MPE (MIDI Polyphonic Expression) is an official extension of the MIDI 1.0 specification that allows expressive instruments like LinnStrument to have three dimensions of continuous touch control (pressure, left/right and front/back movement) independently for each of simultaneous touches.
This overcomes a fundamental limitation of MIDI: when sending over a single MIDI channel, Pitch Bend messages (used for left/right finger movement) and Control Change messages (used for front/back finger movement) must apply to all touches played, preventing independent 3D control over each touch of a chord. MPE gets around this by sending each touch over a separate MIDI channel.
Here's a brief summary of the official MPE Specification:
There are two MPE zones, the Lower and Upper Zones. Normally only the Lower Zone is used, to play a single sound on the entire playing surface. The Upper Zone is used for split keyboard play, where you would use the lower zone for the left split and the upper zone for the right split. Here is how the two zones use MIDI channels:
Master Channel (called Main in LinnStrument): 1
Member Channels (called Per-Note in LinnStrument): 2 through as high as 16
Master Channel (called Main in LinnStrument): 16
Member Channels (called Per-Note in LinnStrument): 15 through as low as 2
The Master channel is used to send messages that apply to all touches, like Sustain Pedal, Program Change, Volume/CC7, etc.
The Member channels are used to send 5 messages that apply to a single touch: Note On, Note Off, Pitch Bend for left/right, CC74 for front/back, and Channel Pressure for finger pressure. Each touch is sent over one of these Member channels, rotating through the assigned Member channels.
Example 1: For playing a single MPE sound across LinnStrument's entire playing surface, you'd use the Lower Zone with a Master Change of 1 and member channels of 2 through as high as 16, commonly channels 2 though 8.
Example 2: For playing LinnStrument in Split Keyboard mode with MPE on both splits, you'd use the Lower Zone for the left split with a Master Channel of 1 and member channels of 2 through 8. For the right split, you'd use the Upper Zone with a Master Channel of 16 and Member Channels of 9 through 15.
Tip: to quickly set all of LinnStrument's MIDI settings for MPE, in Per-Split Settings, MIDI Mode column, HOLD the ChPerNote button for 1 second. It will change color, indicating that you're set up for MPE play. Doing this for the left split's settings sets the Master/Main channel to 1 and the Member/Per-Note channels to 2 through 8. Doing this in the right split's settings sets the Master/Main channel to 16 and the Member/Per-Note channels to 9 through 15.