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

rsdio at audiobanshee.com rsdio at audiobanshee.com
Wed Dec 26 21:58:55 CET 2018

Bumping up from DB9 to 13-pin would allow such a design to be compatible with either the Akai S900 sampler to VX90/AX73 (6-voice) or the popular Roland synth guitar (technically 6-string, but with 7 audio channels). Unfortunately, despite sharing the 13-pin connector, the Akai and Roland cables are not compatible, because they put the voices on different pins. Also, another obvious disadvantage is that the 13-pin jacks and plugs are large.

Then again, DB9 (DE-9) is not exactly “small” either. Unless you were talking about mini-DIN size connectors, where you can find 10-pin versions that fit in the 9.5 mm plug body.

Whether you want to standardize on 6-voice, 7-voice, or 8-voice would be an interesting choice.

As for cheater’s comment that analog switch ICs are terrible - wait until you try frequency domain multiplexing! It’s going to be way more of a design challenge to get good quality out of frequency domain multiplexing - unless you’re happy with telephone quality audio - than getting analog switches to work. There are some very high-end analog audio devices that use analog switch ICs, but they all use the more expensive chips that aren’t exactly replacements for the standard analog switch ICs found in vintage analog synths. These newer chips have expanded voltage ranges and much lower resistance, as Mr. Levy alluded. Intersil and Siliconix were the original high-end analog switch companies, and now Renasas and Vishay seem to share that market, having purchased the former leading companies, respectively.

Brian Willoughby

On Dec 26, 2018, at 12:12 PM, Oren Leavitt <obl64 at ix.netcom.com> wrote:
> Some years ago, I had an idea of using 9 pin DB9 data connectors as patch cords for a polyphonic all-analog modular synth. Technically, you could patch up 8 voices + common ground.
> - Oren (another Oren!)
> On 12/26/18 1:29 PM, oren levy wrote:
>> Sorry about my earlier recommendations; didn’t realize you wanted all analog.
>> As far as switches go, I really like the Intersil (now Renesas) BiCMOS 16x8 crosspoint switch IC. Low ron and very very low crosstalk. I used it when designing the Music Man ‘Game Changer’ guitars for pickup switching, but, it maxes out at 15V.
>> https://www.renesas.com/us/en/products/audio-video/video-switching/unbuffered-crosspoint-switches/device/CD22M3494.html
>> Rock & Roll,
>> Oren Levy
>> On Dec 26, 2018, at 09:09, cheater00 wrote:
>>> analog switch ICs are terrible, let's just skip them. You're not
>>> making an analog switch ic based system anywhere near close a medium
>>> sized modular, it's insanity and prone to shitloads of interference.
>>> same with mod busses. the switch ICs are very proprietary. I haven't
>>> found a digital multiplexer IC that was good, has anyone got any good
>>> suggestions?
>>> On Wed, Dec 26, 2018 at 5:16 PM Tom Wiltshire wrote:
>>>> Why not make a number of “analog virtual patch cables” with analog switch ICs? Each cable has a switch at both ends, one selects between X sources, and the other selects between Y destinations. This is conceptually very similar to a genuine modular with hardware cables, but the control signals can be fanned out to however many voices you have.
>>>> Alternatively, you could have a “mod buss” type system, where each module output could be sent to a particular destination buss, and each module input could be switch to tap signal from a given buss. Again, control signals for the switches in such a set-up can be fanned out to however many voices are required.
>>>> For any polyphonic system (except the early oberheims?), the control panel is separated from the voice generation, so you can think about the interface separately from the audio-producing part of the circuit.
>>>> Tom
>>>> On 25 Dec 2018, at 22:40, cheater00 wrote:
>>>>> I want to make this work for an all-analog synthesizer, but rather
>>>>> than use crosspoint switches I want to use patch cables which makes
>>>>> things much less annoyingly complex and expensive. The thing is, where
>>>>> on a monosynth you have a single patch cable, on a patchable polysynth
>>>>> you have n patch cables, one for each voice. So I am currently trying
>>>>> to work out how to do this using a single patch cable, and frequency
>>>>> domain multiplexing came to mind. I'm 100% certain an FPGA cannot do
>>>>> FDM, since almost all of this is analog, so I'm looking at dedicated
>>>>> radio transmitter chips. At $4 per chip, it's not so bad. The question
>>>>> is how to make the chips talk to a single medium without fighting each
>>>>> other.
>>>>> On Tue, Dec 25, 2018 at 11:24 PM Ben Bradley wrote:
>>>>>> What you're describing sounds all-digital.
>>>>>> It seems to me a crosspoint switch would be the thing to have on each
>>>>>> (analog) voice, and have them controlled by the usual microcontroller
>>>>>> for a polyphonic analog-signal-path synthesizer. Of course, this is a
>>>>>> woefully incomplete description.
>>>>>> On Dec 25, 2018, at 11:13, cheater00 wrote:
>>>>>>> The objective is to be able to create a way for a single patch cord to
>>>>>>> carry 16 voices. I'm not sure how an FPGA in itself will help me, have
>>>>>>> you got any ideas?
>>>>>>> MADI interfaces are prohibitively expensive.
>>>>>>> On Dec 25, 2018, at 10:01, Mike Beauchamp wrote:
>>>>>>>>> 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.
>>>>>>>> So there's $4000 worth of just one IC in a single complete polysynth?

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