Chapter Three: MIDI

6. MIDI Data Format: Channel Mode Messages

Channel mode messages are intended to control the overall function of ALL voice channels of an instrument. The MIDI 1.0 Specification intended that instruments be designed to operate only under one mode at a time. Channel mode messages use controller numbers 120–127 and are sent on the receiving instrument's basic channel. An instrument's programmable parameter set should include a basic channel setting. This means that you may have a number of the same model of instrument set to different basic channels, paying attention only to the channel mode messages sent on their particular basic channel.

The channel mode messages fall into six types: OMNI, MONO/POLY, ALL NOTES OFF, LOCAL CONTROL, ALL SOUND OFF and RESET ALL CONTROLLERS. OMNI, if set ON, instructs all instrument voices to respond to any and all received channel voice messages regardless of what channel they are sent on. This would be equivalent to your TV set displaying all cable channels simultaneously.

MONO/POLY modes determine whether notes are played polyphonically, several at once (POLY), or monophonically, one at a time on each voice (MONO; see below). A new NOTE ON message in MONO mode ends the previous note and begins the new note. You may wish to set your synth to MONO mode if you want to use a patch with portamento (a continuous glide in pitch from one note to the next).

A channel mode message looks like this:
1011nnnn 0ccccccc 0vvvvvvv
n=basic channel, c=controller numbers 120–127, v=value (see the table below).

The full implementation is:

Status Byte Data Byte 1 Data Byte 2 Description
1011nnnn 01111000 (120) 00000000 (0) All Sound Off
1011nnnn 01111001 (121) 00000000 (0) Reset All Controllers
1011nnnn 01111010 (122) 00000000 (0) = off
01111111 (127) = on 		Local Control
1011nnnn 01111011 (123) 00000000 (0) All Notes OFF
1011nnnn 01111100 (124) 00000000 (0) Omni Mode OFF (all notes off as well)
1011nnnn 01111101 (125) 00000000 (0) Omni Mode ON (all notes off as well)
1011nnnn 01111110 (126) 0mmmmmmm
(m=number of channels)*		 Mono Mode ON (Poly Mode OFF, all notes off as well)
1011nnnn 01111111 (127) 00000000 (0) Poly Mode ON (Mono Mode OFF, all notes off as well)

*A special case: m=0 tells the instrument to assign its voices one per channel, starting with the basic channel and continuing up through channel 16 or until it runs out of voices.

There are four OMNI-MONO/POLY mode combinations:

Mode Omni POLY/MONO Result
1 ON POLY Messages are received from all channels and assigned to all voices
2 ON MONO Messages are received from all channels and control one voice monophonically
3 OFF POLY Messages are received only on a specified channel, but are assigned to all voices
4 OFF MONO Messages are received on a specified number of channels, starting with the basic channel, and each channel controls its own monophonic voice

Drawn for an instrument with four voices and its basic channel set to 1; the dot is where incoming notes are pooled and handed out to the voices.

Four panels showing how an instrument with four voices, basic channel 1, handles incoming MIDI in each channel mode. Mode 1, OMNI ON and POLY: all channels, 1 through 16, feed a single point that hands notes out to all four voices. Mode 2, OMNI ON and MONO: all channels feed that point, but it passes notes only to voice 1; voices 2 to 4 are grayed out. Mode 3, OMNI OFF and POLY: only channel 1, marked basic, is accepted, and its notes are handed out to all four voices; channels 2 through 16 are grayed out. Mode 4, OMNI OFF and MONO, with m equal to 4: channels 1 through 4 each connect straight to their own voice, channel 1 to voice 1, channel 2 to voice 2 and so on; channels 5 through 16 are grayed out.

Mode 4 is the only one that listens on several channels at once while keeping them apart, one voice per channel.

ALL NOTES OFF is useful for sequencing programs, where a missed NOTE OFF message may leave a note 'hanging,' sounding indefinitely. This is equivalent to the PANIC button on many synths and DAWs.

Update

The original 1983 specification reserved controllers 122–127 for channel mode messages. Two more were added later. ALL SOUND OFF (120) silences everything sounding on the channel at once, including release tails and notes held by the sustain pedal, which ALL NOTES OFF leaves ringing. RESET ALL CONTROLLERS (121) returns pitch bend, modulation, sustain and similar performance controllers to their defaults, though by convention not volume or pan. A PANIC button typically sends one or both along with ALL NOTES OFF.

LOCAL CONTROL (on/off) is designed to separate the MIDI-generating keyboard function of a synth/sampler from its sound-producing capability. This is useful when you want only notes returning from a computer to cause the instrument to sound, avoiding a common problem of doubled notes. With LOCAL CONTROL off, notes and other data are still sent to the MIDI OUT port.

The same setup in both panels, with MIDI thru turned on in the sequencer; only the LOCAL CONTROL setting changes.

Two panels, both with MIDI thru on in the sequencer. Each shows a synthesizer containing a keyboard and a sound engine joined by a switch labeled LOCAL; the keyboard also sends out the MIDI OUT jack to a MIDI sequencer, which sends the notes back in the MIDI IN jack to the sound engine. Top panel, LOCAL CONTROL ON: the switch is closed, so each key reaches the sound engine twice, directly and by way of the sequencer; the engine uses two voices per key and every note sounds doubled. Bottom panel, LOCAL CONTROL OFF: the switch is open, so notes reach the sound engine only through the sequencer; it uses one voice per key and every note sounds once.

Sequencers, including the MIDI tracks of every DAW, have a MIDI thru setting (Patch Thru in Digital Performer) that routes the notes you play on the keyboard back to the instrument. With LOCAL CONTROL on, each note then sounds twice, once locally and once returning from the sequencer, producing a sort of flanged sound; it also robs your instrument of half its polyphony, since each note you play uses two voices. Normally, LOCAL CONTROL is best set on the instrument itself, not through MIDI.

LOCAL CONTROL matters only for an instrument that is both a keyboard and a sound source, such as a synthesizer, workstation or digital piano. Many setups today pair a controller keyboard, which makes no sound of its own, with software instruments in a DAW. There, the same MIDI thru setting that doubles notes on a hardware synth is what lets you hear what you play at all, and there is no LOCAL CONTROL to turn off.

Update

Most current hardware synths and virtual instruments are multitimbral: each of the 16 channels plays its own sound, polyphonically. This arrangement, often called Multi mode, is one the four original modes did not anticipate, and many instruments simply ignore OMNI and MONO/POLY messages. The one-voice-per-channel idea behind Mode 4 lives on in MPE (MIDI Polyphonic Expression), which gives each sounding note its own channel so it can carry its own pitch bend and pressure (see the Update on the channel voice messages page).