In the MIDI standard, one of four modes that specifies how the receiving synth is to respond to note and control messages arriving on various channels. The four modes are specified by the combination of the Omni mode and Mono/Poly mode messages. These are sometimes referred to by number, according to this formula:

  • Mode 1: Omni mode on, Poly mode on
  • Mode 2: Omni mode on, Mono mode on
  • Mode 3: Omni mode off, Poly mode on
  • Mode 4: Omni mode off, Mono mode on (message specifies number of channels)

The behavior of these modes is:

  • Mode 1: The synth plays polyphonically, responding to note and control messages on any channel.
  • Mode 2: The synth plays monophonically, responding to note and control messages on any channel.
  • MOde 3: The synth plays polyphonically, responding only to note and control messages on a specific channel (which is usually specified by a setup parameter in the synth).
  • Mode 4: The synth's voice are divided among a range of consecutively numbered channels, one voice per channel. Each voice responds to note and control messages on the channel it is assigned to. Additionally, the standard allows for a "global channel" for which certain control messages (e.g., Program change) affects all voices.

Today, these definitions are not often used, and few synths today respond to the Omni or Mono/Poly mode messages, at least not in the way the original MIDI spec intended. The problem is that these modes did not take into account the possibility of multitimbral synths, which did not exist when the original MIDI spec was being written in 1981. Nearly all synths today operate in a manner analogous to Mode 3, although a few support a Mode 4-like facility for being controlled by guitar controllers, which need to have individual voices assigned to specific guitar strings in order to properly track playing technique and provide facilities such as virtual tuning.

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