A reputed designer of effects in the early 70's, Tom Oberheim whose company sold ARP products at the time got requests for a device that would peform sequencing and he released the DS-2 sequencer. The SEM (Synth Expansion Module) product arose from requests to increase the number of synths available for playing from the sequencers without cluttering up the stage with more keyboards, or to layer with other keyboards like the Odyssey.
The sound of Tom's products has always been loved and they are all very much saught after. Hooking up SEM modules with an assigner circuit resulted in the first polyphonic synth; the 4-voice.
SEM Two Voice Four Voice Eight Voice OB-1 OB-X OB-Xa OB-8 Xpander Matrix-6/R Matrix-12 Matrix-1000 OB-Mx