Cathode Ray Tube and Raster Scanning
To every engineer, the cathode ray tube (CRT) will be familiar enough from the oscilloscope. The evacuated glass envelope contains an electrode assembly and its terminations at its base whose purpose is to shoot a beam of electrons at the luminescent screen at the other end of the tube. This luminescent screen fluoresces to produce light whenever electrons hit it. In an oscilloscope the deflection of this beam is affected by means of electric fields—a so-called electrostatic tube. In television the electron beam (or beams in the case of color) is deflected by means of magnetic fields caused by currents flowing in deflection coils wound around the neck of the tube where the base section meets the flare. Such a tube is known as an electromagnetic type.
Just like an oscilloscope, without any scanning currents, the television tube produces a small spot of light in the middle of the screen. This spot of light can be made to move anywhere on the screen very quickly with the application of the appropriate current in the deflection coils. The brightness of the spot can be controlled with equal rapidity by altering the rate at which electrons are emitted from the cathode of the electron gun assembly.
This is usually affected by controlling the potential between the grid and the cathode electrodes of the gun. Just as in an electron tube or valve, as the grid electrode is made more negative in relation to the cathode, the flow of electrons to the anode is decreased. In the case of the CRT the anode is formed by a metal coating on the inside of the tube flare. A decrease in grid voltage—and thus anode current—results in a darkening of the spot of light. Correspondingly, an increase in grid voltage results in a brightening of the scanning spot.
In television, the bright spot is set up to move steadily across the screen from left to right (as seen from the front of the tube). When it has completed this journey it flies back very quickly to trace another path across the screen just below the previous trajectory. (The analogy with the movement of the eyes as they “scan” text during reading can’t have escaped you!) If this process is made to happen sufficiently quickly, the eye’s persistence of vision, combined with an afterglow effect in the tube phosphor, conspires to fool the eye so that it does not perceive the moving spot but instead sees a set of parallel lines drawn on the screen. If the number of lines is increased, the eye ceases to see these as separate too—at least from a distance—and instead perceives an illuminated rectangle of light on the tube face. This is known as a raster. In the broadcast television system employed in Europe, this raster is scanned twice in 25 of a second. One set of 312.5 lines is scanned in the first 50 of a second and a second interlaced set—which is not superimposed but is staggered in the gaps in the preceding trace—is scanned in the second 50 . The total number of lines is thus 625. In North America, a total of 525 lines (in two interlaced passes of 262.5) are scanned in 30 of a second.
This may seem like a complicated way of doing things and the adoption of interlace has caused television engineers many problems over the years. Interlace was adopted in order to accomplish a 2 to 1 reduction in the bandwidth required for television pictures with very little noticeable loss of quality. It is thus a form of perceptual coding—what we would call today a data compression technique. Where bandwidth is not so important—as in computer displays—noninterlaced scanning is employed. Note also that interlace is, in some respects, the corollary of the double exposure system used in the cinema to raise the flicker frequency to double the frame rate.