Standard TVs use an interlacing technique when painting the screen. In this technique, the screen is painted 60 times per second but only half of the lines are painted per frame. The beam paints every other line as it moves down the screen -- for example, every odd-numbered line. Then, the next time it moves down the screen it paints the even-numbered lines, alternating back and forth between even-numbered and odd-numbered lines on each pass. 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 per 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. (Some people can actually hear this frequency as a very high-pitched sound emitted when the television is on.)
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. The TV station or VCR therefore sends a well-known signal to the TV that contains three different parts:
- 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
So how does this information get transmitted to the TV? Read the next page to find out.