[PD] change in compression detection

Roman Haefeli reduzent at gmail.com
Tue Jan 11 23:41:29 CET 2011

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.
> http://en.wikipedia.org/wiki/Dynamic_range_compression

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?


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