John Monro wrote:> Ron N. wrote: > > Yazz wrote: > >> Hopefully some of you guru's here can set me straight here. > >> > >> In an attempt to reduce quantization spurs, I have read about > >> dithering. I have seen wideband dithering and narrowband dithering. > >> All of this is done in the analog domain prior to the sampling. > >> > >> My question is: Why can we not simply add a random LSB (+/- 1) to each > >> of the samples AFTER sampling to reduce quantization spurs ? > > > > You can not regain information once you've thrown it away. > > > > If you add dither after rounding or truncating (which throws > > away any fractional information), then you just add noise. > > > > If you dither before rounding or truncation, then you spread > > out the fractional information (a +0.25 fraction could become > > from -n.75 to +n.25) into a probability distribution where > > the fractional information might be able to leak thru the > > quantization process, given a statistically large enough number > > of samples. The randomization of the dither noise actually > > prevents the information from being completely thrown away, > > but, in exchange, hides it in noise. > > > > In both cases you get noise, but in the pre-dither case, you > > get more information about the signal hidden in the noise. > > A patterned dither, or fraction saving process, instead of > > a random dither, allows one to deduce even more information > > about the signal as it was prior to rounding or truncation, > > by making some of the noise more removable. > > > > > > > > IMHO. YMMV. > > An interesting and instructive answer from the information / statistics > point of view. > > Another way of looking at the question is to examine what happens when a > repetitive, constant-amplitude test signal is applied. If its frequency > is a simple fraction of the sample frequency, the quantisation noise is > also repetitive, and repeats over a few cycles of the test signal. In > the frequency domain the effect is that 'spurs' appear, and these are > harmonically related to the test signal. If dithering is now applied to > the test signal it disrupts the regular nature of the quantisation > noise, spreading out the spurs in the frequency domain. > > This brings me to my main point. Is dithering have any other use apart > from improving the test-signal results? It seems to me that dithering > does not provide any benefit at all when 'real-life' audio or video > signals are being sampled, and I would be interested if anyone disagrees > with this.In real life, the ears and eyes of mammals use a process very much like dithering to improve sensitivity of certain kinds of inputs. (Eyeballs, unlike cameras with very slow film, don't work very well when clamped and aimed at still target images). IMHO. YMMV. -- rhn A.T nicholson d.0.t C-o-M