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 y_b(n) is the sign of the quantizer output, multiplied with constant b. Compared to a nonadaptive modulator, the amplitude of y_b(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 y₀(n) according to a particular adaptation algorithm. Except for the DC-level of s(n), sequence y₀ (n) contains the full digital information of the modulator input signal. From y₀(n), a digital multilevel sequence w₀ (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. Here, simulation results and an example for a practical implementation of an adaptive sigma-delta modulator of first order are presented