Re: MIDI Controller Basics
By Andy Latorre
Ed Gloeggler, in the 11/20 issues of MMI wanted to know about the MIDI to note conversion process. I have gone down this road and will be happy to detail the process. I am going to make it very simple (I hope) since Ed is not familiar with computers. My MIDI-to-note converter was designed specifically for use in the Fairground Organs that I build. Basically it takes the MIDI data, examines it for channel 10 and 11 and sends the data out to the percussion instruments: 25-not glock on channel 11 and the eight traps (snare drum, cymbal, etc) on channel 10. It ignores all other channels.
The process starts by the detection of the MIDI stream of bits. The data comes into the controller in a serial fashion: one bit at a time. It is done very fast: 31,250 bits per second. I have used a special UART chip to catch the data and convert it to an 8-bit word. This chip uses a FIFO memory element (first in-first out) so data is not lost while the computer spends time trying to analyze the data.
The data that comes in, I sometimes equate it to the catching of bullets from a machine gun, is sorted out and stored. This data stream contains the following information about the note being played:
1- whether the note is ON or OFF
2- what note number it is (#1 to 127)
3- what channel it is assigned to
4- the volume level of the note (a number from 1 to 127)
Now, the controller, made up of a microprocessor circuit with memory and input and output ports, makes decisions as to what to do with the data. If it is for the traps (channel 10) it sends the signal to the correct instrument (snare drum, block, triangle, etc.) If it is for the glockenspiel (channel 11) then it sends a signal to one out of 25 wires to the glock.
This signal is weak because it comes directly from an input/output chip. So it does not have enough current drive to actuate any type of power device. It must be amplified in some way. A switching transistor can be used to actuate the device (a solenoid in my case).
In order to do all this, the microprocessor must be told what to do by a program which is stored in nonvolatile memory (EPROM for example). It is like taking the hand of a student and bringing him to each station and telling him what to do. The advantage of the microprocessor (over a student) is that all this decision-making occurs at lightening speed. It has to, otherwise data might be lost. And if you lose data your notes might not go on, or worse, might cipher (stuck ON).
In my controller design, I have used additional features such as glock, snare, block reiteration, and other features designed specifically for my MIDI-controlled Fairground Organs.
So, Ed, how is that? My controller is available for sale but I do not push it. I have to charge too much since I am not in the business of selling controllers. But there are real reasonably-priced units on the market. You might look into the MTP-1 which was discussed a week or so back on the digest.
Andy LaTorre
LaTorre Orgue Mecanique
Cullowhee, NC USA |
(Message sent Thu 21 Nov 1996, 15:37:57 GMT, from time zone GMT-0500.) |
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