[sdiy] Patchable polyphonic synth with FM or AM transmission idea

cheater00 cheater00 cheater00 at gmail.com
Thu Apr 25 13:10:56 CEST 2019

Some good points, thanks.

On Fri, Jan 4, 2019 at 11:17 PM Gordonjcp <gordonjcp at gjcp.net> wrote:
> On Thu, Jan 03, 2019 at 09:24:11AM +0100, cheater00 cheater00 wrote:
> >
> > You don't need racks of equipment to make a single modular voice of a
> > poly synth sound good over FM transmission. It's a single synth voice,
> > not a piece of classical music performed by a philharmonic orchestra.
> > 50 dB headroom is more than plenty. If you don't count "off", many
> > synths don't have more than 45 dB dynamic range, and they're used to
> > make music by people who win the grammy, so I'm not worried.
> >
> Okay, but you're not passing the audio over the patch cable, you're
> passing DC control signals.
> > One of the reasons to explore this idea is that I've explored all
> > others and this actually seems like it would be possible to build. But
> > there are also many pro's:
> > - compared to separate 1/4" leads, or multi-conductor cables: most of
> > the complexity can be taken care of by a pick and place fabrication
> > service, rather than hand-soldering front panel jacks, especially
> > esoteric jacks; cables can be inexpensive;
> >
> > - compared to digital: FM transmission has no delay (not even 1ms),
> > ability to perform feedback, repeated A/D-D/A conversion with
> > affordable ADC and DAC chips will result in a lot of aliasing
> > artifacts as well as in cross talk (due to using codec chips)
> The delay in conversion to and from digital would be in the order of
> microseconds.
> If you need to shave off one millisecond from your latency, sit 30cm
> closer to the speaker.
> > - compared to crossbar switches: crossbar switches just cannot handle
> > a flexible modular synthesizer with a lot of modules, have poor
> > crosstalk and headroom, and need a lot of massaging to work in the
> > first place, plus they're pretty expensive and difficult to source.
> > but if anyone has notes to the opposite i'd love to hear them. bear in
> > mind the design goal here is 50 inputs and 50 outputs, times 16
> > voices.
> Every analogue polysynth you've ever played uses analogue switches to
> route the control voltages.
> > - compared to CAN, LAN, WiFi: all of those are either too expensive or
> > won't be able to handle runs (distances) within a single moderate size
> > modular or won't be able to handle all the channels due to bandwidth
> > limitations at which point you need involved topology. Delays are also
> > an issue
> CAN bus runs over something the size of a car, or lorry, or bus.
> How big a modular are you planning?  I'd love to see a modular bigger
> than a city bus.
> > Handling near-DC component transmission is an interesting question.
> All your control signals in a practical synth are slowly-varying DC.
> > A few ideas on implementation that i would love to hear (constructive)
> > opinions on:
> >
> > - patch cables could be normal BNC cables. this is inexpensive. 1/4"
> > cables might not be good enough.
> Huge, expensive and fragile, but doable
> > - outputs of each transmitter should have balancing resistors of ~ 1K
> > (series to output) and terminating resistors (~50 Ohms across TX pair,
> > before 1K balancing resistor). This among other means the ability to
> > tie together multiple outputs, and distribute them to multiple inputs,
> > just by using T junctions.
> This will give you intermod problems.
> > - you need one FM carrier oscillator per voice. Those should be
> > central and distributed to every synth module. This means 16 good
> > sinewave oscillators, they can be synthesized with DSS.
> > - each module would need some sort of connector for each of the 16
> > carrier oscillators, possibly BNC as well, but could also be an edge
> > connector.
> You'd actually need an FM carrier per module per voice, if you're going
> to do frequency channels.  You'll need a different frequency for every
> single control signal, and they will all need to be chosen so they don't
> interfere with each other.
> > - given you don't need local oscillators, the FM transmitter becomes
> > in essence just a few transistors and an op amp, which might be
> > possible to drop. It could be easier to use a pick and place service
> > rather than source a more expensive FM IC that contains a running
> > oscillator just the presence of which will degrade quality. A
> > transmitter IC could be $2, op amp and transistors could be quite a
> > bit less, which adds up quickly.
> You will need local oscillators for the transmitter to work correctly.
> A transmitter consisting of "a few transistors and an opamp" will not be
> clean and stable enough for this to work.
> > - using your own oscillators also means you can lower the carrier
> > frequency to outside the commercial band, making it easier to create
> > non-radiating patches, i.e. less cross talk between modules.
> > - the receiver can be a bit more involved to create, but still
> > attainable, especially with modern, stable passives.
> > - you might want the modules in separate metal enclosures, just due to
> > the weight of 16 PCBs.
> If you want to avoid leakage between modules, you'd actually be better
> going up into the many-GHz region where screening will be more
> effective.  This will be (more) expensive.
> There are a lot of considerations which make this less than entirely
> practical.
> I'm not entirely sure why it would be useful, since it won't be able to
> interoperate with any "normal" modules.  Why would this approach be
> better than something like the Oberheim Matrix 1000, where you just
> generate all the modulators digitally including the mod matrix, and
> emit control voltages to the appropriate analogue circuitry?
> --
> Gordonjcp
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