To cause the MIDI device to change to a particular Program (which some devices refer to as Patch, or Instrument, or Preset, or whatever). Most sound modules have a variety of instrumental sounds, such as Piano, and Guitar, and Trumpet, and Flute, etc. Each one of these instruments is contained in a Program. So, changing the Program changes the instrumental sound that the MIDI device uses when it plays Note On messages. Of course, other MIDI messages also may modify the current Program's (ie, instrument's) sound. But, the Program Change message actually selects which instrument currently plays. There are 128 possible program numbers, from 0 to 127. If the device is a MultiTimbral unit, then it usually can play 16 "Parts" at once, each receiving data upon its own MIDI channel. This message will then change the instrument sound for only that Part which is set to the message's MIDI channel.
For MIDI devices that don't have instrument sounds, such as a Reverb unit which may have several Preset "room algorithms" stored, the Program Change message is often used to select which Preset to use. As another example, a drum box may use Program Change to select a particular rhythm pattern (ie, drum beat).
0xC0 to 0xCF where the low nibble is the MIDI channel.
One data byte follows the status. It is the program number to change to, a number from 0 to 127.
On MIDI sound modules (ie, whose Programs are instrumental sounds), it became desirable to define a standard set of Programs in order to make sound modules more compatible. This specification is called General MIDI Standard.
Just like with MIDI channels 0 to 15 being displayed to a musician as channels 1 to 16, many MIDI devices display their Program numbers starting from 1 (even though a Program number of 0 in a Program Change message selects the first program in the device). On the other hand, this approach was never standardized, and some devices use vastly different schemes for the musician to select a Program. For example, some devices require the musician to specify a bank of Programs, and then select one within the bank (with each bank typically containing 8 to 10 Programs). So, the musician might specify the first Program as being bank 1, number 1. Nevertheless, a Program Change of number 0 would select that first Program.
Receipt of a Program Change should not cut off any notes that were previously triggered on the channel, and which are still sustaining.