Backward Adaptive Lattice and Transversal Predictors in ADPCM

Four different backward adaptive predictors and a fixed predictor are compared for use in an adaptive differential pulse code modulation (ADPCM) system for coding speech at 16 kilobits/second (kbits/s). For noise-free channels, the four adaptive predictors, a least squares lattice, a least mean square lattice, a Kalman transversal form, and a gradient transversal form, all exceed the fixed predictor performance as well as the performance of a continuously variable slope delta (CVSD) modulation system. For bit error rates (BER's) of 10-3or greater, the transversal predictor performance falls below that of the fixed predictor and CVSD; however, the lattice structures maintain their performance advantage. The least squares lattice predictor has the best objective and subjective performance for both noiseless and noisy channels. All systems perform poorly for a BER of 10-2. To extend the performance of ADPCM with a least squares lattice predictor down to a BER of 10-2, the sampling rate is reduced and a selective coding scheme is devised. The resulting ADPCM system maintains excellent performance through a BER of 10-2and outperforms CVSD for noise-free and noisy channels. The dynamic range, tandeming performance, and behavior for noisy inputs for the ADPCM system and CVSD are investigated.