above image from the archives at Hyperreal
"Following the traditions of Moog, Buchla, and other early synth developers, the EML engineers crafted an analog sequencer that bears little resemblance to today's MIDI sequencers. 'The 400 series sequencer was my baby,' [EML designer, John] Borowicz admits. 'I loved that box. It was killer. I'd say that was one of the first hybrids, because it was analog and digital. One of the problems with analog circuits was tuning the bloody things. So we built a voltage quantizer, and then ARP jumped on the bandwagon, since they were the big boys on the block. Our quantizer was basically set up for 12-tone diatonic, but you could tune it to play microtones too, and it would still maintain its quantizing feature, which was pretty hip.
"'It also had voltage-controlled envelope generators, which as far as I know was a first, too. You could patch that thing, load it up in increments of 16 stages, and the control voltages could be either locked in - quantized - or you could set it to be variable, either way. By patching, you could get the thing to do retrograde permutations: It would go through a sequence as you programmed it with all the rhythm, and then you could start to do permutations by jumping around in a very progammatic fashion to different stages. It could go through stages 1-16, and then 1, 15, 2, 14, 3, 12, and so on. You could line up the patterns however you wanted. That was an amazing little box.
"'It also had a voice module with it, the 401, which was basically a scaled-down 500: dual oscillators with switch-selectable waveforms, a multimode filter, and the voltage-controlled envelope generators. At that point, you only had AR (attack/release) envelopes, but you could get multiple stages by gating the voltage correctly.'"
[excerpted with permission from the book Vintage Synthesizers by Mark Vail, copyright Miller Freeman, Inc]