Number: 11 (coarse) 43 (fine)
Affects:
This is a percentage of Volume (ie, as set by Volume Controller). In other words, Expression divides the current volume into 16,384 steps (or 128 if 8-bit instead of 14-bit resolution is used). Volume Controller is used to set the overall volume of the entire musical part, whereas Expression is used for doing crescendos and decrescendos. By having both a master Volume and sub-Volume (ie, Expression), it makes possible adjusting the overall volume of a part without having to adjust every single MIDI message comprising a crescendo or decrescendo. When Expression is at 100% (ie, maximum or 0x3FFF), then the volume represents the true setting of Volume Controller. Lower values of Expression begin to subtract from the volume. When Expression is 0% (ie, 0x0000), then volume is off. When Expression is 50% (ie, 0x1FFF), then the volume is cut in half.
If a MultiTimbral device, then each Part usually has its own Expression level.
Value Range:
14-bit coarse/fine resolution. 0x0000 to 0x3FFF where 0 is minimum effect.
NOTE: Most all devices ignore the Fine adjust (#43) for Expression, and just implement Coarse adjust (#11) because 14-bit resolution isn't needed for this. In this case, maximum is 127, 50% is 64, and off is 0. So assume that a channel's volume is 100. After receiving an Expression Controller of value 64, the volume is reduced to 50 (ie, 50% of 100). After receiving another Expression of 127, the volume is restored to 100. Now, assume the volume is changed (via Volume Controller) to 80. After receiving an Expression Controller of value 64, the volume is reduced to 40 (ie, 50% of 80). After receiving another Expression of 127, the volume is restored to 80.