[sdiy] moog high pass flter

[harrybissell]

Hiya Magnus...

good point about the feedback topology (fixed passband vs fixed peak level)
I'll have to experiment some. The Sallen Key would like to have a peak level
of about +/-15V (if I let it)

I didn't understand the part about the "double space"...   ???  I am indeed
part German so maybe that affects things :^P

H^) harry



Magnus Danielson wrote:

> 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