21 polyphony, but just a little at the time very well. 180 sounds divided into 3 banks combis + 60, with an extension to memory card drive. 1 multi stereo effects. Keyboard very good quality.
Configuration and edition are machinery Ensoniq 90s, ie confusing for Japanese used synths but very logical for others ...
The sounds are very very particular grain, the SD1 (like VFX) sounds hot and "dirty" at the same time. You can get any kind of synthetic sounds typed by acoustic cons for this is not it. We choose a SD1 for music style electro and not fiddle the guitar or trumpet ... It is not made for this.
I would put the SD1 really apart from other digital synths, it's really special to any point of view, because of its tone color, ergonomics, design, keyboard short, for me it is a single machine and is rare ... It is also very robust and reliable.