Some MIDI devices can be switched in and out of Omni state.
When Omni is off, a MIDI device can only respond to Voice Category messages (ie, Status bytes of 0x80 to 0xEF) upon a limited number of channels, usually only 1. Typically, the device allows the musician to pick one of the 16 MIDI channels that the device will respond to. This is then referred to as the device's Base Channel. So for example, if a device's Base Channel is set to 1, and a Voice Category message upon channel 2 arrives at the device's MIDI IN, the device ignores that message.
Note: Virtually all modern devices allow the musician to manually choose the Base Channel. A device may even define its own SysEx message that can change its Base Channel. Remember that SysEx messages are of the System Common Category, and therefore aren't (normally) tied to the Base Channel itself.
When Omni is on, a device doesn't respond to just one MIDI channel, but rather, responds to all 16 MIDI channels. The only benefit of Omni On is that, regardless of which channel any message is received upon, a device always responds to the message. This mades it very foolproof for a musician to hook up two devices and always have one device respond to the other regardless of any MIDI channel discrepancies between the device generating the data (ie, referred to as the transmitter) and the device receiving the data (ie, referred to as the receiver). Of course, if the musician daisy-chains another device, and he wants the 2 devices to play different musical parts, then he has to switch Omni Off on both devices. Otherwise, a device with Omni On will respond to messages intended for the other device (as well as messages intended for itself).
Note: Omni can be switched on or off with the Omni On and Omni Off controller messages. But these messages must be received upon the device's Base Channel in order for the device to respond to them. What this implies is that even when a device is in Omni On state (ie, capable of responding to all 16 channels), it still has a Base Channel for the purpose of turning Omni On or Off.
One might think that MultiTimbral devices employ Omni On. Because you typically may choose (upto) 16 different Patches, each playing its own musical part, you need the device to be able to respond to more than one MIDI channel so that you can assign each Patch to a different MIDI channel. Actually, MultiTimbral devices do not use Omni On for this purpose. Rather, the device regards itself as having 16 separate sound modules (ie, Parts) inside of it, with each module in Omni Off mode, and capable of being set to its own Base Channel. Usually, you also have a "master" Base Channel which may end up having to be set the same as one of the individual Parts. Most MultiTimbral devices offer the musician the choice of which particular channels to use, and which to ignore (if he doesn't need all 16 patches playing simultaneously on different channels). In this way, he can daisy-chain another multitimbral device and use any ignored channels (on the first device) with this second device. Unfortunately, the MIDI spec has no specific "MultiTimbral" mode message. So, a little "creative reinterpretation" of Monophonic mode is employed, as you'll learn in a moment.
In addition to Omni On or Off, many devices can be switched between Polyphonic or Monophonic operation.
In Polyphonic operation, a device can respond to more than one Note On upon a given channel. In other words, it can play chords on that channel. For example, assume that a device is responding to Voice Category messages on channel 1. If the device receives a Note On for middle C on channel 1, it will sound that note. If the device then receives a Note On for high C also on channel 1 (before receiving a Note Off for middle C), the device will sound the high C as well. Both notes will then be sounding simultaneously.
In Monophonic operation, a device can only respond to one Note On at a time upon a given channel. It can't play chords; only single note "melodies". For example, assume that a device is responding to Voice Category messages on channel 1. If the device receives a Note On for middle C on channel 1, it will play that note. If the device then receives a Note On for high C (before receiving a Note Off for middle C), the device will automatically turn off the middle C before playing the high C. So what's the use of forcing a device capable of playing chords into such a Monophonic state? Well, there are lots of Monophonic instruments in the world, for example, most brass and woodwinds. They can only play one note at a time. If using a Trumpet Patch, a keyboard player might want to force a device into Monophonic operation in order to better simulate a Trumpet. Some devices have special effects that only work in Monophonic operation such as Portamento, and smooth transition between notes (ie, skipping the VCA attack when moving from one Note On that "overlaps" another Note On -- this is often referred to as legato and makes for a more realistic musical performance for brass and woodwind patches). That's in theory how Mono operation is supposed to work, but MultiTimbral devices created long after the MIDI spec was designed, had to subvert Mono operation into Polyphonic operation in order to come up with a "MultiTimbral mode", as you'll learn.
Note: A device can be switched between Polyphonic or Monophonic with the Polyphonic and Monophonic controller messages. But these messages must be received upon the device's Base Channel in order for the device to respond to them.
Of course, a MIDI device could have Omni On and be in Polyphonic state. Or, the device could have Omni On but be in Monophonic state. Or, the device could have Omni Off and be in Polyphonic state. Or, the device could have Omni Off but be in Monophonic state. There are 4 possible combinations here, and MIDI refers to these as 4 Modes. For example, Mode 1 is the aforementioned Omni On / Polyphonic state. Here are the 4 Modes:
Mode 1 - Omni On / Poly
The device plays all MIDI data received on all 16 MIDI channels. If a MultiTimbral device, then it often requires the musician to manually select which one Patch to play all 16 channels, and this setting is usually saved in "patch memory".
Mode 2 - Omni On / Mono
The device plays only one note out of all of the MIDI data received on all 16 MIDI channels. This mode is seldom implemented because playing one note out of all the data happening on all 16 channels is not very useful.
Mode 3 - Omni Off / Poly
The device plays all MIDI data received on 1 specific MIDI channel. The musician usually gets to choose which channel he wants that to be. If a MultiTimbral device, then it often requires the musician to manually select which one Patch to play that MIDI channel, and this setting is usually saved in "patch memory".
Mode 4 - Omni Off / Mono
In theory, the device plays one note at a time on 1 (or more) specific MIDI channels. In practice, the manufacturers of MultiTimbral threw the entire concept of Monophonic out the window, and use this for "MultiTimbral mode". On a MultiTimbral device, this mode means that the device plays polyphonically on 1 (or more) specific MIDI channels. The Monophonic controller message has a Value associated with it. This Value is applicable in Mode 4 (whereas it's ignored in Mode 2), and determines how many MIDI channels are responded to. If 1, then on a non-MultiTimbral device, this would give you a truly monophonic instrument. Of course, on a MultiTimbral device, it gives you the same thing as Mode 3. If the Value is 0, then a non-MultiTimbral device uses as many MIDI channels as it has voices. So, for an 8 voice synth, it would use 8 MIDI Channels, and each of those channels would play one note at a time. For a MultiTimbral device, if the Value is 0, then the device uses as many MIDI channels as it has Parts. So, if a MultiTimbral device can play only 8 patches simultaneously, then it would use 8 MIDI Channels, and each of those channels could play polyphonically.
Some devices do not support all of these modes. The device should ignore controller messages which attempt to switch it into an unsupported state, or switch to the next closest mode.
If a device doesn't have some way of saving the musician's choice of Mode when the unit is turned off, the device should default to Mode 1 upon the next power up.
On final question arises. If a MultiTimbral device doesn't implement a true monophonic mode for Mode 4, then how do you get one of its Parts to play in that useful Monophonic state (ie, where you have Portamento and legato features)? Well, many MultiTimbral devices allow a musician to manually enable a "Solo Mode" per each Part. Some devices even use the Legato Pedal controller (or a General Purpose Button controller) to enable/disable that function, so that you can turn it on/off for each Part over MIDI.
Note: A device that can both generate MIDI messages (ie, perhaps from an electronic piano keyboard) as well as receive MIDI messages (ie, to be played on its internal sound circuitry), is allowed to have its transmitter set to a different Mode and MIDI channel than its receiver, if this is desired. In fact, on MultiTimbral devices with a keyboard, the keyboard often has to switch between MIDI channels so that the musician can access the Parts one at a time, without upsetting the MIDI channel assignments for those Parts.