400 Electronic Communication and Data Systems
In-line gun assemblies were invented after the
shadow mask tube. Figure 23-14 shows four in-line
gun assemblies with different aperture grill and screen
patterns.
A color television receiver is a very complex
instrument. It has to be able to produce both color and
black-and-white pictures. But not all televisions are color
televisions, so the incoming signal must also work with
black-and-white sets. Figure 23-15 shows a block
diagram of a color television receiver.
Television Channel
The FCC has assigned a portion of the radio fre-
quency spectrum for each television channel. There are
two types of television channels: very high frequency
(VHF) and ultra high frequency (UHF). Each channel is
6 MHz wide. The VHF channels, 2 to 13, are listed
in Figure 23-16 along with frequency bands and carrier
frequencies.
Examine channel 4 in Figure 23-17. The basic video
carrier frequency is 67.25 MHz. Recall that when an RF
carrier wave is amplitude modulated, sideband frequen-
cies appear. These stand for the sum and difference
between the carrier frequency and the modulating
frequencies.
To send a very clear, sharp picture, frequencies of
at least 4 MHz are needed for modulation. These
frequencies combine in channel 4 for a band occupancy
of 63.25 MHz (67.25 – 4 MHz) to 71.25 MHz
(67.25 + 4 MHz). It is a total channel width of 8 MHz.
However, the FCC allows only 6 MHz, therefore, a com-
promise must be made.
In commercial TV broadcasting the upper sideband
is transmitted without attenuation. The lower sideband is
partly removed by a vestigial-sideband filter at the trans-
mitter. The curve in Figure 23-17 shows the basic
response traits of the TV transmitter. The sound is trans-
mitted as a frequency-modulated signal at a center fre-
quency 4.5 MHz above the video carrier. In channel 4, the
sound is at 71.75 MHz.
The ultra high frequency (UHF) television band cov-
ers from channel 14 to 83. As in VHF, the bandwidth of
each channel is 6 MHz. The same frequency bandwidths
are used for the picture carrier as VHF.
The UHF channels used for commercial TV are set
by the FCC. See Figure 23-18.
Delta-type
electron gun
Circular
aperture mask
Phosphor screen
Figure 23-13. A delta-type gun, shadow mask, and tri-
dot screen arrangement. (Sylvania-GTE)
Figure 23-12. Red, blue, and green are the basic colors used in television.
Red
filter
Blue
filter
Green filter
Light sources
Yellow (red/green)
Red
Magenta
(red/blue)
White
center
(all 3 colors)
Blue
Green
Screen
Cyan (blue/green)