Sines breaking down

[Joachim Verghese]

Juergen scribed:

> One of the real interesting things of any self oscillating VCF is how it
> behaves when overdriven while self oscillating. It's also a matter of
> taste: Do we want the input signal to drown the self oscillation, or do
> we want both signals to sum up more or less independently, or do we
> want the self oscillation sine to partly block out an input signal. All
> of these options have their benefits.
> I think that was also part of the difference between the Moog ladder and
> the ARP ladder filter.

I believe it was to some extent, yes. ARP put this extra diff.
pair in front of the ladder which makes the filter behave in a
more predictable manner under the circumstances you mention.
In addition to providing input limiting, or clipping, the
input stage acts as a level shifter which allows the signal to
be DC-coupled into the ladder. (Actually, the whole signal chain
of the 4012 filter is DC-coupled). I seem to recall that some of
Moog's modular filters also had the extra diff. pair although
they weren't DC-coupled.

Anyhow, the one filter that passes the input signal almost
unaltered while oscillating is the ARP 4075/4072 (the one with
LM3900 integrators), as I believe tomg mentioned. It's almost
impossible to make this filter scream -- at least with the
limited overdrive that an unmodified ARP allows. Whether this
is good or bad is of course a different matter.

-joachim