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

cheater00 cheater00 cheater00 at gmail.com
Thu Jan 3 09:24:11 CET 2019

OK so this thread currently has over 60 posts.

--> 30 posts talk about how I'm dumb for not having considered these approaches:
- multiple cables
- multi conductor cables
- digital transmission
- crossbar switches
- can bus, LAN, wifi
- MADI and other digital workstation interface systems
- probably something I forgot

and then continue to lecture about "the XY Problem" (i.e. a veiled way
of saying "haha, you're just so stupid, maybe learn to ask

--> 20 posts talk about how a poly synth is a stupid idea, try to
catch22 me with "but there are no patchable polysynths", talk about
how the technology is absolutely not suited (and then others prove it
is suited after all), and other stuff like that.
--> 10 posts talk about technology remotely related to the idea posted
in the original question.

I've considered each of the technologies i listed above years ago. No,
I don't want to re-visit those avenues just to validate myself towards
people who won't let me explore my ideas "because they're dumb". I
have looked at them long enough. I started considering this over 10
years ago. So if anyone thinks I somehow need to jump through
someone's hoops in order to validate my questions, uhh, yeah, no. That
goes especially for Scott and Gordon, your responses just sucked.
Reading this sort of negative, you-can't-do-it stuff on our list makes
me deeply sad, especially when contrasted with responses from more
positive members of the community who have been able to come up with
constructive ideas and ways to accomplish things.

Especially with comments like Roman's "You wanted multichannel FM over
regular $0.10 patchcord - do it. You asked for solutions, instead got
a bunch of voices saying it's a wrong way. Too bad. Do your own R&D
and proove us wrong. Then patent it and get rich."

Roman I seriously don't recognize you, are you sure you're in the right mood?

--> 1 post was useful content and came from Ingo who pointed me to a
cheap receiver chip. Thanks a lot, Ingo. I've only looked into
transmitters so far.


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.

The price isn't an impact. A transmitter chip might be about $2-4. A
receiver about $2. There's going to be some RF stuff inbetween that
one needs to do. That's maybe $5-6 per link, plus cable and jacks of
some sort for $10 total. That's less than good jacks ($2.5*2) + good
patch cables that you'll be replacing over a time span of a few years
($10 at an attrition rate of 50% over 5 years, meaning $15). Compared
to $20 per link using separate cables, $16 is well economical. On top
of this, if you're talking about a polyphonic modular synthesizer, $16
will pale in comparison to the cost of the design time electronics,
mechanical parts, and mainframe.

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)

- 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

- 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

Handling near-DC component transmission is an interesting question.

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.
- 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.
- 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
- 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.
- 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.

On Wed, Jan 2, 2019 at 2:25 PM Mattias Rickardsson <mr at analogue.org> wrote:
> On Mon, 31 Dec 2018 at 08:01, <rsdio at audiobanshee.com> wrote:
>> A Gate, however, is a signal that’s separate from the physical switches, and it’s a digital signal. In fact, a Gate signal may come from a switch (keyboard), or it may come from some other source such as a sequencer, a clock, or some other module. In that sense, Gate is a pure digital signal, as pure as any definition of digital could be.
> Without trying to either avoid or adhere to any definitions, just my gut feeling, I'd call the Gate signal a LOGIC signal. It doesn't really matter if it's considered analog or digital, because DIGITAL essentially implies that the signal is supposed to be *used* in a digital fashion - as discretely encoded numbers of some sort. And a Gate in itself doesn't have to be. It can be just a timing signal, for instance when triggering a non-gated AD envelope. :-)
> /mr
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