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

Scott Gravenhorst music.maker at gte.net
Sat Dec 29 00:07:15 CET 2018

cheater00 cheater00 <cheater00 at gmail.com> wrote:
>I don't know Roman, I can receive more than 16 FM stations over the
>air and they're all pretty listenable and have more than 3.4 kHz

And those radio stations have advanced transmitter systems that by law cannot emit energy
outside of their bandwidth.  The radio you receive it on is an advanced circuit called a
superheterodyne receiver which has critical layout, power and shielding requirements.  It
is the superheterodyne circuit that allows the receiver to be so incredibly selective
that there's never interference even with it's full hearing range audio bandwidth.  There
is a big difference between the radio transmitter and receiver systems for FM music and
something for FDM and be sure to check those FDM chips for a spec on usable bandwidth. 
3.4 kHz is pretty much crap for full range music.  Almost any modular can make
fundamental pitches well above 3.4 kHz and then add harmonics on top of that..  Sure, you
can do superhet, but (and I'm sorry, but it's necessary to mention cost) imagine the
price tag of 16 superhet receivers and 16 advanced transmitters per module (for the 16
voice system you imagined).

You also write of "complex patches".  I have no idea what you mean by that, but I'm sure
we're talking more than 2 VCOs, a VCF, an EG and VCA for each voice - and obviously
there's not much point in making that patchable, but (using your own voice count of 16)
just that is 32 VCOs, 16 VCFs, 16 EGs and 16 VCAs.  Have you done pricing checks on just
those analog components?  And just that small system would need 80 receivers and 80
transmitters.  Now start talking about a truly patchable system that does more than a
Fatman and I think you'll see that without lotto winnings, it won't happen.

You asked a question, you're hearing the answers based in reality and cost and
feasibility is part of reality.  But you're also invited to prove us all wrong - do the
research on how to use those FDM chips and build up a simple system of say, just 4 or
even just 2 voices - and make it work with quality full audio range output.

>On Thu, Dec 27, 2018 at 2:55 PM Roman Sowa <modular at go2.pl> wrote:
>> Frequency domain multiplexing was used long time ago in telephony.
>> How nice and easy solution this was can be simply put in two questions:
>> - why nobody remembers that?
>> - why it was abandonned so quickly?
>> I mean you have ready to use multichannel digital standards and
>> multichannel ADc and DACs, why bother using an ancient idea that was so
>> troublesome to manage that even 0.3-3.4kHz bandwith audio was a challenge.
>> And if you want full analog, I'd go with multipin connectors, there are
>> lots to choose from. OK expensive patch cable then, but still a tiny
>> fraction of the price of any 16-voice module.
>> Polysynths are hardwired for a reason.
>> Roman
>> W dniu 2018-12-25 o 20:49, cheater00 cheater00 pisze:
>> > Hi everyone,
>> > I was thinking again about how you could do a modular polysynth well,
>> > and it occurred to me that you might be able to use frequency domain
>> > multiplexing (FDM) to put several signals over a single conductor
>> > pair. A quick search found the Si4710 which is a bit pricey at ~$12 on
>> > digikey but it's a tiny 3x3mm QFN device that performs complete FM
>> > transmission. In a 16 voice system, at about 5 output functions per
>> > module, and 12 modules, you can easily use up ~1000 of those, which
>> > drops the price to $4. I was wondering what everyone thinks about this
>> > sort of scheme.
>> >
>> > Given that metal patch cables could function as antennas and cause
>> > both cross talk and external signal pick up, one could also use
>> > multimode glass fiber and transmit signals this way. The question is,
>> > does anyone know whether support electronics could be found that are
>> > integrated enough (small in footprint) and inexpensive? The idea would
>> > be to build an electronics module that takes 16 analog audio channels
>> > and outputs a signal that can be then converted to glass media
>> > signalling.
>> >
>> > Merry Christmas everyone!
>> > _______________________________________________
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>> >
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-- ScottG
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22

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