"Throughout the early '70's, Oberheim Electronics [founded by Tom Oberheim] was a highly reputed manufacturer of electronic effects devices....In addition to designing and building effects boxes, Oberheim electronics was an ARP dealer for about a year and a half. Many of their early ARP customers asked about a sequencer for their synth, so Oberheim designed their DS-2, one of the first digital sequencers."
"As soon as the DS-2 was on the market, Tom Oberheim noticed that many of his customers wanted to set up a sequence, then play their synth from the keyboard at the same time that the sequence was playing back. In addition, many ARP Odyssey and Minimoog owners were looking for a way to fatten up their sound. So Tom drew up plans for a fairly simple analog voice module that could serve" both needs, the SEM (Synthesizer Expander Module). "The development of the entire Oberheim synthesizer line, right up to the present time, has been a continuous evolution that began with the SEM modules."
[excerpted with permission from the book Vintage Synthesizers by Mark Vail, copyright Miller Freeman, Inc]