Original version SEM. Courtesy Yves Usson @ yusynth.net
Acronym for "Synthesizer Expansion Module", a product developed by Oberheim in 1973. The SEM contained two VCOs, a multimode VCF, a VCA, and a LFO. Most parameters were controllable by external control voltage inputs. The SEM had no keyboard of its own, being intended to be played by control voltage and gate signals from another analog synthesizer.
The SEM was originally intended to be used to add capability to other synths. However, Oberheim soon realized that they could use a bank of SEMs, with a scanning keyboard, to construct a polyphonic synth of sorts. The result was the Four Voice series. Ironically, far more SEMs were sold integrated into Two/Four/Eight voice synths than were sold as expanders.
The original SEM went out of production around 1980, as Oberheim was phasing out the Four Voice series in favor of more integrated synth models. Tom Oberheim has since released an updated version of the SEM with new features, including a MIDI interface with full control over all settings.