[LAD] Does noiseshaping affect quantisation noise?

[Philipp Überbacher]

Excerpts from fons's message of 2010-06-07 23:10:27 +0200:
> On Mon, Jun 07, 2010 at 10:41:07PM +0200, Philipp wrote:
> 
> > This is probably a stupid question.
> 
> Not stupid, but maybe worded in a way that makes
> answering it quite impossible.

You managed anyway, thanks ;)

> > My guess is that quantisation noise is only something present between
> > the input signal and its digital representation, and hence no change of
> > the digital representations can do anything about it.
> 
> Noise shaping and dithering make sense only for 16 bit
> or lower.
> 
> For input (A/D conversion), if your converter is only 16 bit
> then very probably the analog part isn't really high quality,
> so even in that case analog noise will probably dominate any
> quantisation noise, making the latter irrelevant. If your A/D
> converter is 24 bit, then analog noise will always dominate,
> so again quantisation noise is irrelevant.
> 
> The only case that remains is when a digital signal is converted
> to analog using a 16 bit D/A, or converted to 16 bit digital, e.g.
> for CD.
> 
> The purpose of dithering in that case is to convert systematic
> quantisation errors (i.e. errors that would be correlated with
> the signal, and therefore appear as distortion and not as noise)
> to noise. In its simplest form this is done by adding noise,
> resulting in a S/N ration that would be 3 dB worse than without
> dithering. This completely removes any correlation between signal
> and error. 

So the actual problem isn't the noise but its correlation with the signal?

> Noise shaping and error feedback are used to avoid that S/N
> ratio degradation. It works by moving most of the noise energy
> to frequency regions where it matters less.  
> 
> You can see some examples of this here:
> 
> <http://www.kokkinizita.net/linuxaudio/dithering.html>
> 
> The last one (using noise shaping) has the worst S/N
> ratio if you measure it without any psychoacoustic
> weighting. But it will sound the best.

I'm a bit curious about the first graph. The actual signal is the ~1kHz
one, but what are all the other 'spikes'?


-- 
Regards,
Philipp

--
"Wir stehen selbst enttäuscht und sehn betroffen / Den Vorhang zu und alle Fragen offen." Bertolt Brecht, Der gute Mensch von Sezuan