Voice Cards for DIY Polysynths

[Obsoletetechnology]

Hi,
I'm in the middle of developing a control CPU board for a polyphonic synthesizer. The first prototype is already running and I am currently waiting for my first PCB to be manufactured in order to proceed with the firmware development and testing. I intend to release the CPU board as Open Hardware/Open Software project, but this as soon as I have completed the system.
The system is intended to be highly customizable, so it will be possible to adapt it to DIY monosynths (like a DIY Minimoog Voyager) as well as to polysynths (like a DIY Prophet5 or similar). The CPU hardware is laid out to support something like 80 analog inputs for frontpanel controls, more than 80 digital ins (buttons, encoders etc.), 90 analog outs for CV's to the voice boards and more than 80 digital outs for indicators and routing switches. Of course it will have full midi implementation and LCD Display support as well.
Oh, and I forgot, it is supposed to be a lowcost module, 200Euros max would be the target cost.

Now to my question: Is anybody here aware of a DIY Voice card which holds all required stages necessary to create a monosynth, like it has been used in the Prophets or the OB's? (see attached blockdiagram for the required stages).
SSM had something like that a zillion years ago:
http://www.emulatorarchive.com/Synthesizers/SSM/SSMVC/ssmvc.html

I would for sure like to adapt my CPU voltage levels to something already existing instead of designing my own voice card. I have already been thinking about adapting it to the Doepfer DIY Synth card but this one requires all the routing switches to be added on a separate board, which comes almost to the same as developing an all new card.

Maybe there's also somebody here who would like to develop such a card, as I know that there are some masters of analog circuit design here in this forum!

What I also need to know would be the standard control voltages used on the most common synth modules. I know that the control voltages for the Prohet 5 voice boards were mostly positive railed, since this reduces the part count considerably, but since I would like to keep my module a bit general, what would I really need?
Bear in mind that I would like to keep it as lowcost as possible too, so just the unavoidable features please!

Hoping to get some good input,
Cheers,
Obsoletetechnology

www.obsoletetechnology.wordpress.com

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Voice Card_2.pdf
Description:	Voice Card Blockdiagram	Download (listen)
Filename:	Voice Card_2.pdf
Filesize:	9.25 KB
Downloaded:	222 Time(s)

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[ru_klo]

Here are some ideas,
1) is to use a ready made synth and adapt it to your project, for example
like this from MFOS
http://www.musicfromouterspace.com/mfosweb/navigate.action?MAINTAB=SYNTHDIY&PROJARG=SOUNDLABMINISYNTH/page1.html&CATPARTNO=SLMS001PCB

2) build from the CEM3396 chip (in the datasheet is a circuit to start from)
PRO: few chips and you have a synth which is controlled mostly with a CPU
CON: CEM3396 is obsolete

[Neil Johnson]

Hi,

Interesting post. It seems you're trying to do what many others have already done, for example:

http://members.cox.net/ebrombaugh1/synth/index.html

http://www.milton.arachsys.com/nj71/forums.html?menu=2&submenu=4&subsubmenu=4

http://www.keyparts.co.uk/news/52-open-synthesizer-firmware-project

...and so on.

A few observations if I may:

It looks like your board will have more than 320 connections to the outside world. Way too many. The cost and complexity of interconnect will be huge. If you look at typical synth front panels there will either be a separate front panel microcontroller (modern) or some analogue multiplexing locally, so far fewer signals to/from the control board

Building a polysynth is all about keeping the cost of the voice cards down, as every dollar spent on the voice card design actually adds N dollars (for N voice cards) to the overall costs. Granted, in the early days the only way to control the voice cards was with analogue CVs, but then these were a limiting factor. Nowadays the better solution is to have a local CPU on each voice card that the control processor sends commands to over a high-speed serial bus, e.g. SPI (fast) or RS-485 (twisted-pair so lower noise). Are you really expecting to carry 90 analogue signals and lets say 40 digital signals (don't forget some controls will be on/off) to all voice cards? Assuming you include enough grounds that's going to be 260-wide ribbon cable and very expensive connectors.

Writing open firmware is tricky. Very tricky. See what has been done on the Arduino platform, and try to gauge how much effort that took.

For such a huge platform that you are developing I think your price is both optimistic, and too high for potential customers. My own custom board (the Siel one) cost far less than your solution, is custom to my specific needs, and does things which your board would not.

But, that said, it will be interesting to see what you have produced. When, and where, do you plan to make the design information online?

Cheers,
Neil

[Obsoletetechnology]

ru_klo, Thanks for your recommendations, I have already looked into the minisynth before and i'll keep it in mind. For the CEM based solution, that way too obsolete component for a new design but thanks for the idea anyway.

Neil Johnson, Thanks also for your remarks, I have been twisting my mind around exactly these questions before I started the design and considered most of your observations.
Fortunately, since I plan to make this for my own enjoyment only, I am free to implement whatever I like. I do not have in mind to create a 'Synth Arduino' suitable for everybody and in all flavors. If afterwards somebody likes what I did, well, even better but that's not the primary goal.
Also, rest assured that I have implemented SPI towards all peripherals in order to reduce the wire count.

I'll post some picture once I've populated my proto-pcb.

Regarding my question related to the voice boards, I think I'll try to contact adamstan, who seems to have good experience on this subject.

Regards,
Obsoletetechnology

[adamstan]

Well, my control board has only 32 analogue inputs for front panel knobs and 32 analogue outputs (half of them are EG outs). It supports up to 64 buttons, but they are scanned by separate processor. It's much smaller than what you are planning.