Chapter 8 - Elements of Television

Abstract

The methods of coding the picture information for digital transmission are very different from those used in the older (analog) system, but many of the basic principles of television are much the same. This chapter concentrates on the older analog system because these receivers can have a long life and may still be in use (using a digital set-top box converter), and then focuses on the differences brought about by digital television. The analog television waveform is not a symmetrical wave of fixed shape. The synchronizing pulse for each line occurs at regular intervals, but the waveshape that follows this pulse depends on the distribution of light and shade in that line of the picture. The use of interlacing, scanning only half of the total number of lines in each vertical sweep, reduces the bandwidth by half without degrading the picture quality. Interlacing is not required for flat-screen displays, but is still used on the broadcast signal for compatibility with cathode ray tube (CRT) receivers. The analog video signal is generated from a camera tube or a CCD panel and is amplified and combined with synchronizing signals to form the composite video signal. The CRT can use either electrostatic or magnetic deflection, so that the beam of electrons can have its direction altered, allowing the light spot to appear anywhere on the face of the tube. Magnetic deflection has been used for large televisions, computer monitors, and radar tubes; electrostatic deflection for the smaller tubes for measuring instruments.