Vector Synthesis is a type of audio synthesis introduced by Sequential Circuits in the Prophet VS synthesizer during 1986. The concept was subsequently used by Yamaha in the SY22/TG33 and similar instruments and by Korg in the Wavestation.
Vector synthesis provides movement in a sound by providing dynamic cross-fading between (usually) four sound sources. The four sound sources are conceptually arranged as the extreme points of X and Y axes, and typically labelled A, B, C and D. A given mix of the four sound sources can be represented by a single point in this 'vector plane'. Movement of the point provides sonic interest and is the power of this technique. Mixing is frequently done using a joystick, although the point can be controlled using envelope generators or LFOs.
Vector synthesis implementations
There have been a number of different implementations of vector synthesis. These differ in what they use for the four sound sources, and what processing is done to the sound after the vector synthesis stage. The actual vector synthesis concept is identical.
Prophet VS vector synthesis
The Prophet VS used four digital wavetable oscillators based on those in the PPG Wave as its four sound sources. The limitations, particularly the digital aliasing, of this design, coupled with its use of Curtis analogue filter ICs to process the mixed sound, gave the Prophet VS its distinctive sound.
Yamaha SY series
The Yamaha SY22 added to the Prophet's implementation of vector synthesis by providing two types of sound source. Each axis of the vector had both an FM sound source and a sample-playback source. Although it lacked any filtering, it added digital effects for processing the results of the vector synthesis.
The Korg Wavestation went further still, allowing each of the four sound sources to produce not just a static tone, but a complex wave sequence, by playing back or cross-fading one wave after another.
The Korg OASYS workstation is one of the first synthesizers for over a decade to feature vector synthesis. It also features an updated form of wave sequencing, like the Wavestation. The Korg Kronos also features this synthesis.
The Prophet V, an hybrid VST version of the original Prophet 5 and VS, is featuring vector synthesis, as well as Arturia's Origin hardware synthesizer, which is using an automated mixer, the 2D Envelope.