All TVs use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second but only half the lines are painted per frame. The beam paints every other line as it moves down the screen, for example every odd-numbered line. The entire screen, in two passes, is painted 30 times every second. The alternative to interlacing is called progressive scanning , which paints every line on the screen 60 times a second.
Most computer monitors use progressive scanning because it significantly reduces flicker.
Because the electron beam is painting all 525 lines 30 times per second, it paints a total of 15,750 lines per second. When a television station wants to broadcast a signal to your TV, or when your VCR wants to display the movie on a video tape on your TV, the signal needs to mesh with the electronics controlling the beam so that the TV can accurately paint the picture that the TV station or VCR sends. Intensity information for the beam as it paints each line Horizontal retrace signals to tell the TV when to move the beam back at the end of each line Vertical retrace signals 60 times per second to move the beam from bottom right to top left. A signal that contains all three of these components is called a composite video signal . If your VCR has a composite video input capability, it is normally a yellow RCA jack.