Video Synchronization 811This 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 fl ow 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 fl are. 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 fl ies 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 suffi ciently 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 251 of a second. One set of 312.5
lines is scanned in the fi rst 501 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 501. 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 301 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
fl icker frequency to double the frame rate.
28.4 Television Signal ...................................................................................................
The television signal must do two things, the fi rst is obvious, the second less so. First, it
must control the instantaneous brightness of the spot on the face of the CRT in order that