[sdiy] Fwd: Shruthi 4PM (was re something else...)

[Dave Manley]

rburnett at richieburnett.co.uk wrote:
>That's the nightmare of working with filters made from real-world 
>analogue components (>.<)
>
>You could always trade component tolerance problems for those of 
>coefficient rounding and state rounding instead! ;-)
>
>That multi-mode filter technique works very well in the digial domain 
>where you can specify the pole positions and the mixing ratios with 
>excellent accuracy and repeatability.
>
>Regardless of whether the implementation is analogue or digital, the 
>resulting band-pass and high-pass responses might not be quite the
>shape 
>you would initially expect though.  Due to the way the 4 poles split in
>
>that x-formation in the "4-pole cascade with feedback" the bandpass 
>response becomes a little lopsidded as resonance is applied.  And the
>HP 
>response has a dip in the passband to the right of the resonant peak!
>
>These are surely not show-stoppers musically, but since we are obsessed
>
>about accuracy of the filter shapes it's probably worth mentioning :-)
>
>-Richie,

I ran spice sims with different tolerances years ago.  I'll look if I still have the files. It didn't look good.

If someone has one of these filters, a a ramp LFO, a sine wave VCO and a scope that works in XY mode then it would be easy to do some simple sweeps and see what the shapes look like.  

LFO ramp -> VCO CV and x-input.
Sine into filter, filter output into y-input. Sine input should have constant amplitude across freq.  Crude, plot won't be log/log, but simple enough to see if you get a notch or bandpass.

Pretty exotic test equipment :-)

-Dave

P.S. note these questions aren't specific to, or intended to be critical of Oliver's design, but any filter of this general architecture.