[sdiy]Do you need to add more functions to your favorite LP4P VCF ?

[harrybissell at copper.net]

The DG chips should be better in terms
of charge injection etc... I don't know
why anyone would ~not~ choose them for
DIY or low volume apps (damn the extra
cost - who cares ?)

The other point might be there will be
phase shifts with different filter
topologies... so a switch from a 4p
to a 1 or 2p might well have a guaranteed
'pop'.  I think you might need to use
separate recovery amps (15 different
summing amps :^) and crossfade between the
outputs to get true pop-free performance.

The A/D will be an issue with dithering
between steps (will happen even using the
high order bits only.  A workaround is to use
a matching D/A converter (could be a very 
simple opamp summing network) and feed back
a small amount (less than one bit you are using)
into the input summer to create hysteresis. I've
simulated this but have not built a live model.

The same issue is found on devices like the Wiard
MiniWave... if you modulte with a slow input
you can hear the dithering easily.

H^) harry


>
>
>---- Original Message ----
>From: m.bareille at free.fr
>To: synth-diy at dropmix.xs4all.nl
>Subject: RE: [sdiy]Do you need to add more functions to your favorite
>LP4P VCF ? 
>Date: Mon, 29 Jan 2007 09:30:19 +0100
>
>>Hello,
>>
>>I have updated my site ...
>>
>>An answer to the topic's question is here proposed here :
>>
>>http://m.bareille.free.fr/modular1/vcf_multimode/extrapole.htm
>>
>>Have a nice day :!
>>
>>Cheers,
>>
>>Marc B.