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
