An advanced ADPCM system with a variable sampling rate, multiple quantization and error protection
Ryuji Kohno, Hideki Imai, Mitsutoshi Hatori, Subbarayan Pasupathy · 1st IASTED International Symposium on Signal Processing and its Applications · 1987
This paper proposes and investigates an advanced ADPCM system with a multi-sampler and quantizer (ADPCM-MSQ) for speech and data transmission. The sampling rate and the number of quantization levels are adaptively changed in order to reduce average transmission bit rate to less than l6kb/s while maintaining toll quality. A/D converters with different sampling rates and ADPCM coders with different levels of quantization are used in parallel. The output of an ADPCM coder which has a SNR greater than a desired threshold within a frame and achieves the lowest transmission bit rate, is selected frame by frame. In the receiver, an ADPCM decoder and a D/A converter are switched according to the supplemental information from the transmitter. A scheme to adaptively vary the frame length and a scheme to omit transmission of the supplement information are proposed. Moreover, in order to improve the robustness of ADPCM-MSQ against bit errors in noisy channels such as in mobile radio, an error protection scheme which maintains I6kb/s constant transmission rate by using error-correcting codes with different code rates is described. Computer simulations show that the ADPCM-MSQ system can improve the average SNR and transmission rate over a conventional ADPCM-MQ system for any frame length, and that an error-protected 16kbps system can achieve a average SNR of 12dB for a channel BER=10-2.