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In-Depth Information
Trace
Retrace with
electron gun off
F I GU R E 19 . 4
The path traversed by the electron beam in a television.
Even field
Odd field
F I GU R E 19 . 5
A frame and its constituent fields.
First, one field consisting of 262.5 lines is traced by the electron beam. Then, the second
field consisting of the remaining 262.5 lines is traced
between
the lines of the first field. The
situation is shown schematically in Figure
19.5
. The first field is shown with solid lines while
the second field is shown with dashed lines. The first field begins on a full line and ends on a
half line while the second field begins on a half line and ends on a full line. Not all 525 lines
are displayed on the screen. Some are lost due to the time required for the electron gun to
position the beam from the bottom to the top of the screen. We actually see about 486 lines
per frame.
In an analog color television, instead of a single electron gun, we have three electron guns
that act in unison. These guns excite red, green, and blue phosphor dots embedded in the screen.
The beam from each gun strikes only one kind of phosphor, and the gun is named according to
the color of the phosphor it excites. Thus, the red gun strikes only the red phosphor, the blue
gun strikes only the blue phosphor, and the green gun strikes only the green phosphor. (Each
gun is prevented from hitting a different type of phosphor by an aperture mask.)
In order to control the three guns we need three signals: a red signal, a blue signal, and
a green signal. If we transmitted each of these separately, we would need three times the
bandwidth. With the advent of color television, there was also the problem of backward com-
patibility. Most people had black-and-white television sets, and television stations did not want
