Adding MIDI to a Schober organ
by Peter A. Stark
This page discusses MIDI in general, and adding
MIDI to a Schober organ in particular. But before you get carried away,
let me start by saying that adding MIDI to a Schober organ is difficult,
and in many cases may not be worth the effort. So here goes:
With sheet music, you need three things - someone to write or generate the sheet music, the sheet itself, and someone to perform it on an actual instrument. Same in MIDI - you need some device to generate the MIDI signal (which is most often a keyboard, but can also be a computer), the MIDI connection itself, and something to "play" the MIDI signal and generate the actual sound (which is most often done by an electronic device called a sound module or sound card.) Just as a live performer can choose how to play from sheet music, so the sound module can change the MIDI information by playing it on a different instrument, or in a different key. (In some situations, MIDI may even be used to control other devices, such as stage lighting.)
The MIDI connection itself is on two wires, but
generally uses a 5-pin round DIN connector, of which only two pins are
actually used. In technical terms, the connection uses a current loop driving
an optical coupler at the receiving end; in lay terms, this simply means
that it is a special kind of connection different from most other computer
connections, used specifically because it avoids grounding problems in
complex audio setups. Although most modern computers can send and receive
MIDI, they are not electrically compatible, and so need a small adapter
to connect to the 5-pin connector.
Generating a MIDI OUT signal is not trivial. For example, each time you press a key on the keyboard, the MIDI circuit has to send out thirty binary bits (a bit is either a 0 or 1) in a particular pattern called a "note-on message", and at an exact timing. Later, when you release the key, a similar set of thirty bits (the "note-off message") is sent out. Play a three-note chord, and you're talking about 90 bits (three times 30) when you start, and another 90 when you release it.
Most of the inexpensive keyboards with MIDI OUT provide it strictly as an extra feature which most buyers may not even use. These keyboards contain their own sound generation circuits (called "sound modules"), so people play the keyboard and hear the sound directly out of its speakers.
Sequencer programs for PC computers are quite inexpensive and powerful. If the computer has a sound card, it can even play the output directly without sending it out the MIDI OUT to an external device.
The idea here is to add an interface circuit, which connects to each keyboard switch (and, hopefully, to other switches, such as stop switches, as well), and generates the appropriate note-on and note-off signals (and perhaps other control signals as well) in response.
Most large organs generate DC voltage outputs from their keyboards, which are then sent to separate sound generators or even to pipe valve magnets. These DC signals can be tapped off and sent to a MIDI OUT controller; conversely, MIDI IN circuits can receive MIDI signals and send out DC control signals that operate the organ as if they had come from the keyboard.
On the other hand, Schober organs (as well as many other home organs) key audio directly -- both their keyboards, as well as their stop switches, coupler switches, etc. are directly connected to audio lines. There is no DC control voltage that can be tapped off to control MIDI circuitry.
Schober keyboard contacts are also connected backward from what we need. In simple terms, what we need for a 61-key MIDI keyboard is a one-to-many connection, where one DC voltage entering the keyboard can be sent to 61 different output wires, one for each note. But Schober (and other home organs) use a many-to-one connection, where 61 different signals coming into the keyboard are switched into one audio output wire as needed.
The gist of this is that adding a MIDI OUT to a Schober organ requires a complete rewiring of the keyboard from a many-to-one connection into a one-to-many, which makes it completely incompatible with the normal Schober sound generation circuits.
So there are very few ways of adding a MIDI OUT to a Schober:
1. Completely gut the organ, get rid of all Schober sound generation circuits, and go for just MIDI. You can reuse the keyboards, pedals, and other controls, but are then totally committed to using MIDI sound modules or computers to generate all sounds. This is the approach that I am currently using; it is documented here.
2. Rewire the keyboards to use Devtronix keying (see later), and then add MIDI to that.
3. Rewire only one of the keyboards to
MIDI, and keep the Schober sounds in the other keyboard. But it may be
easier to just mount a cheap Casio keyboard somewhere nearby and play it
4. Schober had two addons to their organs - the Dynabeat, and the Percussion Group for the Theatre Organ - which required an extra set of contacts to be added at the front of a keyboard to control these addons. If you have those contacts, then you could use them to add MIDI to that keyboard. My recollection is that they did not cover the entire 61-note compass of the keyboard, but that is probably not that significant. The photo at the right shows what they looked like. (But ... let me add that a friend has experimented with this approach, and found that it does not work well at all. These extra contacts are intermittent, and they are also difficult to adjust so that all the keys require the same amount of movement to play.)
(There may be another approach. I thought
about it once, and discarded the idea as being too messy, but this same
friend has recently brought it up again. It may be possible to leave
the original Schober wiring as is, but detect the pressing of a key by
looking at either the signal level or the impedance at a switch contact.
Some additional circuitry would be needed to 'massage' that signal to key
the MIDI circuits, and I'm just bringing it up for your consideration in
case you have masochistic tendencies!)
The Devtronix boards were actually a very good solution, and a number of Schober owners have already done that conversion. If this includes you, then you are in luck -- adding MIDI OUT would be a cinch.