envelope follower

Audio signals change drastically and quickly. A 100 Hz sine tone goes from (absolute value) peak to peak in 5 milliseconds. An attempt to detect its peak amplitude in a lesser time interval might incorrectly evaluate a non-peak values as a peak. What we really care about is the shape of the peaks over time, so a certain amount of latency is necessary to represent that shape properly. And, if we plan to use the envelope to shape other sounds, some smoothing of the envelope is necessary to minimize the audio-rate modulation it causes when applied to other signals.

This patch is very similar to the "Simple envelope follower", with the difference that in this patch the line~ object's time of interpolation to get to a detected peak value can be different from the peakamp~ object's interval of evaluation. If, for example, the peakamp~ interval is 10 ms and the line~ ramp time is 2.5 ms, the envelope follower latency will range from only 2.5 to 12.5 ms.

An "envelope follower" provides a smoothed global representation of the extreme amplitudes of a signal. It can be as rough or as detailed as you want it to be, depending on how much of the original signal you disregard in the evaluation of the peaks. Because sound signals tend to vary in both positive and negative directions around a central 0 value, it's best to evaluate the absolute values of the samples, so that peaks in the negative direction are easily compared to peaks in the positive direction.