The Burst Electronics Model VSA detects the presence of composite video at its A input and outputs this video. If loss of video on the A input is detected, then the unit switches to the B input, lights a Red LED, and energizes an internal "tally" relay. This relay may be used to control external devices. The tally relay has both normally closed and normally open contacts (Form C relay). This Model includes stereo audio switching, audio follow video.
The video detection process utilizes a sophisticated sync separator circuit and a micro controller IC to analyze the A input. The composite sync tip amplitude, vertical sync, horizontal sync, and frame pulse are all analyzed by the micro controller to determine the quality of the incoming video on the A input.