[sdiy] Linear VCF control

Mattias Rickardsson mr at analogue.org
Fri Nov 9 17:53:21 CET 2018


Hi all,

following the recent discussions about temperature compensation of
exponentially VCFs, one could imagine that the easiest way of avoiding the
temperature dependence would be to control the filter linearly (Hz/V)
instead of exponentially. I can't remember seeing it done in a VCF, though.
Even if there are VCOs with linear control in currently produced synths -
the latest example might be the Moog One if I'm correctly informed.

A quick search in the SDIY archives returned some hits in an interesting
old thread called "Filter frequency cutoff with linear response?" from
2012, where Olivier Gillet, Florian Anwander and Tom Wiltshire pointed out
that the CEM3394 has linear filter FM from the onboard VCO's triangle wave
via an onboard modulation VCA. The datasheet of the CEM3394 describes it
some more. I remember liking the filter FM in the Akai AX-60, and this was
it. It still relies on exponential VCF in the regular filter CV input
though.

PAiA Fatman was mentioned as having linear control of both VCO and VCF. Any
other pure Hz/V filters out there? Or do people consider it a genuinly bad
idea (except for the lack of V/oct CV input and the need of increased CV
resolution/precision in the lower frequency range)? The bonus possibilities
of linear FM might make it worthwhile. :-)

/mr
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