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

rsdio at audiobanshee.com rsdio at audiobanshee.com
Fri Jan 4 07:45:55 CET 2019

Before this latest post, I was already wondering how a cable television studio handles mixing 500 channels onto a single coaxial cable for “broadcast.” They don’t really need an antenna for every channel, nor do they need the kind of high power necessary to cover the typical reception area with electromagnetic waves. Of course, a reasonable amount of power is needed to push those 500 channels down the miles of coax needed to fill a city, but that amplification is not all handled centrally - it’s done every few miles down the road. (*)

I imagine that a lot of the cable television content comes directly from a satellite, where the channels are already multiplexed together. Then, the local channels are added. So, I imagine that the cable studio has racks full of equipment that can shift certain channels to different frequencies to make room for every channel to be on its own frequency. They might even have gear that can change a carrier frequency on a channel without full demodulation and remodulation.

All of this may or may not be applicable to a 16-voice synth. In one sense, it seems easier to handle 16 channels than 500! In another sense, though, it’s rather difficult to make audio circuits that handle more than 20 kHz frequencies, so the whole FM concept requires analog circuits that go up into the kilohertz or megahertz range. Those op-amps are going to be way more expensive.

I would recommend against using radio and television frequencies. Those are way too high, and the lower frequencies would be wasted. It seems likely that a 16-voice, 20 kHz synth would not need to modulate any higher than 320 kHz. Thus, it seems likely that everything would fit under 1/3 of a MHz, and that makes all of the analog circuit design a lot easier.

Let us know what you work out, cheater.


* p.s. Since cable television is only using high frequencies, there’s nothing really happening on the coax at normal audio frequencies. So, they actually mix 60 Hz AC power in there along with the television (and radio) stations. This turns out to be useful in rural areas where this is no power available to boost the broadcast power. The cable equipment on the side of the road simply uses the 60 Hz AC power like a wall outlet, puts a high pass filter on the radio/tv channels, boosts the analog signal and passes it on down the road to the next amplification station. I suppose you could have a modular synth where the patch cables not only carry 16 channels, but they also power the entire module. I mean, if every module needs modulation and demodulation on every patch point, you might as well have an AC power supply in there, too. I guess the question then becomes: Which patch cable to depend upon for power, assuming each patch cable is optional?

On Jan 3, 2019, at 9:40 AM, Chromatest J. Pantsmaker <chromatest at azburners.org> wrote:
> 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.

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