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A controller on the panel of the Electronic Musical Instrument. This controller lets you adjust the volume of the overall sound, as output from the OUTPUT jacks as well as the PHONES jack (or the speaker, if so equipped).
A MIDI Channel Message that determines the volume for each individual part. No sound is produced when the value is set to "0", and "127" produces maximum volume. This message is used to adjust the relative balance among all the parts of the tone generator block.
In XG, this parameter is defined as Multi Part parameter change (System Exclusive Message).