[sdiy] the white whale - modular synth patch memory

Neil Johnson neil.johnson71 at gmail.com
Wed Mar 18 22:39:02 CET 2015


*sigh*

This old chestnut, rearing its ugly head once more.  Time to get the
dead horse out again....

Richard Moore wrote:
> I think you've hit the softest spot, Paul.  Right now my best thought is to replace the pots on a retrofitted module with a little board containing a pot and a digipot.  The digipot could be controlled by the pot or could receive a value from the CAN bus.  Between the digipots and the analog muxes there could be full patch memory.  I don't like the resolution of switching noise of the digipots, though, so I'm not sold on this concept yet.

Pots are used in many ways in analogue synth modules, some as simple
potentiometers, others a variable resistors.  Sometimes strapped
across the supply rails, others in op-amp feedback loops, others in
critical signal junctions.  The application of potentiometers is
sufficiently wide that a single digipot solution will not fit all, and
digitpots have their own issues (stepping, maximum voltage/current,
noise, non-linearities, etc).  The only truly transparent replacement
would be motorised pots, in all the different track values (say 2k2 up
to 2M2 in both linear and log, and some dual track as well).

The routing aspect is another tricky problem you won't solve.  There
have been several comments along the lines of you won't use all the
patches all the time, just a subset.  Which is true.  But that subset
changes enormously.  And just thinking about the spec you need to beat
- your switching/routing system needs to be as good as a length of
shielded copper - no noise, no offsets, no distortion, extremely low
coupling.  You need excellent DC performance for pitch CVs (gain,
offset, minimal drift), and AC performance >50kHz if you want to keep
those snappy envelope attacks and not suffer appreciable audio
degradation after a few trips through your routing matrix.

And then there a multitude of switch types: push buttons, rotary,
toggle, slide, and so on. A general solution would also need to cope
with the "intelligent" synthesizer modules that have menus and sub
menus - retrofitting into them could be entertaining.

If you want to get an idea of where you're heading check out the
professional audio realm.  The closest would be mixing desks, and they
either went down the VCA route and/or flying faders.  I think the
closest to where you're heading would be the gear made by Bettermaker:
http://www.bettermaker.eu/
But they're not cheap.

Neil
--
http://www.njohnson.co.uk


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