[sdiy] the white whale - modular synth patch memory

Pete Hartman pete.hartman at gmail.com
Wed Mar 18 22:50:08 CET 2015

If I were to go after this I would put aside any ideas of replacing
pots, etc, and focus on a solution that let me route CVs to already
CV-able parameters on each module.  Obviously the issues of bandwidth
etc still apply (audio modulation for example), but at the "head end"
if you will I'd see a board with several DACs hooked up to the normal
inputs of all the jacks (or some similar arrangement, obviously that
won't work for absolutely everything either).

Still a formidable problem, but I think that's a lot more solvable
than the global everything approach of fully retrofitting modules.



On Wed, Mar 18, 2015 at 4:39 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> *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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