Idle channel tone and periodic noise suppression for sigma delta modulators using high level dither.

摘要

A dither signal (18) is applied to a sigma-delta modulator (9) to remove low level tones and periodic noise in the desired passband of the modulator when no, or a very low, signal is present (idle). The dither signal is a high-level signal added to the input of a quantizer (16) in the modulator, the normalized in the AC component thereof being at least about 2<-2(N-1)> DIVIDED 12 of the square of the quantizer step, where N is the order of the modulator. No significant reduction in the dynamic range of the modulator results. The technique may also be applied to multiple order sigma-delta modulators as well as to multiple stage sigma-delta modulators. Further, the dither may be added at any point in the modulator with suitable filtering of the dither. The transfer function of the filter (111 ... 11N+1) is proportional to the noise shaping transfer function of the modulator between the point of addition of the dither and the input to the quantizer.