability of a frame erasure occurring in a transmission across said first communications channel.
9. A multi-descriptive decoder system for decoding a plurality of multi-descriptive bit streams each comprising a representation of a single source signal and for producing a single reconstructed source signal therefrom, the decoder system comprising:
a plurality of decoders corresponding to the plurality of multi-descriptive bit streams to be decoded, each of said decoders generating a corresponding decoded bit stream; and
a mixer for combining said decoded bit streams to produce the reconstructed source signal,
wherein the plurality of multi-descriptive bit streams have been generated by a multi-descriptive encoder which comprises,
(i) a source signal input port for supplying a single source signal,
(ii) a first coder applied to the source signal input port, the first coder for generating a first multi-descriptive bit stream from the source signal, the first coder employing a first coding algorithm which includes a first quantization process in which a first data value based on the source signal is coded with use of a corresponding first quantized data value, thereby resulting in a corresponding first quantization error representative of a difference between said first data value and said first quantized data value, and
(iii) a second coder applied to the source signal input port, the second coder for generating a second multi-descriptive bit stream from the source signal, the second coder employing a second coding algorithm which includes a second quantization process in which a second data value, based on the source signal and corresponding to said first data value, is coded with use of a corresponding second quantized data value, thereby resulting in a corresponding second quantization error representative of a difference between said second data value and said second quantized data value,
and wherein said second quantized data value as produced by said second quantization process is based at least in part on said first quantization error resulting from said first quantization process,
wherein said each of said multi-descriptive bit streams have been encoded with use of a code excited linear predictive coding scheme.
10. The decoder system of claim 9 wherein said source signal comprises a speech signal.
11. The decoder system of claim 9 wherein each of said plurality of decoders has a corresponding internal state thereof, and wherein said internal state corresponding to a first one of said plurality of decoders is updated based on said internal state corresponding to another one of said plurality of decoders when said multi-descriptive bit stream corresponding to said first one of said plurality of decoders experiences a frame erasure.