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Low cost analog to MIDI convertor
In the summer of 1996 I developed an inexpensive 8-channel analog to MIDI convertor. It was designed to process (slowly changing) voltages between 0 and 5 volt. The apparatus continuously samples these signals and outputs them as 8 MIDI controller values (between 0 and 127). It can be build for around $30!

With some patience, converter and power supply can be built together on a small piece of prototype-board. The eurocard, shown on the right, measures 10 x 16 cm.

Below, the complete design is shown, so you can build one yourself. It will take you some days of soldering and testing, and I think you will need an oscilloscope, a (digital) voltmeter and some datasheet-books. 		picture of my prototype
This apparatus has some disadvantages. Because it does not contain a microprocessor (which makes it inexpensive and easy to build), it is not programmable! It keeps sending MIDI all the time (even when controllers do not actually change). This may require a lot of your MIDI-resources in case you select a high samplerate.

I have built in some switches to slow down the MIDI-transmission rate, because some Macintosh software (like MAX) is not capable of processing 480 controller messages per second (MAX may eventually crash with the message: Spurious Interrupt).

Wiring:


At powering on, several weird MIDI-bytes may be sent. But after that, the analog-to-MIDI-convertor gets into the right rythm and everything goes all right (study the time-diagram below). For most applications these few nonsense-bytes at startup will not be a problem. However, this can be easily solved by using the (still unused) clock-inhibit-input of the 74LS165 for example and a simple condensator.
Timing:

Tips:

First build the crystal-oscillator and test it. Then attach the clock-dividers and measure (or listen to) their frequencies. Then connect the shift-register and other control-logic and watch the signals on an oscilloscope. When you have a MIDI-computer at your disposal, you can now already inspect the MIDI-output! Finally insert the ADC-convertor and give it some analog signals between 0 and 5 volt to process and watch the MIDI.

Good luck with soldering!

© Pieter Suurmond, 1996