Quote:

Originally Posted by

**Yuri Korzunov**
1. Dithering is method of suppressing of non-linear distortions, that risen after decreasing bit depth (truncation) with/without rounding.

2. Before truncation and rounding low level noise added to signal.

3. Dither don’t work properly without rounding.

4. As result energy of non-linear-distortion «sticks» distributed into nearest frequency area. Total level noise is increased slightly, but «sticks/peaks» of non-linear distortions drown in surrounding noise.

5. Sometimes applied noise shaping (pushing noise energy to high frequencies about ultrasound) for decreasing of noise level in «more audible» lower range.

I prefer a more functional/practical description (which you may find says similar things)

1 When you reduce word length, the audio gets forced to 'the nearest voltage' that can be recorded in the new word length. Like if it's one bit, your output voltage can only be ON or OFF, if it's two bits you get to add HALF and so on. This is coming from a continuously varying voltage (or as near as you can get it)

2 If you add noise just right, this output will kinda sputter back and forth at the speed of the sampling rate. The result is, it averages out to be similar to what the underlying voltage ought to be. Any given moment is still going to be ON or OFF, but if it's dithered, it will average out to be exactly halfway. Or, if you started with 3/4 voltage, then ON will be picked 3/4 of the time and OFF 1/4 of the time.

3 If you didn't dither, the signal tries to follow the desired curve but it fails. It gets stuck on 'the nearest voltage' and it's really easy to hear the presence of the resulting artifacts, if you're listening for the right thing. There's no one specific frequency that's wrong, the whole sound gets garbaged up and can't be put right. Dithering completely stops this happening because the signal's never, ever stuck on 'nearest voltage' anymore. It's always bouncing around among the nearest possible values, so it's always causing the impression of an output between them.

4 Noise shaping is if you keep track of how wrong you were, and try to fix it faster than it would be fixed by just waiting. Ordinary dither (TPDF) will return a perfect result if you filter away all the noise, but we like hearing higher frequencies. Noise shaping can force things to sound quieter noise-wise, but it can only do it by adding energy (typically way up top). Adding extra noise isn't good, but if it's right up near the limits of digital to reproduce (called the Nyquist frequency), often the digital playback will try to filter out that noise (it will never try to filter out midrange or ordinary treble noise unless you specifically ask it to). So, noise shaping is really dependent on the playback system's filters and treble roll-off.

It's not really about redistributing 'sticks' of digital artifacts (that's more commonly seen in aliasing, which is a whole separate issue!

), it's about avoiding any places where the signal wants to go one way, and the output file says 'no, we're gonna stay on this value for a while, even though it's wrong'. 'sticks' aren't a cause, but a result of this: they're what you get when spectrum analyzing the audio. And again, they're much more of a thing when you're talking about aliasing. I'm pretty sure if you just looked at spectrum analyzer 'sticks', it would look like truncation was the winner because adding noise just adds loads more 'sticks'. But it makes it possible to keep track of where the underlying voltage was supposed to be… statistically.

Covering anything up with noise is totally not what's happening. We're arranging so that you can still hear where the voltage was supposed to go, not concealing anything. It just so happens that putting in noise, lets us do this keeping-track, and putting in not enough noise or the wrong sort (or too much!) fails to let us keep track of where the voltage was. TPDF is exactly two noise sources of exactly one bit in size, so any TPDF variation gives us this statistical result where we can hear 'what was supposed to happen'.