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Wire and Wireless Communication Applications Chap. 8
8–9 TELEVISION
Television (TV) is a method of reproducing fixed or moving visual images by the use of electronic
signals. There are numerous kinds of TV systems, and different standards have been
adopted around the world. Moreover, as discussed subsequently, TV is in a state of flux since
there is a worldwide push for digital television (DTV). On-the-air broadcasters in the United
States switched to DTV in June 2009, although there are a few low-power TV stations that are
still analog. U.S. cable TV systems are still using analog TV on the lower-tier channels to
accommodate subscribers with analog TV sets, but are using QAM DTV technology for the
digital channels. As we shall see, analog TV uses an analog video signal that is interspersed
by digital pulses (for line and frame synchronization) to form the composite analog TV
signal. DTV uses samples to the analog TV video signal and passes these samples to an ADC
(analog-to-digital converter). The synchronization data is interspersed on the ADC output
signal to produce the composite digital TV signal.
The following sections describe U.S. television technology as it has developed historically,
by first describing analog black-and-white TV, then analog color TV, and, finally,
digital TV.
Analog Black-and-White Television
The black-and-white image of a scene is essentially the intensity of the light as a function of
the x- and y-coordinates of the scene and of time. However, an electrical waveform is only a
function of time, so that some means of encoding the x- and y-coordinates associated with the
intensity of light from a single point in the scene must be used. In broadcast TV, this is
accomplished by using raster scanning, which is shown in Fig. 8–30. It is assumed that
the whole scene is scanned before any object in the scene has moved appreciably. Thus, moving
images can be transmitted by sending a series of still images, just as in motion pictures.
As the scene is scanned, its intensity is signified by the amplitude of the analog video signal.
A synchronizing pulse is inserted at the end of each scan line by the electronic switch, as
shown in Fig. 8–30, to tell the TV receiver to start another scan line. For illustrative purposes,
a black triangle is located in the upper left portion of the scene. In Figs. 8–30b and 8–30c,
numbers are used to indicate the corresponding times on the video waveform, with the location
of the point being scanned in the scene at that instant. The composite analog TV video
signal is actually a hybrid signal that consists of a digital waveform during the synchronizing
interval and an analog waveform during the video interval. Operating personnel monitor the
quality of the video by looking at a waveform monitor, which displays the composite video
waveform, as shown in Fig. 8–30c.
The picture (as viewed) is generated by scanning left to right to create a line, and the
lines move from the top to the bottom of the screen. During retrace, indicated by the dashed
lines from right to left, the TV screen is turned off by the video level that is “blacker than
black” during the sync pulse interval. The scene is actually covered by scanning alternate
lines from the top to the bottom (one field) and returning to the top to scan the missed lines
(the next field). This technique is called interlacing and reduces the flicker effect on moving
pictures, giving two fields (passes down the screen) for every frame. In the United States there
are approximately 30 framess transmitted, which is equivalent to 60 fieldss for the 2:1