[sdiy] what is the amplitude envelope of a signal

[Richard Wentk]

At 15:26 29/09/2003 +0200, Czech Martin wrote:
>Yes, that is what I also tryed.
>The peaks of a rectified signal seems easy,
>but if you define them as local maxima
>things get messy when noise comes into play.
>Connecting such peaks will not look like the envelope, but
>more like the rectified signal.
>So, only the larger peaks should be used, but what does
>that mean in a formal way?
>
>The filtering approach must guarantee to have not too much
>lag, and especially an unsymmetrical lag does not make sense,
>since the incomming signal could start slow and end very suddenly.
>
>But I will dig out this EN paper.
>Thanx!
>
>As I said: what seemed to be so easy turns out to be very difficult!

Traditionally you just put a manually controllable time constant after the 
rectifier. That way you can tune the response to suit the application. E.g. 
short for drums, longer for things like guitars and speech. A log scale of 
1ms to 1s is a reasonable range. If you know the approximate envelope (e.g. 
drums) than an assymetric time constant *would* make sense, and would be a 
nice improvement on most env followers. I've never seen one like that and 
it would make a nice little project.

If you're getting signal breakthrough it means either there's some signal 
breakthrough in the circuit due to a design or construction fault, or the 
filtering isn't working properly, or both. The output should be a slow *DC* 
variation. There should be no effective signal content above 100Hz at the 
very maximum, and 10Hz would be more typical for many applications.

The only way to create a perfect envelope follower is to do it digitally in 
non-real time, where you can go forward and backtrack with an adaptive 
time-constant and some clever logic. This is more trouble than it's worth 
for most jobs, so any real analogue circuit (that isn't impossibly complex) 
will compromise on perfection. But it's not so terribly hard to get 
something that's good enough for most applications, like swinging a filter 
or mixing in another signal that follows the envelope of your input.

There are circuits that will approximate RMS level to good accuracy, and 
I'm sure a google search will show a few. But that's usually more 
complexity and accuracy than you need in a synth.

Richard