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Wire and Wireless Communication Applications Chap. 8
where B is the video bandwidth, T h is the horizontal interval, and T b is the blanking interval.
(See Fig. 8–30.) For U.S. standards, B = 4.2 MHz, T h = 63.5 ms, and T b = 10.5 ms, so that
the horizontal resolution is
n h = 445 pixels
(8–60b)
Furthermore, because of poor video bandwidth and the poor interlacing characteristics of consumer
TV sets, this horizontal resolution of 445 is usually not obtained in practice. † At best,
the U.S. standard of 445 × 483 does not provide very good resolution, and this poor resolution
is especially noticeable on large-screen TV sets. [For comparison, the super VGA (SVGA)
standard for computer monitors provides a resolution as large as 1024 × 768.]
For broadcast TV transmission (U.S. standard), the composite video signal of Eq. (8–58)
is inverted and amplitude modulated onto an RF carrier so that the AM signal is
s v (t) = A c [1 - 0.875m c (t)] cos v c t
(8–61)
This is shown in Fig. 8–31. The lower sideband of this signal is attenuated so that the spectrum
will fit into the 6-MHz TV channel bandwidth. This attenuation can be achieved by using a
vestigial sideband filter, which is a bandpass filter that attenuates most of the lower sideband.
The resulting filtered signal is called a vestigial sideband (VSB) signal, s v0 (t), as described in
Sec. 5–5. The discrete carrier term in the VSB signal changes when the picture is switched
from one scene to another, because the composite baseband signal has a nonzero DC level that
depends on the particular scene being televised. The audio signal for the TV program is transmitted
via a separate FM carrier located exactly 4.5 MHz above the visual carrier frequency.
The rated power of a TV visual signal is the effective isotropic radiated peak envelope
power (EIRP, i.e., effective sync tip average power), and it is usually called simply the
effective radiated power (ERP). The EIRP is the power that would be required to be fed into
an isotropic antenna to get the same field strength as that obtained from an antenna that is
actually used as measured in the direction of its maximum radiation. The antenna pattern of
an omnidirectional TV station is shaped like a doughnut with the tower passing through
the hole. (The antenna structure is located at the top of the tower for maximum line-of-sight
coverage.) The ERP of the TV station (visual or aural) signal is
P ERP = P PEP G A G L
(8–62)
where
P PEP = peak envelope power out of the transmitter,
G A = power gain of the antenna (absolute units as opposed to decibel units) with respect
to an isotropic antenna,
G L = total gain of the transmission line system from the transmitter output to the antenna
(including the duplexer gain).
For a VHF analog high-power TV station operating on channel 5, some typical values are
G A = 5.7, G L = 0.873 (850 ft of transmission line plus duplexer), and a visual PEP of 20.1 kW.
† Typically, a horizontal resolution of about 300 lines is obtained.