18 Oct
2006
18 Oct
'06
10:02 a.m.
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.
John
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.