Low-delay VXC at 8 kbit/s with interframe coding

The authors present a novel version of low-delay vector excitation coding (LD-VXC) operating at 8 kb/s that provides very good speech quality with a coding delay below that of the CCITT requirement. The coder integrates techniques such as vector quantization and analysis-by-synthesis, without any excessive buffering of speech samples in the encoder, by incorporating backward adaptive linear prediction. Compared with LD-VXC at 16 kb/s which was reported earlier, this version features an increased vector dimension, closed-loop pitch prediction with a three-tap pitch filter, codebook orthogonalization techniques, and dual-mode interframe coding of the pitch trajectory. Informal listening tests demonstrate that its quality is comparable to that of VSELP operating at the same rate, while maintaining a delay constraint an order of magnitude smaller than that of VSELP