Sub-band coding has been shown to provide high quality speech at rates of 16 kb/s and above. In this paper we describe how one particular subband coder has been modified to give good quality speech over a noisy channel. The coder is a dynamic bit allocation sub-band coder of the type first proposed by Ramstad in 1982. It is structured in such a way that the relative importance of all bits is established as a byproduct of the dynamic bit allocation. It is shown that there is a difference in error sensitivity of four orders of magnitude between the most and the least important bits of the bit stream on an average. A very flexible unequal error protection scheme is used to match the error sensitivity of the sub-band coder bitstream. This is accomphshed using the concept of rate compatible punctured convolutional coding. These trellis codes are decoded by means of the Viterbi algorithm using soft decisions on the noisy received signal If necessary, the decoder can also include information about the time varying channel characteristics in a rather straightforward way to improve performance. All this is accomplished without any change in the encoder and the decoder strocture during changes in the level of error protection.The resulting coder is more robust to channel errors than one in which all the error protection is spread evenly among all bits or one in which all error protection is used for just a small number of important bits. The resulting coder gives robust perfonnance over a simulated noisy channel.Its nominal bit rate is 16 kb/s, with 12 kb/s assigned for speech coding and 4 kb/s assigned for error correction. The entire encoder and decoder have been implemented on a single AT&T DSP-32 digital signal processor.
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