linuxaudio.org

On Wed, Oct 18, 2006 at 10:45:02AM +0100, Steve Harris wrote:

quoted text
> > 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