[sdiy] moog high pass flter

[Magnus Danielson]

From: harrybissell <harrybissell at prodigy.net>
Subject: Re: [sdiy] moog high pass flter
Date: Sun, 08 Dec 2002 23:38:56 -0500

> Have not tried that one. Do you know what the topology of that one is ???

To the best of my knowledge (and it seems to check out when I looked into a
CS-80 service manual) the dedicated IG00156 VCF chip implements a 2-pole (and
thus 2-zero, to follow my latest writing on signal theory) state-variable like
filter. I have however no detailed description (i.e. datasheet) of the chip.
I think some form was floating around before (I think Jürgen was in on it).

> I've tried HPF using the State Variable and Sallen Key methods. (both are
> 12dB/oct). I just finished the EFM  MS-20 clone and the HPF has some appeal.
> I guess time will tell how often I patch the HPF in. I used toggles so
> I can switch real fast if I want to.

(You'r like the Germans during the war... you double-press on the SPACE all to
often... it was things like that which made it easy to break their crypto,
besides being flawed to begin with...)

I haven't checked hour the feedback of either type works, but if you recall
there's two scools of feedback, one which keeps the DC gain of the passband
stable while the peak rises, the other keeps the peak att the same level and
let's the DC gain drop?

If you only fiddle with the lower harmonics, as in the case of a lowpass
filter, then that may or may not be such a big deal, in a highpass however...

Could that _possibly_ be an issue you have not considered? The effect might be
of greater importance here I think.

> TO clear the air... I really do not think HPFs 'suck'.  I reserve that word
> for BBDs....

Thanks for clarify that important aspect for us Harry, we where a bit worried
there for a minute or two ;O)

I however think that the use of highpass filters might be less since some of
its uses you can emulate by switching waveform and use a lowpass to raise the
lower harmonics with. Not all the things you can do works like that thought.
I think there lies quite alot truth in that waveform overtone slopes and the
peak treatment matches what the ear is used to experience in many cases.

But arguing about it is not worth the effort, it's a religions issue. It's not
the functionallity needed en masse, but some people have need for it. I like it
and have had great use for it myself in my MS-20. It is possibly more
interesting when processing noise than VCO waveforms. Come to think of it, I
haven't used the one in the CS-40 as much... gotta use it ;O)

Cheers,
Magnus