As far as I could compare, the MT-55 is the fully developed version of the MT-21. Still, only 2/3 of the auto accompaniment ICs' pins have been made use of, so I guess that a lot more functions should be possible.
The drum sounds are quite electronic and analog, although their generation is only partly analog. Same thing goes for the square wave bass sound and the chords. All in all they closely resemble the PT-100. The BD and the toms are generated from square wave signals which rely on digital control signals that can only be fully interrupted and hardly allow tampering. The HH and snare sounds are rendered with the bass sound from one IC pin, making manipulation or seperation close to impossible. Same thing goes for the chord section, which can only be muted or slightly distorted.
Nevertheless, I think that, next to a heap of easter eggs, this keyboard should also allow a lot of possibilities for data manipulation, which are, however, hard to determine without a circuit diagram. Up to now, I was only able to carry out the following modifications:
- Mute and distortion switch for BD and toms respectively percussions
- Mute and distortion switch for SN, HH and bass
- Mute and distortion switch for chords