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characteristics are necessary to assure uniform
The Y signal goes to the matrix and the C signal goes
amplification of the high-frequency color subcarrier
to the Q and I demodulator. The 3.58-MHz oscillator
is synchronized by the color sync burst from the
sidebands that carry chrominance information.
transmitter. The demodulator separates the
The video amplifier stage, which in monochrome
chrominance C signal into the individual I and Q
receivers usually consists of one stage of amplifica-
signals. The matrix circuit forms the original red,
tion, usually has three stages of amplification.
green, and blue signals.
Additional stages are necessary because the
The color picture tube is specially made for color
luminance signal is used to drive the cathodes of all
reproduction. Three electron guns, one for each color,
three electron guns in the color cathode-ray tube
are used. The tube's screen consists of small, closely
(CRT) as compared to the single gun in a monochrome
spaced phosphor dots of red, green, and blue. The dots
CRT.
are arranged so that a red, green, and blue dot form a
A video delay line is usually located between the
small triangle. The shadow mask provides a centering
video output stage and the CRT simultaneously. The
hole in the middle of the triangle of dots. The
fixed delay (fig. 8-14) is necessary because the
convergence electrode causes the three separate
chrominance signals pass through additional stages
electron beams to meet and cross at the hole in the
before being applied to the control grids of the CRT.
shadow mask.
Were it not for the delay of luminance information, the
Each electron gun is electrostatically focused by a
two signals would not arrive at the same time. A
common grid voltage. In other words, each gun has its
distorted video presentation would be the result.
own electrode, but all three are connected together
The use of an aperture mask type of picture
requiring only one grid voltage. The three electron
tube makes the brightness of a color receiver
beams scan the screen controlled by the deflection
yoke mounted externally around the neck of the tube.
characteristically low. Therefore, higher voltage is
As the three beams scan the phosphor screen in the
necessary to maintain adequate brightness. The output
standard scanning pattern, the dot trios are lighted
voltage of the high-voltage supply is nominally 20 to
according to the video input signals.
25 kV as compared with 15 to 18 kV for monochrome
receivers.
The purifying coil produces a magnetic field
within the tube, which aligns the electron beams
All three electron guns must be sharply focused
parallel to the neck of the tube. Rotating the purifying
onto the screen to obtain good monochrome and color
coil adjusts the electron beams so they strike their
reproduction. The focus rectifier in color receivers
respective color dots without striking the neighboring
provides a variable focus voltage (4 to 5 kV) that is
dots. When this adjustment is made for the red dots,
applied to the electrostatic focus elements of the CRT.
the other two electron beams are aligned as well.
Another factor that requires design techniques much
different than monochrome is the load of the
The high-voltage anode is a metallic ring around
high-voltage rectifier must be held fairly constant.
the tube. The field neutralizing coil aids color purity
Otherwise, severe blooming or shrinking of picture
at the outer edges of the picture tube. A metal shield,
called a mu-metal shield, is placed around the bell of
size will occur during the reception of signals with a
the tube to prevent stray magnetic fields from
varying brightness level. The voltage regulator circuit
affecting the electron beams.
provides a fairly constant anode voltage regardless of
the brightness level of incoming signals,
The color TV monitor/receiver has many
adjustments in addition to those of a black-and-white
The color demodulator section is the "heart" of the
unit. Consult the manufacturer's maintenance
color television receiver. In this section, the 3.58-MHz
handbook before you make adjustments. A color
subcarrier sidebands are demodulated to produce
television receiver, as shown in figure 8-14, contains
color information signals. The color information
many circuits that are different from the circuits used
signals are then applied to a matrix. In the matrix,
in monochrome (black-and-white) receivers. The
color difference signals are produced by matrixing
differences are outlined in the following paragraphs.
proportionate amounts of the demodulated signals.
The color difference signals are amplified and applied
The tuner and amplifier stage in color receivers
to the control grids of the CRT in the proper
are designed to pass a wider band of frequencies than
proportions to reproduce the televised scene.
conventional monochrome receivers. Wideband
8-16

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