[PD] editing pd silence - dither

Marvin Humphrey marvin at rectangular.com
Sun Oct 23 23:39:42 CEST 2011

On Sun, Oct 23, 2011 at 04:04:58PM -0400, Mathieu Bouchard wrote:
> I understand all of that already, but my impression is that it's more 
> like making a 24-bit gradient use dithering so that it looks more like a 
> 48-bit gradient. Would it make a perceptual improvement if you did so ?

No, of course not -- such a difference, though measurable, would fall below a
human's perceptual threshold.  But truncate over and over again, and
eventually, the error accumulates and rises above threshold.

It's hard to hear the first pass of a perceptual codec.  But run audio through
a codec multiple times, and you get a "cliff edge" effect: nothing...
nothing...  nothing... oh wow now I hear it.

Truncation distortion, being enharmonic, is pretty nasty.   It's not like
analog tape overload.  A little truncation distortion goes a long way, and
unless you are going for glitch, best practice to keep it at bay by managing
gain structure wisely and dithering when appropriate.

> E.g. if you have a fully 16-bit-digital volume control on an amp, and the 
> amp has a big volume range and you only use the quiet range, the 
> effective number of bits can down a lot.

It's also not uncommon to capture a killer take under less than ideal
recording conditions -- including input gain structure.

It's worthwhile for developers of audio software to think about such things,
so that downstream users benefit from the additional headroom.

Marvin Humphrey

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