Cathode Ray Tubes
If you think about it, you might come to realize that there
is a lot of similarity between the operation of cathode ray
tubes and the mechanical scanning systems already
described. For instance, a CRT has vertical and horizontal
sweep signals that develop the raster. The scanning disk
also provides these sweeps. Because it is rotating, each
scanning hole performs a sweep and with each hole
being displaced one hole width from the others, they
together provide a second sweep, perpendicular to the
first. The same is true of all of the other scanners. In a
CRT, the electron gun has a means of controlling the
density of electrons and therefore the brightness of the
scanning spot on the screen. In a similar way, mechanical
systems have the means to control spot brightness too.
And like the CRT, the control of brightness is done before
the scanning takes place.
The only problem with this comparison is that long before the CRTs
could produce television images, the mechanical systems were out
there doing it. And for a long time, they did it better!
For Example: Except for the vibrating mirror systems, all of the
rotating disc mechanical scanning techniques, produce linear
sweeps or scans. In the 1920s when Farnsworth, Zworykin and the
others that were working with CRTs, they were using sine wave
sweeps, usually supplied by small motor generators. This caused
shading problems in the image and synchronizing problems
between the sweeps. At some point, someone ( maybe Farnsworth
first ) must have looked at an operating mechanical system and
realized that the sweeps were in fact linear and they needed to do
the same with CRTs. This suggests that the mechanical systems
may have often been a model for the electronic versions.
The cathode ray tube of today has many important advantages over
the mechanical scanners. The first of which is that it can operate at
extreme speeds. Another is that it is has a low cost for what it
does. It has good image qualities, such as brightness, contrast, and
resolution as well as long life. Another important reason for the
experimenter to consider using the CRT is that unlike mechanical
scanners, which are generally limited to the format they were
designed for, CRTs can be easily changed from one format to
another, usually by the twist of a knob.
Because of the popularity of television over recent years, many
varieties of CRTs and their accessories are available on the new and
used market at reasonable prices. This is an ideal situation for the
television experimenter. Many times, they even throw away sets
that often contain many useful components.