How Television Works

Composite Video Signal

A typical composite video signal
A typical composite video signal

A signal that contains all three of these components -- intensity information, horizontal-retrace signals, and vertical-retrace signals -- is called a composite video signal. A composite-video input on a VCR is normally a yellow RCA jack. One line of a typical composite video signal looks something like the image on this page.

The horizontal-retrace signals are 5-microsecond (abbreviated as "us" in the figure) pulses at zero volts. Electronics inside the TV can detect these pulses and use them to trigger the beam's horizontal retrace. The actual signal for the line is a varying wave between 0.5 volts and 2.0 volts, with 0.5 volts representing black and 2 volts representing white. This signal drives the intensity circuit for the electron beam. In a black-and-white TV, this signal can consume about 3.5 megahertz (MHz) of bandwidth, while in a color set the limit is about 3.0 MHz.

A vertical-retrace pulse is similar to a horizontal-retrace pulse but is 400 to 500 microseconds long. The vertical-retrace pulse is serrated with horizontal-retrace pulses in order to keep the horizontal-retrace circuit in the TV synchronized.