Video Circuits

Video circuits generate, process, switch and display the electrical signals that carry moving pictures. A basic analogue video signal encodes a sequence of image lines together with synchronisation pulses that tell a display where each line and frame begins. In a composite video signal the brightness (luminance), colour (chrominance) and synchronisation information are combined onto a single line, which is why a single cable can carry a complete picture.

Different regions have used different analogue television standards. PAL and NTSC are two such colour systems; they differ in the number of lines, the frame rate and the way colour information is encoded, so equipment built for one standard is not directly compatible with the other. Video circuits that handle these signals must preserve their wide bandwidth and exact timing, since errors in synchronisation or amplitude show up directly as visible distortion, rolling or loss of colour.

Common video circuits include amplifiers that boost or buffer the signal without distorting it, switchers that select between several sources, and distribution amplifiers that drive multiple displays from one source. Digital sources such as DVD players produce video that is decoded from a compressed digital stream before being output, and modern equipment increasingly uses digital interfaces. Careful attention to impedance matching, usually 75 ohms for video, and to bandwidth keeps the picture sharp and free of artefacts.

Frequently asked questions

What is composite video?

It is an analogue signal that combines brightness, colour and synchronisation information onto a single line, allowing a complete picture to be carried over one cable.

What is the difference between PAL and NTSC?

They are different analogue colour television standards that use different line counts, frame rates and colour encoding, so equipment made for one is not directly compatible with the other.

Why is impedance matching important for video?

Video signals are usually carried in 75-ohm systems. Matching impedance prevents reflections that would otherwise blur or ghost the picture and degrade the high-frequency detail.