[sdiy] passive MIDI merge?

[Andre Majorel]

On 2001-10-31 15:24 -0800, Simon Gatrall wrote:

> I have what I think should be a straight-forward MIDI problem, but I 
> can't find a good solution.  I have 13 MIDI devices in my studio and 
> one Opcode Studio 4 interface (8 in/out).  I'd like to ba able to 
> hook all of the external MIDI equipment to my computer with both the 
> MIDI in's and out's.  If I didn't care about sending MIDI back from 
> every device to the computer, I could hook everything up just fine 
> using the thru's that some of my equipment has.  However, I like to 
> be able to save patches and use the controllers on all of my gear, 
> not just selected pieces.  Given that problem, I can think of a few 
> options:
> 
> 1) Buy another multiport MIDI interface (I just saw a used Studio 4 for $150)
> 2) Buy a MIDI merge unit that can handle at least 6 inputs. (around $200 new)
> 3) Build my own simple MIDI merge unit.
> 4) Sell all my MIDI gear and go all [software/analog] (take your pick);)
> 5) Build/buy a MIDI patchbay.
> 
> I think there must be a simple solution for #3 because I don't need 
> an active merge.  There are two people at most in my studio at any 
> given point, and we aren't both playing/tweaking knobs at the same 
> instant.  Isn't there some simple way to combine MIDI signals if you 
> assume that no two pieces of gear will be transmitting at the same 
> time?

I doubt it. MIDI is a current loop.

You could or together the outputs of the opto isolators but
since the encoding is NRZ, that wouldn't work unless you invert
them first.

Maybe this ? This is completely untested and my EE is rusty at
best.

                                   _______                     ^
   MIDI IN 1                       \      \                    |
                 2 _______ 4  |\    \      \                   <
   5 o--------+---|       |---| >o---|      \              220 >
              |   |       |   |/     |       |                 <
        1N914 v   | 6N138 |          |       |                 >
              -   |       |          |       |                 |
        220   |  1|       |5         |       |                  --o 4
   4 o--^v^v--+---|_______|---       |       |
                             |       |       |
                            ===      |        \    |\      220         MIDI
                                     |         >---| >o---^v^v^---o 5
   MIDI IN 2                  ... ---|        /    |/                  OUT
                                     |       |
   MIDI IN 3                  ... ---|       |
                                     :       :                  --o 2
   ...                                                         |
                                     :       :                 |
   MIDI IN 8                  ... ---|      /                 ===
                                    /      /
                                   /______/

I can't find an 8-input or gate in my TTL book but you can
make a "tree" of smaller or gates.

Come to think of it, this is never going to work. A MIDI output
is never idle because MIDI devices are supposed to send an
active sensing message every 300 ms and these messages are
fatally going to collide with each other and with your
controller messages and SYSEX. At best, transmission is going to
be unreliable. At worst, it could upset the MIDI device on the
receiving end. I'm afraid there's no sane way to merge MIDI
inputs at the electrical level.

-- 
André Majorel <amajorel at teaser.fr>
http://www.teaser.fr/~amajorel/