AW: [sdiy] Dual Frequency Shifter update

[jhaible at debitel.net]

 > Even if you have a nice non-linearity which is bipolar (like direct
> compression
> without timeconstant) the distorsion it causes will probably be similar to
> what you have simulated. You can only compand a small amount with such a
> system
> before the distorsion exceeds the S/N gains that you hoped to acheive. 

Magnus, I'm completely with you.
Predistortion (before the FS, i.e. before the dome filter) causes a lot
of harmonics, which won't be harmonics after the FS process anymore,
so a static waveform won't be static after the shift process anymore
(the now non-harmonics will beat against the original signal), and I
can't see how this could possibly be made undone with another nonlinear
function.
Maybe with the predistortion before the multipliers (after the dome
filter? - Haven't thought this thru yet.)
I just say sometimes I'm blind, and as I understood Tim has a solution
that has worked in practice.

Anyway. Compander will work. I have a compander in my old FS-1.
A very simple one, too, with long release time constant.
All that I'm fighting with now is the not-so-optimal behaviour of
a NE572-based compander. I tried 1uF attack and 10uF release caps
frm the datasheet. This works very clean on some recorded music I've 
tested, but thumps ridiculously with a fast-attack synth singnal.
Attack time constant is way too high. Tried a shorter attack last
night, now I got distortion, but the thumping is gone. Gotta increase
the release time constant next. (Thumping: I mean compresor overshot.
Before the compressor can react, the output goes up until the zener
diodes clip it. See NE572 data sheet for reference.)
Maybe I'll try a capacitance multiplier for the attack time constant
(old NE570 trick). Will act fast with sudden change of dynamics, and
slow on steady levels.
If everything fails, I throw out the whole companding board and 
build something I know will work. (Like my old FS-1 compander,
or the vactrol-hungry Moog 12-stage phaser compander)

JH.


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