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

[Chromatest J. Pantsmaker]

I don't often post to the list, but I'm going to throw out a credential
answer.... (what does that mean?)

So, I'm a broadcast engineer.  I've been doing this as my full-time career
since 2002.  There are vast amounts of stuff that I don't know.  With that
said, I maintain high-power FM radio transmitters among my other duties.

I think what you are proposing is technically possible, but it's going to
be nowhere as simple as you are imagining it.  One of the things that we
deal with when installing a transmitter that is physically close to another
transmitter is the cross-talk and frequency intermodulation.  (making up
numbers here) Say your first carrier frequency is 10kHz and you're planning
on spacing them out with 10kHz between each one... so you'd have 10kHz,
20kHz, 30kHz, etc...  You'll have to design your system very carefully to
block any intermodulation from each signal.  With just the 10 and the 20,
you'll have a 3rd order intermod product at 10 and 30, which is a
combination of the original 10 and 20... which is going to play havoc.  You
were planning on having voice 3 at 30kHz, and you already have an intermod
signal there from the 10 and 20.  Adding that 30 creates an additional
intermod signal at 10, and one at 20 and 40... Now extrapolate that to 16
voices.  Unless every FM signal is carefully isolated and filtered from
each other, at each transmitter, you *will* have problems.

Oh, and that's just for one module.  How many modules do you plan on
using?  Each transmitter on every module in the entire synth has to be
isolated from each other.  Also, each receiver has to be ensured to only
receive the signal from the intended transmitter.  You might get some crazy
cool sounds, or you might get garbage.

You might also get problems with your patch cords being resonant at certain
frequencies and not at others.  Imagine having a patch cord that works fine
for 11 voices, marginally for 3, and barely works at all for 2 voices.
Imagine moving a patch cord and (not unplugging it) and your sound changes.

Further, standard FM broadcast is limited to 15kHz frequency range.
Off-the-shelf transmitters are likely frequency range limited to transmit
50Hz to 15kHz.  Certainly you'd want your synth to be able to play lower
and higher notes than that.  Now you are designing your own FM circuit, and
the combiner circuitry to push all of those 16 signals into one wire
without letting any of them have any cross-talk.

It's likely that issues a high-power transmitter can have won't be an issue
for your project, but I wouldn't bet on it.

On Thu, Jan 3, 2019 at 1:25 AM cheater00 cheater00 <cheater00 at gmail.com>
wrote:

> 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
> questions").
>
> --> 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
> voices.
>
> - 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
> connector.
> - 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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