|CH. A||-7 to +8||-127 to +128|
|CH. B||-7 to +8||-127 to +128|
|CH. C||-7 to +8||-127 to +128|
A positive offset means to increase the frequency of tone A, B, or C.
A negative offset means to decrease the frequency of tone A, B, or C.
In the TUNE MENU you can put a positive or negative OFFSET to each TUNE REGISTER (press [+]/[-]).
Notice: the tune ratio of the AY 3 8912 chip is not linear, so any offset will DETUNE A, B and C (not transpose!). The +/- offset can put the tune out of range.
The frequency of each square wave (A, B, and C) is defined trough the respective value of the COARSE TUNE REG (4bit) and the FINE TUNE REG (8bit).
For Details see the MIDIBox AY 3 8912 MIDI to Hz Chart: