revtor@... schrieb:
> Hey, I was reading the great EMU modular guide pdf doc,
> and noticed that their HPF didn't have res control
> because it introduced distortion. How's our friend the
> 420 do in this respect in HP mode? Don't have one (yet)
> to fiddle with but I was just wondering..
I don't have the EMU docs in front of me (and yes,
that's some excellent reading !). But in general,
the main reason for reluctance to give a 4pole
HPF resonance or even self oscillation, is
that unlike a 4pole HPF, the oscillation is not
pure, or there would be oscillations at higher
frequencies rather than the desired cutoff frequency.
This is a general problem of "polygon" type HP
filters, as Bernie Hutchins showed in an old
electronotes article. (Sorry, don't know the number.)
The Moog HPF falls under this category, maybe the
Emu HPF as well. (But I haven't seen the schemos,
so there is some guessing.) BTW, I think this
is more about self oscilation rather than mere
resonance peaks. And it can be overcome with other
filter structures, such as the SEM filter (or two
in series, like The Roland Jupiter-6).
Distortion can occur in ∗every∗ resonant filter,
and it can be avoided with a low enough input
level for every filter.
With a HPF, the cutoff (and resonance, if there is any)
is more ∗likely∗ to be in the lower frequency range,
so it's more ∗likely∗ to overload the filter with
a relatively low input level (lower harmonics
are typically stronger than higher ones). So I see
some reason in this distortion argument. But
OTOH other HPF designs have resonance nevertheless,
and some of them sound ∗good∗.
It's more a matter of making distortion a pleasant one
rather than to avoid it alltogether. This, again, is
true for all types of filters, but HPFs have the
drawback that distortion products are not filtered
inside the same HPF.
There is a reason why in most
synths with fixed architecture the HPF comes ∗before∗
the LPF. On a Korg MS-20 (or a pair of MOTM-420's in
HPF -> LPF configuration) you can choose between
(focussing on HPF overdrive here):
(1) distortion free resonance -> Low input level
(2) screaming HPF distortion -> High input level,
high resonance, LPF wide open
(3) tamed HPF distortion -> High input level, LPF
set to filter out some of the HPF's distortion
Even more fun when self oscillation comes in ...
All this "pleasant distortion" mainly aplies to
filtering constant level VCO waveforms. Complex
musical signals (filter to process external signals)
are normally more critical to distortion.
JH.