MC-202 MIDI (warning - long, technical, yet inept post follows)

[Sean Costello]

Andrew Schrock wrote:

> I've been debating this as well... Frankly there just isn't enough space
> inside the 202 for the internal midi->cv to be accomplished. (maybe if you
> removed the sequencing/digital board, but that would be a real shame)
> Blacet Research has a midi->din24 sync chip which (+ a few discretes)
> would _probably_ fit inside. I was seriously considering changing 1 of the
> sync outs to a midi in, putting this mod in, and thus having an automatic
> internal midi sync. This option also has the nice feature of not ripping
> up your mc-202's case at all, and is VERY VERY easy to do. (2 resistors, 2
> caps and a xtal are all the other hardware you need, i think.. check
> metro.net/blacet/music.html)

My idea is similar to Blacet's idea: use a PIC chip, or some other
little microprocessor (I know nothing about PICs, and am not sure what
they are capable of).  However, instead of having it control simply
DIN/Sync, use it to control the synth section itself.  If you design it
right, I could see a MIDI modification for a 202 consisting of nothing
more than an optoisolator (on, say, one of the Sync out jacks, to allow
it to receive MIDI in), a PIC or other uP, a crystal, and a few
resistors and capacitors. 

The key is having the new uP directly access the same areas of the synth
that the original uP does.  For reference, go to
http://www.synthfool.com/mc202digital.gif and check out the digital
section of the MC-202.  From what I can tell, the relevant information
is on the right side of the diagram - the little arrows marked 1 through
18, and the uP pins they indicate.  A quick summary as I see them:

-Pins 50 and 49 of the uP control the S/H.  By sending different bit
combinations, the S/H switches between receiving voltages from the
output of the D/A for either the internal or external synth.  I presume
that the S/H has two channels, one for the internal synth, one for the
external output channel.
-Pins 43 through 48 lead to the D/A.  A 6-bit D/A, apparently (much like
the 303).
-Pin 16 leads to the A/D comparator out.  Don't know what this is for.
-Pin 37 leads to the on/off switch for the ext. synth cv portamento.
-Pin 38 leads to the on/off switch for the built in synth portamento.
-Pin 39 controls the on/off of the vcf accent (Aha! This is seperate
from the VCA accent!)
-Pin 40 controls the on/off of the VCA accent.
-Pin 34 controls the gate of the built-in synth.
-Pin 33 controls the gate of the external synth channel.

My idea would use an additional uP to directly access these traces on
the main board.  By doing so, you could control the internal and
external channels of the 202, as well as having direct control over
portamento, VCA accent, and filter accent.  

Of course, this is not an easy task.  I don't know jack about uP's, and
wouldn't know how to program one to convert MIDI into the note
information needed.  Besides dealing with the above pins, the uP would
also have to somehow prevent the MC-202's own uP from spitting data out
at the same time data information is being received.  Additionally, it
would be nice if the new uP also converted the MIDI clock into DIN Sync
info.

Still, I think it is doable.  And I think the results would be VERY
worthwhile. Turning the MC-202 into a 2-channel MIDI/CV convertor, with
a nice built-in synth and DIN Sync output, would be very useful for
anyone who wants to perform music live.

> Remember, enterprising 202 people: you can also sync the 202 with a tape
> click track! (like that output from an mmt-8 and many drum machines) You
> may have all you need to sync right now! (you mentioned using an mmt-8
> before.. you could do a live acid/techno performance with just the 202, an
> mmt-8, and a sampler.)

I've tried that; it didn't work.  I can see that it might, but it
wouldn't work for my applications, as I need that TAPE/SYNC IN jack to
load in new sequences after each song (sigh). 

Sean Costello