Single-chip per channel codec with filters utilizing Δ-Σ modulation

Abstract

A single-chip per channel codec with filters, fabricated using a single poly-Si NMOS technology, is discussed. In the encoder, the analog signal is converted to a 2.048 M samples/s digital signal by a /spl Delta/-/spl Sigma/ modulator. Filtering necessary for the sampling rate 8 k sample/s and compression by the /spl mu/255 law are performed digitally. In the decoder, the 8 k samples/s PCM is successively resampled and converted into the 2.048 M samples/s /spl Delta/-/spl Sigma/ signal, which is then decoded by a /spl Delta/-/spl Sigma/ demodulator. All the high-frequency images, which appear around multiples of 8 kHz, are removed by digital filters. The chip has continuous-signal antialiasing and smoothing filters for the 2.048 Samples/s sampling rate. It also has reference voltage generators for /spl Delta/-/spl Sigma/ modulation/demodulation. Some of the observed characteristics are given. The NMOS /spl Delta/-/spl Sigma/ modulator requires only two on-chip matched capacitors as precision components, and does not require a linear amplifier. A deliberate quantization step imbalance is introduced to allow a low sampling rate. The main band limiting for the 8 k samples/s is done by the recursive filter. This is realized with the serial-parallel pipeline multiplier (SPPM) in four-phase logic. The whole system is integrated on a 296 mil/spl times/342 mil chip.