optical limiter problem

[media at mail1.nai.net]

>Vactrols have a (decent) fast time to get low impedance, and a rather
>high time to return to higher impedance. Now that's exactly what is
>needed for a peak limiter.

More or less -- it really depends on your application.  Vactrol driven gain
reduction is excellent for subtle limiting like tracking vocals to analog
tape and smooth deliberate compression, but they aren't typically found in
modern devices used for peak or program limiting.

Firstly, their release time is often too slow.  Secondly, their attack time
is also too slow -- really hard transients can still pass through.  In many
ways this overshoot is good because it will prevent hole punching.
Unfortunately, the inputs on your DAT aren't as forgiving.

>Because of the very nonlinear R / I characteristic of a vactrol, I have
>to use a feedback rather than a forward compressor topology. So I have
>a feedback loop, and inside this feedback loop one lowpass filter
>(the inertia of the vactrol). But if I add an electrical
>smoothing as well (i.e. RC-filter), I have a second pole inside the
>feedback loop, and this will cause overshot in the regulation response.

That makes sense to me.  You could add an RC filter so that the ripple was
way above the cutoff frequency, but then you would have an awfully slow
device to call it a limiter.  This would take you from "limiter," past
"compressor" all the way into "leveling preamplifier."  Come to think of
it, all of those sexy, syrupy, leveling pre-amps were optical so you may be
onto something :)

I'm afraid the only way to go around might be to convert the response of
the vactrol to a change in voltage and then use that to drive a VCA.  Then
you could add all sorts of envelope controls between the detector and the
gain stage.  However, this seems to defeat the elegance and simplicity of
your intended design.

Many of the older optical devices used inductors, tubes or both -- perhaps
they were used in a way that smoothed out the ripple.

Another thing you might want to note, is that many of the optical
compressors had HPF at the input to the side-chain.  Since low frequencies
have the most energy, an impulsive sound such as a kick drum could "suck
down" the vocals.  To avoid these sort of problems, hardcore peak limiters,
such as an Aphex Dominator, are multiband devices.

>So far, I have not experimented with a real electrical peak detector in
>front of the LED driver, but I have just connected elkos across the
>LED of the Vactrol.

Again, what is an "elkos" ??

Do you think adding a LPF before the vactrol might reduce ripple??  I'm
just taking a wild guess here.

>How are the commercial optical limiters optimized? (BTW: *are* there
>actually optical hard peak limiters, or do they have limited gain reduction
>in the first place ?)
>
>Any ideas ?

I know you don't want to hear this, but I do not know of any optical
brickwall limiters that have ever been made.  So maybe you should use the
vactrols for a production compressor and build a peak limiter with a solid
state detector.  Sorry, I wish I could be more helpful.

PEACE OUT :)
MARK