Why transpose the sub voice?
This function may not seem very necessary at first glance. After all, transposing one voice up
or down a few semitones while leaving the other untransposed would only complicate playing
unnecessarily — unless you want to try sightreading a part written for. say, a saxophone with
your right hand and a bass line with your left.
However, there are in fact many interesting ways to use this feature. In Dual mode, for example, you could use
it to play two-note harmonies, lowering the sub voice so that it plays a fifth or an octave below the main voice.
In Split mode, too, you could transpose the sub voice up or down two octaves and play it in the same pitch range
as the main voice. This would allow you to play a two-handed duet, for example, performing a melody with one
hand and echoing it using a different voice played by the other.
24