On Tue, 2011-01-11 at 13:33 -0500, Mathieu Bouchard wrote:
On Mon, 10 Jan 2011, Roman Haefeli wrote:
Assuming that the more compression is applied, the more the RMS amplitude [1] approaches the Peak amplitude [2] of an audio signal,
Why do you assume that ? Let's say I take a signal and divide it by its recent peak volume. The output of [osc~] will stay unchanged. A signal made of plenty of sharp spikes will have a much lower RMS/peak ratio and still be unchanged.
Are you confusing this with waveshaping such as [expr tanh($v1)] ? It may be a special case of compression, but is not what is usually meant by that.
I hope I'm not confusing dynamic range compression with wave shaping. Actually, depending on the compressor settings (short attack times, etc.) a dynamic compressor indeed acts a bit as a wave shaper.
You mention constant oscillator signals as an example. On those a compressor wouldn't have (or only little, I should say in order to not contradict what I said above) any effect. Instead of constant amplitude signals think of signal with ever changing amplitudes which I believe what we call music normally belongs to, especially acoustic (as opposed to synthesized) music. Since compression/limiting allows to increase the RMS value of a signal without amplifying the peaks, I think compression decreases the difference between RMS amplitude and peak amplitude.
The problem with calculating an average with peak amplitude is that peaks by definition occur only at certain points in time. In order to get an average you need a constant signal which is constructed by holding a reached peak value for a certain time and the slowly lowering the amplitude until a new peak value appears. Assuming that hold time usually is longer than one period of the measured signal, this would again decrease the difference between average peak amplitude and average RMS amplitude. Does that make sense?
Roman