The DK Synergy (1981-1985) is a USA digital keyboard that's the descendant of the GDS, a programmable digital synthesizer developed at Bell Labs in the 70s.
Several hundred of Synergy I and IIs were built — fewer than a hundred are believed to still be in service.
I acquired this instrument from Condor Electronics in Seattle. It was the most complete of the two "parts" machines they had. It was upgraded with a fanless power supply and has MIDI implementation.
The sound of the GDS / Synergy is associated in popular music with Wendy Carlos (think the 1982 Tron soundtrack) who played both the GDS and the Synergy, and who developed sound banks for the Synergy. Legend has it she tuned the notes by ear. These sound banks were delivered in the form of ROM cartridges that have the same physical shape as the cartridges used in Atari 400 and 800 home computers.
There's a delicateness to the Synergy's personality, although it can growl and get aggressive as well.