Adaptive sigma-delta modulation with one-bit quantization

A method for improving the signal-to-noise ratio (SNR) of sigma-delta modulators with one-bit quantization is presented. The two-level feedback signal of a standard sigma-delta modulator is replaced by a multilevel signal, which is a superposition of two parts. One part s(n) represents a rough estimate of the instantaneous amplitude of the input signal (prediction signal), and the other yb(n) is the sign of the quantizer output, multiplied with constant b. Compared to a nonadaptive modulator, the amplitude of yb(n) is reduced. Therefore, less noise power is introduced in the quantizer, and the SNR is considerably enhanced. Signal s(n) is derived numerically from the quantizer output yo(n) according to a particular adaptation algorithm. Except for the de-level of s(n), sequence yo(n) contains the full digital information of the modulator input signal. From yo(n), a digital multilevel sequence omega(0)(n) can be calculated, which represents the digital modulator output. The price paid for the improved SNR is a moderate slew rate limitation of the input signal. The approach is basically suited for a wide class of sigma-delta modulators. sere, simulation results and an example for a practical implementation of an adaptive sigma-delta modulator of first order are presented.