[linux-audio-dev] Paper on dynamic range compression

[John Rigg]

On Wed, Oct 18, 2006 at 10:45:02AM +0100, Steve Harris wrote:
> > True, but if the audio signal contains significant HF energy near
> > the band limit, it doesn't take a very fast gain change to push it
> > past that. Bear in mind that the ear is _very_ sensitive to aliasing
> > artifacts, so `significant' can be a very small amount.
> 
> These are aliasing artifacts in the sidechain though, right? So they will
> show up as modulations in the output, rather than directly audible
> aliasing.

I was referring to aliasing in the compressed audio. Harmonic distortion
is introduced during the attack (and decay, but usually to a lesser extent
because it's slower).

Consider a sine wave audio input. In the attack, the gain is decreasing,
so the rising parts of each input half-cycle are reduced in slope, and
the falling parts are increased in slope. The audio waveform starts to take
on a sawtooth shape if the rate of gain change is high enough.

This is a form of harmonic distortion and happens with any compressor,
analog or digital. In a digital compressor aliasing will occur if the sample
frequency is insufficient to deal with the introduced harmonics (which
have been added _inside_ the digital domain remember, so no amount
of anti-aliasing filtering on an input ADC will have any effect on it).
It will only last for the duration of the attack/decay, but that doesn't
make it inaudible. A small amount of harmonic distortion on its own is not
objectionable, but when it is accompanied by aliasing it's bad news.

John