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

Oren Leavitt obl64 at ix.netcom.com
Wed Dec 26 21:12:19 CET 2018 

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 cheater00 <cheater00 at gmail.com 
> <mailto:cheater00 at gmail.com>> 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 <tom at electricdruid.net 
>> <mailto:tom at electricdruid.net>> 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
>>>
>>> ==================
>>>       Electric Druid
>>> Synth & Stompbox DIY
>>> ==================
>>>
>>>> On 25 Dec 2018, at 22:40, cheater00 cheater00 <cheater00 at gmail.com 
>>>> <mailto:cheater00 at gmail.com>> 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 
>>>> <ben.pi.bradley at gmail.com <mailto:ben.pi.bradley at gmail.com>> 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 Tue, Dec 25, 2018 at 4:58 PM oren levy 
>>>>> <orenlevysticky at gmail.com <mailto:orenlevysticky at gmail.com>> wrote:
>>>>>>
>>>>>> You can use the FPGA to combine all the data you are trying to 
>>>>>> transmit into a single stream that you can transmit over a single 
>>>>>> cable.
>>>>>> MADI interfaces are expensive as a unit. There are various ways 
>>>>>> to implement MADI at a board level with microcontrollers and FPGAs.
>>>>>> Other options would be to make your own protocol. Using a TRRS 
>>>>>> cable should be able to provide enough bandwidth at more 
>>>>>> manageable speeds that won’t require you to think about 
>>>>>> transmission line theory.
>>>>>>
>>>>>>
>>>>>> Rock & Roll,
>>>>>> Oren Levy
>>>>>>
>>>>>>> On Dec 25, 2018, at 11:13, cheater00 cheater00 
>>>>>>> <cheater00 at gmail.com <mailto:cheater00 at gmail.com>> 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 Tue, Dec 25, 2018 at 9:19 PM oren levy 
>>>>>>>> <orenlevysticky at gmail.com <mailto:orenlevysticky at gmail.com>> wrote:
>>>>>>>>
>>>>>>>> I think you’d be better off using FPGAs so you can mux the 
>>>>>>>> signals however you want along with data. Either a bunch of 
>>>>>>>> small ones or one big one per module.
>>>>>>>> If you just want to share audio and don’t want to mess around 
>>>>>>>> with FPGAs, you can probably use a protocol like MADI. Not sure 
>>>>>>>> if MADI has a DC coupling requirement but if not, CV could also 
>>>>>>>> be passed.
>>>>>>>> You’d probably want a very stable clock to sync all the modules 
>>>>>>>> to and optimize clock phase delays so everything can mux/demux 
>>>>>>>> in sync.
>>>>>>>>
>>>>>>>> Rock & Roll,
>>>>>>>> Oren Levy
>>>>>>>>
>>>>>>>>> On Dec 25, 2018, at 10:01, Mike Beauchamp 
>>>>>>>>> <list at mikebeauchamp.com <mailto:list at mikebeauchamp.com>> 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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