MIDI consists of both a simple hardware interface, and a more elaborate transmission protocol.

Hardware

MIDI is an asynchronous serial interface. The baud rate is 31.25 Kbaud (+/- 1%). There is 1 start bit, 8 data bits, and 1 stop bit (ie, 10 bits total), for a period of 320 microseconds per serial byte.

The MIDI circuit is current loop, 5 mA. Logic 0 is current ON. One output drives one (and only one) input. To avoid grounding loops and subsequent data errors, the input is opto-isolated. It requires less than 5 mA to turn on.

The standard connector used for MIDI is a 5 pin DIN. Separate jacks (and cable runs) are used for input and output, clearly marked on a given device (ie, the MIDI IN and OUT are two separate DIN female panel mount jacks). 50 feet is the recommended maximum cable length. Cables are shielded twisted pair, with the shield connecting pin 2 at both ends

Messages

The MIDI protocol is made up of messages. A message consists of a string (i.e., series) of 8-bit bytes. MIDI has many such defined messages. Some messages consist of only 1 byte. Other messages have 2 bytes. Still others have 3 bytes.

One type of MIDI message can even have an unlimited number of bytes. The one thing that all messages have in common is that the first byte of the message is the Status byte. This is a special byte because it's the only byte that has bit #7 set. Any other following bytes in that message will not have bit #7 set.

The Status bytes of 0x80 to 0xEF are for messages that can be broadcast on any one of the 16 MIDI channels. Because of this, these are called Voice messages. For these Status bytes, you break up the 8-bit byte into 2 4-bit nibbles.

The high nibble tells you what type of MIDI message this is. Here are the possible values for the high nibble, and what type of Voice Category message each represents:

• 8 = Note Off - Indicates that a particular note should be released. Essentially, this means that the note stops sounding, but some patches might have a long VCA release time that needs to slowly fade the sound out.
• 9 = Note On - Indicates that a particular note should be played. Essentially, this means that the note starts sounding, but some patches might have a long VCA attack time that needs to slowly fade the sound in.
• A = AfterTouch (ie, key pressure) - The Aftertouch message conveys the amount of pressure on a key at a given point. Since the musician can be continually varying his pressure, devices that generate Aftertouch typically send out many such messages while the musician is varying his pressure.
• B = Control Change - Sets a particular controller's value. A controller is any switch, slider, knob, etc, that implements some function (usually) other than sounding or stopping notes (ie, which are the jobs of the Note On and Note Off messages respectively). There are 128 possible controllers on a MIDI device. These are numbered from 0 to 127.
• C = Program (patch) change - To cause the MIDI device to change to a particular Program (which some devices refer to as Patch, or Instrument, or Preset, or whatever). Most sound modules have a variety of instrumental sounds, such as Piano, and Guitar, and Trumpet, and Flute, etc. Each one of these instruments is contained in a Program. So, changing the Program changes the instrumental sound that the MIDI device uses when it plays Note On messages.
• D = Channel Pressure - The Channel Pressure message conveys the amount of overall pressure on the keys at a given point. Since the musician can be continually varying his pressure, devices that generate Channel Pressure typically send out many such messages while the musician is varying his pressure.
• E = Pitch Wheel- To set the Pitch Wheel value. The pitch wheel is used to slide a note's pitch up or down in cents (ie, fractions of a half-step). If the device is a MultiTimbral unit, then each one of its Parts may respond differently (or not at all) to Pitch Wheel. The Part affected by a particular Pitch Wheel message is the one assigned to the message's MIDI channel.