Sequential noise spectral shaping in ADPCM

The output speech from a fixed-tap differential pulse code modulation system with adaptive quantization and adaptive noise spectral shaping (NSS) is compared for block-adaptive and sequentially adaptive NSS filters. Block-adaptive systems incorporate a delay which can build up in analog communications systems and cause echoing problems. The buildup of delay can be eliminated by implementing the sequentially adaptive Kalman algorithm in the NSS filter. Simulations are performed for 4-, 8-, and 16-level quantizers with fourth- and ninth-order Kalman adaptation of the NSS filter. A block-adaptive system is implemented as a reference. Subjective listening tests and spectrograms show the Kalman algorithm to be a viable alternative to the block-adaptive algorithms. Signal-to-noise ratios are also given.