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