810 Chapter 28
frame is projected upon a screen by way of an arrangement of lenses. Experiments soon
established that a presentation rate of about 12 still frames per second was suffi ciently
rapid to give a good impression of continuously fl owing movement but interrupting
the light source at this rate caused unbearable fl icker. This fl icker phenomenon was
also discovered to be related to the level of illumination; the brighter the light being
repetitively interrupted, the worse the fl icker. Abetted by the low light output from
early projectors, this led to the fi rst fi lm frame-rate standard of 16 frames per second
(fps). A standard well above that required simply to give the impression of movement
and suffi ciently rapid to ensure fl icker was reduced to a tolerable level when used with
early projection lamps. As these lamps improved, fl icker became more of a problem
until an ingenious alteration to the projector fi xed the problem. The solution involved a
modifi cation to the rotating shutter so that, once the fi lm frame was drawn into position,
the shutter opened, then closed, and then opened again before closing a second time
for the next fi lm frame to be drawn into position. In other words, the light interruption
frequency was raised to twice that of the frame rate. When the fi lm frame rate was
eventually raised to the 24-fps standard, which is still in force to this day, the light
interruption frequency was raised to 48 times per second, a rate that enables high levels of
illumination to be employed without causing fl icker.
28.3 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 fl uoresces to produce light
whenever electrons hit it. In an oscilloscope the defl ection of this beam is affected by
means of electric fi elds—a so-called electrostatic tube. In television the electron beam (or
beams in the case of color) is defl ected by means of magnetic fi elds caused by currents
fl owing in defl ection coils wound around the neck of the tube where the base section
meets the fl are. 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
defl ection 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.