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

cheater00 cheater00 cheater00 at gmail.com
Thu Apr 25 13:19:23 CEST 2019

Some good points, thanks.

On Thu, Jan 3, 2019 at 10:08 AM Vladimir Pantelic <vladoman at gmail.com> wrote:
> > Now with that out of the way.
> >
> > 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.
> 50dB, fine, lets do that. what lower cutoff? 50Hz enough?
> > A few ideas on implementation that i would love to hear (constructive)
> > opinions on:
> >
> > - 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.
> if you are talking about *FM*, then you need to actually frequency
> *modulate* the carrier, so you need one oscillator per signal output,
> not one per voice common to the whole system. in the simplest case a
> crystal oscillator with a varicap diode. if you distribute a common
> carrier freq, then you can use that to sync and modulate a carrier VCO
> with a PLL, but lower cutoff will be a problem since at some point your
> PLL will just compensate away your near DC signals.
> > - 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
> I think that is not correct, you cannot have a central bank of
> oscillators if you want FM.
> what you can do is AM, that is to shift every voice in frequency into
> adjacent bands, but then you need very good filtering at the source to
> prevent the overtones of one AM signal to hit another band. at the
> receiver end you need a bank of selective filters before the
> demodulators as well. this has all been done in the past, but it was not
> an opamp and a few RCs for a few cents...
> also keep in mind that in synthesis you are not limited to 20KHz, analog
> VCOs can produce a square wave at 20kHZ with overtones much higher. you
> might not need them reproduced at the other end, but you will probably
> have to filter them out, so a decent lowpass filter in front of each FM
> or AM sender is needed.
> feel free to ignore the following:
> if I had to do that in all analog, I would probably opt for pulse
> amplitude modulation at a high switching frequency, distribute a central
> clock to all modules and use it to mux your N signals into a single
> cable using analog switches. at the receiver demux it back into N lines
> with a sample and hold circuit and a simple lowpass reconstruction
> filter assuming the carrier clock is high enough.
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