Consider a requantization system with noise shaping and dithering.
The question is what to do when the sum of the signal, the noise
feedback and dither exceeds the range of the output quantizer.
A differentiator of the Nth order used as a noise shaping filter has the
max. feedback of ~2^N. So it is quite likely that the quantizer will run
out of range at or near the peak values of the input signal.
I can see the following approaches to this problem:
1) Limit the input signal so the requantizer will never run out of
range. This works, however it reduces the available dynamic range. The
reduction can be substantial if the noise shaping of high order is used.
2) Limit the sum of signal, dither and noise feedback to +/-max of the
output quantizer. Calculate the feedback taking this limiting into the
account. The result is horrid; error windup.
3) Limit the sum to +/- max. output, set the feedback to +/- 1 lsb
accordingly.
What do you think is right approach?
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
