A method of synthesis that involves rapid cross-fading between (usually) four different layers, samples, or other sound sources. Vector synthesis was invented by Sequential Circuits and introduced in their Prophet VS in 1986; unfortunately, that model was only in production for a few months before Sequential ceased operations in 1987. However, Dave Smith took the concept with him first to Korg, where he created the Wavestation (basically an improved version of the Prophet VS), and then to Yamaha where he added the concept to the SY22 and TG33 models.
All of the above models include a joystick, each of whose four cardinal directions is tied to one layer or sound source. (On the Prophet VS and Wavestation, the four sound sources are digital equivalents of VCO-VCF-VCA chains with multiple available waveforms. On the SY22 and TG33, two of the sound sources are sample playback voices, and the other two are two-operator frequency modulation voices.) When the joystick is centered, all four sources sound at the same volume; moving the joystick in particular direction emphasizes the sources corresponding to the closest cardinal points, and de-emphasizes the others. After the layers and sources are set up, the performer can play a note while moving the joystick, and the synth will remember the joystick moves and store these as part of the patch. Then, each note played will replicate the joystick movements.