[sdiy] RE: Polyphonic synthesizer (controlling with vactrols)

[mikko.a.helin at nokia.com]

Hi.
Nice synth! I've been recently playing with Linux and I'm considering to build a poly synth with the digital software oscillators and analog filters. Seems many (incl. MiMi) are using the Moog filter for VCF for polyphonic synth, so it may be the one I'll use. MiMi seems to use 12 bit DAC's for control CV's, is it really enough (no stepping?)? I think that by using /dev/shm you could easily generate the CV's in the synth app and put to FIFO and update the DAC's in another separate process (like 'cvgeneratord') which reads the FIFO in shared memory. There should also be enough options for controlling the filters (and  VCA's) like a CV for emphasis and filter modulator (for each VCF module, though  the emhasis could be just one CV output, but the filter mod cv could come from MIDI poly pressure or key note value). 

However, because I'd like to have the circuit as small as possible I'd like to use Vactrols (in place of the OTA's) for controlling the slow-motion signals like filter mod ctrl and emphasis. Any ideas how to use them like in Rick's Moog schem. at http://dropmix.xs4all.nl/rick/Emusic/Moog/moogvcf_schematic.gif ? 

I'd also like to get rid of the LF412 buffers there, how big impedance is needed for the diff. amp, is 100k enough (with a gain of 6.8 the feedback resistance would then be 680k)? The 50k pot would then be replaced by a Vactrol (with appropriate specs). The CV input stage would also have one 1V/oct input summed with another input via the Vactrol (for LFO & S/H). I thought to put VCA's on another board with LFO & S/H. Don't know how well the Vactrols are matched, maybe some trims are needed ther for fine tuning.

In the Linux box I though to use the cheap Guillemot Muse LT (20 EUR) card with 4 outputs (though there are also models which have upto 6 outputs using the CMI8738 6ch version chipset). With patched 2.4.18 kernel the (sustained) latency should be below 1.5 ms with the latest 0.9.0 ALSA drivers (come with  the new SuSE 8.0). Don't know if the stuff could be put on embedded style PC (PC/104 etc. though they are way too expensive for this stuff, 700 EUR and up @300MHz 686 PCI); Linux seems to be quite a huge operating system after all, though you can boot into it with just one floppy (with the compressed kernel). I just upgraded my old Abit BH-6 (intel bx) PC with Powerleap adapter and Celeron (tualatin) 1.2GHz (works pretty well!) so I'm using it anyway.
-Mikko

> -----Original Message-----
> From: ext Magnus Danielson [mailto:cfmd at swipnet.se]
> Sent: 19. April 2002 23:49
> To: chris.crosskey at vicon.com
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: Polyphonic synthesizer (Was: Re: [sdiy] Ladder filter pole)
> 
> 
> From: Chris Crosskey <chris.crosskey at vicon.com>
> Subject: RE: [sdiy] Ladder filter pole
> Date: Fri, 19 Apr 2002 08:20:31 +0100
> 
> > Woop woop woop....Eric Barbour alert :-)).....look around 
> for valve or tube
> > synth-diy on the net, or just look for Eric, he's done lots 
> of valve based
> > synth design, the prophet of that particular part of the 
> religion, in the
> > way that Paul M is the prophet of homebuilt PPG-like 
> stuff....is there a
> > prophet of home-built prophets?....I guess that would be 
> Juergan...he builds
> > polysynths :-))
> 
> You guys have obviously missed out on Richard "Vargen" 
> Wanderlöf Wolf's
> MiMi-a:
> 
> http://home.swipnet.se/~w-21314/mimia.html
> 
> This synth is also the cause for some of us crazy Swedes to
> occasionally say "sylth" instead of synth, since the modulation-wheels
> is made from can-lids of the kind you find on jam-cans (sylt means jam
> in Swedish).
> 
> I can only say one thing: Respect!
> 
> Cheers,
> Magnus
>