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

Thu Jan 3 20:24:01 CET 2019

Potentially interesting concept idea. Even if it might end up too complex
or difficult in a real-world scenario, I'm intrigued by the possibility of
meta-modulation in the form of oscillator FM patched via FM'ed
transmission. (-:

/mr

Den tors 3 jan. 2019 18:41 skrev Chromatest J. Pantsmaker <
chromatest at azburners.org>:

> 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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