About MIDI and the MIDI Specification
MIDI stands for Musical Instrument Digital
Interface. It is a protocol which allows devices
to communicate with each other.
Harmony Central
has a excellent group of free web resources regarding MIDI. The actual MIDI
Specification is not available for free download. It is sold for
approx $50 (US) by the
MIDI Manufacturers Association. When
I designed the MIDI drum machine, I obtained the MIDI 1.0 Specification from
appendix A of "The MIDI Manual", by David Miles Huber, SAMS 1991,
ISBN 0-672-22757-6, which Rod checked out from the
Oregon State University Library.
Several years later I purchased a used copy of this book at
Powells for $16.
Hardware Details
MIDI communication is async serial, 8-N-1 format at 31250 baud, using a
opto-isolated current loop circuit. The
optical isolation prevents the interconnection of digital grounds
between devices, which (usually) have their analog circuit grounds
connected together at an audio signal mixer. Each MIDI cable carries
communication in only a single direction. Devices need separate ports
for input and output, and a "Thru" is usually provided which
is an exact copy of the input, so that multiple devices can be easily
connected to receive the same messages.
Messages
MIDI communication consists of messages, which are usually short
groups of bytes, where the first byte has its Most Significant Bit (MSB)
set (128 to 255) and the following bytes of the message have their
MSB clear (0 to 127). Having the MSB dedicated to marking the
beginning of a message means that every byte can only carry 7 bits of
data. The MIDI Spec defines a number of different messages, but the
MIDI Drum Machine only uses a small subset of all the possible
messages.
The MIDI Drum Machine transmits these five messages:
- Note On
- Note Off
- Program Change
- Bank Select
- All Notes Off
The following sections document the actual bytes sent in each
message by this drum machine. These descriptions are not intended to
serve as a replacement for the MIDI Specification.
Note On
The Note On message is sent when the drum machine detects that the drummer
has struck a pad. The message is always transmitted as 3 bytes. MIDI's
running status feature (allowing 2 bytes instead of 3) is not used.
| Byte # | Bits | Function |
|---|
| 1 | 1001CCCC | C = Channel |
| 2 | 0NNNNNNN | N = Note |
| 3 | 0VVVVVVV | V = Velocity |
The Channel and Note bits in the first two bytes are copied directly
from the user's configuration for the pad that was struck. The
velocity bits are copied from the upper 7 bits of the code from the
A/D converter. The MIDI Spec suggests a log scale for velocity,
but the linear code is copied. Perhaps a conversion implemented as
a 256 byte lookup table would be an improvement?
Note Off
When the Note On message is sent, a software timer that corresponds to
the pad struck is started. The length of time set is controlled by
the pad's "sustain" parameter.
Later, when the timer expires, this
Note Off code is sent.
| Byte # | Bits | Function |
|---|
| 1 | 1000CCCC | C = Channel |
| 2 | 0NNNNNNN | N = Note |
| 3 | 0VVVVVVV | V = Velocity |
The Channel and Note bits in the first two bytes are copied directly
from the user's configuration. The user's configued "release"
parameter is copied into the Note Off velocity bits. With Rod's equipment,
this Note Off velocity didn't have any noticable effect.
When a pad is struck, the firmware
had ought to check corresponging timer
and if it is currently running, a Note Off message should be sent before
another Note On is sent. This simple check was not implemented in the
code, but it could easily be added.
Program Change
When the user assigns a particular sound to a pad, by changing the Voice
parameter on the front panel, the Program Change message is sent.
| Byte # | Bits | Function |
|---|
| 1 | 1001CCCC | C = Channel |
| 2 | 0PPPPPPP | P = Program (voice #) |
Bank Select
When the user select a particular bank of voices for a pad, by changing the
Bank parameter on the front panel, the Bank Select message is sent. This
is actually a vendor specific system exclusive message.
| Byte # | Bits | Function |
|---|
| 1 | 11110000 | Begin System Exclusive Message |
| 2 | 01000011 | . |
| 3 | 0001CCCC | C = Channel |
| 4 | 00010101 | . |
| 5 | 00000100 | . |
| 6 | 00000BBB | B = Bank |
| 7 | 11110111 | . |
All Notes Off
If the All Notes Off button is pressed, these two bytes are sent. Many
MIDI devices immediately turn off all notes when they receive these bytes.
While developing, this can be useful feature if a Note On message is sent
and no Note Off message turns it off.
| Byte # | Bits | Function |
|---|
| 1 | 11110000 | Begin System Exclusive Message |
| 2 | 11110001 | . |
The MIDI Drum Machine, Paul Stoffregen and Rod Seely.
Designed and constructed Fall, 1991. Project status: Complete.
http://www.pjrc.com/tech/midi-drums/midi.html
Last updated: November 28, 2003
-- These drum-machine web pages are still under construction --
Questions, Comments?? <paul@pjrc.com>
Checkout
MIDI Protocol Details
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