Prophet VS was Sequential's first foray into digital synthesis. Actually, you could say the Prophet 2000 was, but technically it was a sampler not a synth. The VS was the first synth to bring two-dimensional crossfading between waveforms into the mainstream. This interesting concept became known as vector synthesis. The Wavestation A/D expands upon the concept by allowing crossfading between wavesequences, not just individual waveforms.