Page 278 - Satellite Communications, Fourth Edition

258 Chapter Nine

Super mastergroup. A super mastergroup is formed by FDM three
basic mastergroups together. The lower sidebands are combined to
form the 900-VF-channel super mastergroup. Allowing for 8-kHz
guardbands between sidebands, the super mastergroup extends from
8516 to 12,388 kHz.

In satellite communications, such multiplexed signals often form the
baseband signal which is used to frequency modulate a microwave carrier
(see Sec. 9.6). The smallest baseband unit is usually the 12-channel VF
group, and larger groupings are multiples of this unit. Figure 9.4 shows
how 24-, 60-, and 252-VF-channel baseband signals may be formed.
These examples are taken from CCITT Recommendations G322 and
G423. It will be observed that in each case a group occupies the range
12 to 60 kHz. Because of this, the 60-VF-channel baseband, which
modulates the carrier to the satellite, differs somewhat from the standard
60-VF-channel supergroup signal used for terrestrial cable or microwave
FDM links.

9.5 Color Television
The baseband signal for television is a composite of the visual infor-
mation signals and synchronization signals. The visual information is
transmitted as three signal components, denoted as the Y, I, and Q sig-
nals. The Y signal is a luminance,or intensity, component and is also
the only visual information signal required by monochrome receivers.
The I and Q signals are termed chrominance components, and together
they convey information on the hue or tint and on the amount of satu-
ration of the coloring which is present.
The synchronization signal consists of narrow pulses at the end of each
line scan for horizontal synchronization and a sequence of narrower
and wider pulses at the end of each field scan for vertical synchroniza-
tion. Additional synchronization for the color information demodula-
tion in the receiver is superimposed on the horizontal pulses, as
described below.
The luminance signal and the synchronization pulses require a base
bandwidth of 4.2 MHz for North American standards. The baseband
extends down to and includes a dc component. The composite signal con-
taining the luminance and synchronization information is designed to
be fully compatible with the requirements of monochrome (black-and-
white) receivers.
In transmitting the chrominance information, use is made of the fact
that the eye cannot resolve colors to the extent that it can resolve inten-
sity detail; this allows the chrominance signal bandwidth to be less
than that of the luminance signal. The I and Q chrominance signals are