A videotape system that records with four video heads mounted 90°apart around the rim of a rotating head wheel. Quad VTRs generally use 2-inch videotape. Also called transverse scan VTR. The first successful professional videotape recorders were quadruplex recorders.

Quadrature error: Horizontal displacement of picture elements in the display of a quad VTR, caused by a misalignment of one or more of the quad machine’s video heads.

Quadrature AM: A process which allows two different signals to modulate a single carrier frequency. The two signals of interest Amplitude Modulate carrier signals which are the same frequency but did differ in phase by 90°(hence the quadrature notation). The two resultant signals can be added together, and both signals recovered at the other end, if they are also demodulated 90 degrees apart.

Quadrature distortion: Distortion resulting from the asymmetry of sidebands used in vestigial sideband television transmission. Quadrature distortion appears when envelope detection is used, but can eliminated by using a synchronous demodulator.

Quad head rotation: The drum carries four heads with coils whose tips protrude and penetrate slightly into the magnetic coating of the tape. The tape is curved to the drum radius by the guide and is held in close contact with the guide by applying a vacuum.