[sdiy] simple "level separator"?

[Magnus Danielson]

From: VINTVOLT <VINTVOLT at terra.es>
Subject: [sdiy] simple "level separator"?
Date: Wed, 30 Apr 2003 13:49:17 +0200

> Hi,
> I'm in need of a simple circuit that would be used to separate high and 
> low
> levels from a signal. It's transfer function should be like this:
> 
> 
>     ^out
>     |         /
>     |        /
>     |       /1:1
>     |      |
>     |      |
>     |      |
> sym  ======---------->in
>           thr
> 
> 
> where the threshold should be adjustable, so very similar to a noise 
> gate,
> but without any time constants. It should be instantaneous.
> I searched the web, but could not find any simple circit. I thought 
> about
> using a fet in the audio-path with the gate connected to the output of a
> comparator. would that work?

Well, I think you might get quicker results by using a analog switch like the
4066 or similar. I think that is the quickest path and will probably work
satisfactory to you for doing that non-linear curve.

However, if you are considering a noise-gate curcuit, the case is a little
more complex, since then it is not the actual voltage-level at one particular
time you want to gate out, but rather when the amplitude of the signal has gone
below a certain point. You could use about the same setup, but a VCA probably
a better choice then, and you need some sort of amplitude sense, like a
full-wave rectifier followed by a lowpass filter (usually adjustable) and a
comparator such that when the detected level is above the threshold level you
have unity gain, and when it is below you have a high damping. You might also
consider to have some form of "Attack" and "Release" shaping setup to shape
the comparators sharp edges before reaching the VCA. Toss a input gain lever
and you have a fairly complete noise-gate.

Actually, this point to one of the type of curcuits which is not very often
discussed here, namely dynamic processors (expanders, noise gates, compressors,
duck processors, deessers, etc. etc.).

Cheers,
Magnus