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

[Vladimir Pantelic]

> 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.