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MIDI Polyphonic Expression
MIDI Polyphonic Expression is an extension of the standard MIDI specification to allow more control over individual notes. It works by sending pitchbend, aftertouch and cc74 messages on a Note per channel basis. These, combined with Note-On and Note-Off velocity give up to 5 controls. Every time a finger hits the surface it is assigned to a MIDI channel, and the associated control messages are sent on that channel until the finger is released.
MPE keyboards typically use the X-axis of their surface to vary pitch, and the Y-axis to send cc74. This gives very fine control of these two data streams. However, Note-On/Off velocity and aftertouch pressure are not so easy to control and the usability depends very much on the device. For example, Note-On velocity on the Roli Seaboard is more difficult to judge than on a standard semi-weighted MIDI keyboard. This is an important consideration when mapping plugin controls to these streams.
Both Derailer and Preparation can be used with MPE devices by selecting the MPE button. Note that the pitchbend range on your keyboard should be set to +/- 48 semitones (the default setting for the Seaboard).
Derailer
The MPE implementation in Derailer varies according to the excitation type.
Sawtooth – Y-axis controls the Tone, and X-axis is continuous pitch control.
Plucked string – Y-axis controls the pluck position along the string, and X-axis varies pitch to +/- 2 semitones. Note-On velocity sets the force of the pluck.
Bowed string – Y-axis controls the bow force, and X-axis varies pitch to +/- 2 semitones. Note-On velocity varies the attack.
Pitched bar – Y-axis controls the strike position along the bar, and X-axis varies pitch to +/- 2 semitones. Note-On velocity sets the force of the strike.
Strikers – Y-axis controls the force of the strike.
Preparation
Preparation uses a single excitation, the force of the pluck/strike. In standard MIDI mode this is mapped to Note-On velocity. However, in MPE mode we decided to map this to the Y-axis, as this allows much finer control of the force value. This is important as the rattling element in Preparation can be very sensitive to the force of the initial pluck/strike. Therefore, Note-On velocity is not used in MPE mode. The X-axis varies pitch to +/- 2 semitones.