Sequential Circuits Prophet VS
Here's a very cool keyboard from Sequential. Notice the joystick for controlling your sound modeling. The Prophet VS uses Vector Synthesis as its revolutionary new means of sound creation. It uses a total of four oscillators per voice with 127 waveforms (32 user) and dynamic waveform crossfading via the joystick. This was Sequential's first digital synth, and it was also their final synth unfortunately. But after Sequential was gone, Vector Synthesis technology went into the popular Korg Wavestation and Yamaha's SY-22 and TG-33.
Pictured above is the common rackmount version of the PVS. The Prophet VS is still a very sought after instrument because it has an incredible range of sounds! Its only problem is that there's practically nobody left that can fix it if something went wrong. It is used by Brian Eno, Trent Reznor, Apollo 440, Depeche Mode, Vince Clarke, Kraftwerk, Vangelis, Erasure and filmmaker/composer John Carpenter.
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Websites of Interest
Images from Perfect Circuit Audio.
Additional information provided by Dave Bellamy.