COURSE NOTES 9
MIDI

MIDI

• Musical Instrument Digital Interface

• A communications protocol created for interfacing synthesizers and other electronic musical instruments including computers

MIDI Hardware

• Typical synthesizer or other computer music instrument contains:
  - performance controllers
  - front panel controls
  - microprocessor
  - memory
  - voice circuitry
  - MIDI in, out, thru ports

• Hardware Specs
  - 31.25 kBaud
  - Asynchronous transfer
  - Start bit/8 data bits/Stop bit
  - DIN 5 pin connector
  - MIDI cable, maximum length 50 ft
  - MIDI messages travel in only one direction along MIDI cable (not bidirectional)

MIDI In, Out, Thru Ports

• MIDI IN
  Allows a device to receive MIDI messages for its microprocessor to execute

• MIDI OUT
  Used to transmit MIDI messages from the device's microprocessor to another device

• MIDI THRU
  Used to pass an unaltered copy of MIDI IN to another device

MIDI Configurations/Devices

• Master/slave

• Multiple slaves
  - daisy-chaining (using THRU ports)
  - tree branching (using a MIDI THRU box)

MIDI Channels

• 16 MIDI Channels

• Possible to transmit as many as 16 independent performances over a single MIDI cable

• MIDI instruments can be set to receive messages on specific channel(s) to play only specific data on the MIDI data cable

MIDI Terms

• Monophonic
  Only one note can be assigned to play on one MIDI channel

• Polyphonic
  More than one note can be assigned to play on one MIDI channel

• Omni On
  Ability to interpret messages on all MIDI channels

• Omni Off
  Ability to interpret messages on a single MIDI channel

MIDI Byte Formats

• Status bytes - contains commands: the most significant bit (bit 7) of a status byte is always set to 1

• Data bytes - the most significant bit (bit 7) of a data byte is always set to 0 (data values 0-127)

MIDI Message Formats (see handout for detail)

• Channel Voice Messages
  voice commands for MIDI channel(s)

• Channel Mode Messages
  commands for polyphonic/monophonic/omni response

• System Common Messages
  commands for all MIDI channels

• System Real-Time Messages
  high-priority messages for all MIDI channels

• System Exclusive Messages
  commands for specific MIDI devices only

Channel Voice Messages

1xxxnnnn 0ddddddd 0ddddddd

xxx = status type
nnnn = channel number
ddddddd = data value

• Note Off

• Note On

• Polyphonic Key Pressure

• Control Change

• Program Change

• Channel Pressure

• Pitch Wheel Change

Channel Mode Messages

1011nnnn 01111xxx 0ddddddd

nnnn = channel number
xxx = controller ID
ddddddd = data value (optional 2nd data value)

• Reset All Controllers

• Local Control

• All Notes Off

• Omni Mode Off

• Omni Mode On

• Mono Mode On

• Poly Mode On

System Common Messages

11110xxx 0ddddddd 0ddddddd

xxx = system common type
ddddddd = data value (optional 2nd data value)

• MTC Quarter Frame

• Song Position Pointer

• Song Select

• Tune Request

• End of Exclusive (EOX)

System RealTime Messages

11111xxx

xxx = system realtime type

• Timing Clock

• Start

• Continue

• Stop

• Active Sensing

• System Reset

System Exclusive Messages

11110000 0ddddddd ............. 11110111
11110000 00000000 0ddddddd 0ddddddd .............. 111101111

dddddddd = manufacturer ID (possibly two bytes)

• Used to define special commands that only particular hardware devices can understand and execute

Writing MIDI Software

• Messages arrive and transmit in the following formats:
  - normal format
  - running status format
  - prioritized format (for system realtime messages)

• Running Status
  Redundant status bytes are not retransmitted for each command.
  For example: if three consecutive MIDI messages have the same status byte, the status byte is transmitted with the first MIDI message only.

• Prioritized
  Can interrupt any other MIDI message
  Example: MIDI Clock messages are generated 96 times for each whole note length. Must be sent without delay in order to keep MIDI devices in sync.

Parsing MIDI Data

• Every incoming byte must be identified as either a MIDI status byte or a data byte.

• Use a status buffer variable to hold the value of the last received channel voice or channel mode status byte, with the following exceptions:
  - the buffer should be cleared on startup or whenever a system common, system exclusive or ignored channel message status is received
  - the buffer should be left unaltered whenever any system realtime message is received

• The parsing routine can use the value of the status buffer to select the appropriate message processing routines.

• The program should ignore all incoming bytes while the status buffer value is zero.

• Use a counter variable to keep track of the order of received bytes. It should be cleared whenever a new, recognized status is received. It should be incremented by one each time a subsequent data byte is received, starting with the first data byte, and set to 1 when the final byte of the message is received.

• The counter should not be altered if a system realtime status byte is received.

• Processing routines for each recognized message can use the counter value to determine which byte of a message they are currently processing.

Standard MIDI Files ( see handout example)

• Consists of 2 types of "chunks"" header and track chunks

• Three defined MIDI File Formats
  - 0: one header, one track
  - 1: one header, one or more tracks
  - 2: one header, independent tracks

• Header chunk must come first

MIDI File Meta Events

• FF 58 04 nn dd cc bb
  - nn,dd: time signature
  - cc: number of MIDI clocks per beat
  - bb: number of 32nd notes per MIDI quarter note (24 clocks)

• FF 59 02 sf mi
  - sf: number of sharps/flats in key (0 = note, 1 = 1 sharp, -1 = 1 flat, etc.)
  - mi: major/minor key (0=major, 1=minor)

• FF 2F 00
  end of track

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