A vocoder operating at a variety of low data rates has been implemented by software emulation. It uses Vector Quantisation (VQ), Variable Frame Rate Coding (VFR) with infil by replication and interpolation, and analysis by synthesis with a formant synthesis model. Experiments with a large talker-dependent codebook gives intelligible speech at 800 bps in context, and similar performance is expected with a talker-independent codebook. Intelligible speech at 600 bps is expected to be possible with further work. At 480 bps the speech is too slurred and further work using the same infil methods is not expected to give sufficient improvement.
Keywords: speech coding, vocoder, formant vocoder, very low rate vocoder.
Bibliographic reference. Sedgwick, Nigel (1993): "Emulation of a formant vocoder at 600 and 800 bps", In EUROSPEECH'93, 523-526.