Enables Monophonic operation (thus disabling Polyphonic operation). See the discussion on MIDI Modes.
If Omni is off, this Value tells how many MIDI channels the device is expected to respond to in Mono mode. In other words, if Omni is off, this value is used to select a limited set of the 16 MIDI channels (ie, 1 to 16) to respond to.Conversely, if Omni is on, this Value is ignored completely, and the device only ever plays one note at a time (unless a MultiTimbral device). So, the following discussion is only relevant if Omni Off.
If Value is 0, then the number of MIDI channels that the device will respond to simultaneously will be equal to how many voices the device can sound simultaneously. In other words, if the device can sound at least 16 voices simultaneously, then it can respond to Voice Category messages on all 16 channels. Of course, being Monophonic operation, the device can only sound one note at a time per each MIDI channel. So, it can sound a note on channel 1 and channel 2 simultaneously, for example, but can't sound 2 notes both on channel 1 simultaneously.
Of course, MultiTimbral devices completely violate the preceding theory. MultiTimbral devices always can play polyphonically on each MIDI channel. If Value is 0, what this means is that the device can play as many MIDI channels as it has Parts. So, if the device can play 16 of its patches simultaneously, then it can respond to Voice Category messages on all 16 channels.
If Value is not 0 (ie, 1 to 16), then that's how many MIDI channels the device is allowed to respond to. For example, a value of 1 would mean that the device would only be able to respond to 1 MIDI channel. Since the device is also limited to sounding only 1 note at a time on that MIDI channel, then the device would truly be a Monophonic instrument incapable of sounding more than one note at a time. If a device is asked to respond to more MIDI channels than it has voices to accomodate, then it will handle only as many MIDI channels as it has voices. For example, if an 8-voice synth, on Base Channel 0, receives the value 16 in the Mono message, then the synth will play messages on MIDI channels 0 to 7 and ignore messages on 8 to 15.
Again, MultiTimbral devices violate the above theory. A value of 1 would mean that the device would only be able to respond to 1 MIDI channel (and therefore only play 1 Part), but would do so Polyphonically. If a MultiTimbral device is asked to respond to more MIDI channels than it has Parts to accomodate, then it will handle only as many MIDI channels as it has Parts. For example, if a device can play only 5 Patches simultaneously, and receives the value 8 in the Mono message, then the device will play 5 patches on MIDI channels 0 to 4 and ignore messages on channels 5 to 7.
Most devices capable of Monophonic operation, allow the user to specify a Base Channel. This will be the lowest MIDI channel that the device responds to. For example, if a Mono message specifies that the device is to respond to only 2 channels, and its Base Channel is 2, then the device responds to channels 2 and 3.
Note: When a device receives a Mono Operation message, it should automatically turn off all playing notes.