[sdiy] Fast envelope follower circuit needed..

Paul Perry pfperry at melbpc.org.au
Sat Mar 30 07:24:24 CET 2013


A little philosophy on envelope followers:
It's easy to see what the envelope should look like - after the original 
signal has been! But if you take a typical input signal, graph it, and then 
cover it with a card, and try to draw an envelope as you slide the card to 
the right........... you might see that is is not just 'difficult', but 
actually 'impossible' to have a circuit that behaves as you would wish.

In my experience, there is no envelope follower that suits all applications. 
In particular, percussion and guitar are very different. I don't doubt that 
Harry's circuit is ideal for a guitar.. and that if he could have got away 
with fewer components, he would have (remember, a hex guitar has SIX 
outputs!)

On the bright side, I have also found that if you are using the derived 
envelope to control a filter or VCA, the actual audio effect can be quite 
musically useful, even though the envelope itself looks really unpromising 
on the scope. In particular, full wave rectification was not much better 
than half wave.

Real problems arise when the frequencies being enveloped are very low, in 
this case the low pass filtering is in conflict with the envelope being 
derived. Again, it is a matter of knowing what the results are going to be 
with a practical envelope, rather than knocking oneself out trying to 
produce an 'ideal' envelope.

paul perry Melbourne Australia




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