[sdiy] CC-Modular

[xmurz at gmx.de]

I woulnd't start doing all LFO's and EG's in a pic, this would make little sense to me,
but you could stick a couple LFO's and EG's in the Pic aditionally to some analog ones
if you got some processing power left.

Anyways, that's the way I'd do it.
The PCI idea is also very slick. Though that would mean you'd have to write drivers for
different operating systems and the card would have to be well shielded. (or only for one
operating system if you want once and only for you..)
As you need so many ins and outs you will end up with an external box anyways.
OR you could do a custom PC chasis with additional linear PSU and stick all stuff
in one PC  custom chasis and have the posts on the front panel of the PC.
You can do that with a 19" 4U chasis that you build youself.
A small pentium would be sufficient, so you could do without a fan (->no noise)
and a zip drive to boot (which goes quet after transfer).
You would have to shield the analog circuitry pretty good, too.

well, just some brainstorming.

- Hans

08.09.02 03:25:49, Seb Francis <seb at is-uk.com> wrote:

>Hi,
>
>It's been interesting reading the discussions about the computer controlled Walsh Module.  I've had a dream for a long time now to build a complete modular synth system where it's possible to save 
and recall patches instantly.  Ok, so it's good to always make new sounds with every new piece of music, but sometimes it's nice to be able to call up and re-use an old sound without having to 
manually copy it from a patch sheet (to say nothing of having to manually "store" the sound on a patch sheet in the first place!)
>
>Being a relative beginner in the world of analog audio electronics I'd like to share my ideas, invite comments, and even ask a few questions to some of the experts on this list :)
>
>
>Design goals:
>- The synth must sound analog (so digital modelling is not a solution)
>- The synth must be as flexible and extensible as a normal modular
>- Electrically compatible with other common modular systems (e.g. MOTM, etc..)
>- Completely "saveable" patches
>- PC based graphical patch editing
>- MIDI control of every parameter
>- Physical control of every parameter
>- The synth must sound good (did I mention sound already .. it's important, otherwise I'd just buy a NORD modular! ;)
>
>
>Overview:
>(see also picture: http://burnit.co.uk:8888/overview.gif )
>
>Clearly no pots can be used, so physical control would be done from a digital device.  I plan to use some nice quality (but not too expensive) ALPS continuous rotary encoders.  The ones I'm 
thinking of have 24 sine pules per rotion, and because they are sine output you can derive much more detail than 24 notches per rotation.  Each "pot" would send pairs of MIDI controller messages 
(14 bits) and have a dispay as you turned it to show the exact digital value.  Exactly how many turns make up the full value scale depends on how fast you turn the knob (i.e. turn slowly for precise 
control, faster for bigger jumps).  When a "patch" is loaded on the PC, the software would send MIDI CC messages to initialise the value of each "pot".  Names for each pot would also be sent by 
SysEx, so you get the name displayed as you turn each control.
>
>So far, so good.  This whole controller could be build around a PIC or other microcontroller and a few auxiliary components.
>
>Also no problem is the PC software (softare I can do!) - some nice GUI app built with something quick like Delphi, working in a similar way to existing "virtual" modular synths.  No need to worry 
about special hardware drivers since MIDI is the method of communication.
>
>But now it comes to the tricky part .. the interface between computer and analog modules.  Every analog module must have no pots or switches (or at least if it does have pots they must be kept at 
the same position - I'm not about to start mounting motors in the analog modules!)  So every parameter must be controlled by CV, and every input must be able to be "patched" from every appropriate 
output.  In addition control voltages generated from the 14bit MIDI CC data must be able to be patched to every appropriate input.
>
>My original idea (see the picture) was to implement this literally - e.g. 32 MIDI CC -> CV converters, and a 64 input x 64 output digitally controlled analog switch matrix (even this amount of CVs and 
switches isn't enough for a very large modular).  Then I started working out how many chips I'd need for the switch matrix (*4096* 16-1 analog multiplexers!) and I realised why there isn't anything like 
this made already.
>
>To try and solve this without compromising the design goals too much I considered 2 things:
>
>(1) Limit the flexibility in ways which would not matter (e.g. mixer outputs never need to connect to mixer inputs, don't need to use noise for things like pulse width control, etc..)
>- It would be hard to do this with an ever extending modular, since the digital circuit design needs to have some idea what inputs can be patched to what outputs.
>- Actually when you look at it, there are not so many outputs/input combinations which might not give some nice synthesis possibility.
>
>(2) Move some of the synthesis functionality into the digital domain (i.e. the microcontroller).  LFOs and envelope generators seem the obvious choice.  It seems unlikely this will compromise the 
sound too much, and neither of these modules generally need input from any other analog modules (except from other LFOs and env gens).  A large number of cross-patchable LFOs and envelope 
generates could be programmed, limited only by the microcontroller speed and how many CV outputs are needed.
>
>I believe a combination of these 2 approaches will make this idea a practical proposition, but I am open to other ideas :)
>
>
>And now I come to my questions ..
>
>Will digital LFOs and env gens sound as good as analog?  I can't think of any reason why they shouldn't, but maybe I've overlooked something?
>
>I want to make it electrically compatible with as wide a range of modern modular systems as possible:
>What are the generally used voltage ranges and current levels?
>Analog switches that handle 0->15V (or -7.5->7.5V) signals are quite a lot cheaper than switches that can handle -15->15V - I wonder if I can get away with using all, or at least some, 0->15V (-7.5->
7.5V) ones.
>How much current must each switch and digitally generated CV signal be able to handle?
>
>How much effect do you think the choice of analog switch will have on the sound - could I get away with using really cheap things like HEF4067 (datasheet says 0.04% sine distortion, but no 
mention of THD), or perhaps a mixture of HEF4067 for CVs and some more expensive (e.g. ADG428) switches for the audio signals?
>
>Could I get away with cheaper 12bit DACs (~3.7mV per click on a 15V scale), or would 14bit DACs (~0.9mV per click) be noticeably better for things like LFO and Envelope ouputs?
>
>Is it ever needed to patch an output into multiple inputs in a modular synth?  Or is this electrically not so good?
>
>Am I mad at even trying to attempt this?
>
>
>Thanks in advance for any replies.  Actually it's been good just writing my ideas down in pen and ink (or do I mean 0 and 1s;)
>Seb
>
>
>