Page 281 - Satellite Communications, Fourth Edition
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Analog Signals 261
information. The chrominance signal subcarrier frequency has to be pre-
cisely controlled, and in the NTSC system it is held at 3.579545 MHz
10 Hz, which places the subcarrier frequency midway between the 227th
and the 228th harmonics of the horizontal scanning rate (frequency). The
luminance and chrominance signals are both characterized by spectra
wherein the power spectral density occurs in groups which are centered
about the harmonics of the horizontal scan frequency. Placing the chromi-
nance subcarrier midway between the 227th and 228th horizontal-scan
harmonics of the luminance-plus-synchronization signals causes the
luminance and the chrominance signals to be interleaved in the spec-
trum of the composite NTSC signal. This interleaving is most apparent
in the range from about 3.0 to 4.1 MHz. The presence of the chrominance
signal causes high-frequency modulation of the luminance signal and
produces a very fine stationary dot-matrix pattern in the picture areas
of high color saturation. To prevent this, most of the cheaper TV receivers
limit the luminance channel video bandwidth to about 2.8 to 3.1 MHz.
More expensive “high resolution” receivers employ a comb filter to
remove most of the chrominance signal from the luminance-channel
signal while still maintaining about a 4-MHz luminance-channel video
bandwidth.
Because the subcarrier is suppressed in the modulation process, a sub-
carrier frequency and phase reference carrier must be transmitted to
allow the I and Q baseband chrominance signals to be demodulated at
the receiver. This reference signal is transmitted in the form of bursts
of 8 to 11 cycles of the phase-shifted subcarrier, transmitted on the
“backporch” of the horizontal blanking pulse. These bursts are trans-
mitted toward the end of each line sync period, part of the line sync pulse
being suppressed to accommodate them. One line waveform including
the synchronization signals is shown in Fig. 9.6.
Figure 9.6 One line of waveform for a color TV signal.
