A variable bit rate operation of LD-CELP mainly for VBD applications in DCME

This paper describes the candidate algorithm for Annex J of ITU-T Rec. G.728. The algorithm achieves compression and near transparent transmission of VBD signals such as V.34 @ 19200 bps (and up to 21600 bps under moderate analog impairments conditions) using a 40 kbit/s transmission rate. Additionally the algorithm provides toll speech quality and smooth mode-switching (speech-to-data/data-to-speech) during voice and signaling signals. The main novelty of the algorithm is the application of trellis coded quantization (TCQ) approach to codebook search that results in sufficient quantization gain, lower complexity (than LD-CELP), and maintains the same algorithmic delay of 5 samples (0.625 msec).