b. Composite Signal. A composite carrier signal contains the signal intelligence from all the channels.
(1) FDM signals. In frequency-division multiplex, the greater the number of channels, the greater is the
required bandwidth capability of equipment. In FDM a direct relationship exists between these two
factors because the bandwidth of each channel takes up its portion of the overall frequency spectrum
in the composite signal. Each channel signal is analog (continuous) in nature. Assuming that the
equipment has adequate bandwidth capability, a theoretically limitless number of voice channels can
be added because the equipment translates the individual channel frequencies to other frequencies.
Thus, to add more channels, it is necessary to use more of the available frequency spectrum to
accommodate the expanded composite signal bandwidth.
(2) TDM signals. In time-division multiplex, there is no direct relationship between channel numbers
and composite signal bandwidth. This condition springs from the fact that TDM signals are digital in
nature (pulses). The time frame which the equipment is designed to handle limits the number of
channel pulses that can be accommodated. Merely increasing the frequency spectrum for the
composite signal does not allow more channels to be added, because the time frame can handle only a
limited number. To add more channels, it is necessary to change to a terminal equipment type that
will allow the use of a different time frame.
3-9. APPLICATION OF CARRIER SYSTEMS
Carrier systems have been adapted to both commercial and military communications. In many cases, both
military and commercial systems have similar applications. Generally, carrier is used in systems that are required
to carry many parallel channels of information.
a. Specialized Military Applications. Most military carrier systems are designed for use in voice
communications networks. Both telephone and telegraph carrier terminals have their appropriate uses in such
systems. But carrier is by no means limited to these applications. Examples of specialized carrier applications in
military systems include:
(1) A landline on-tone, off-tone carrier system conveys radar data between installations of an air defense
(2) A radio-carrier link conveys command signals to a guided missile.
(3) A radio-carrier link conveys combined telemetry signals from a satellite in orbit.
(4) Landlines or radio link carrier systems tie together headquarters elements within an area
communication system. The carrier channels serve primarily as trunk lines between switchboards.
(5) Landlines or radio link carrier systems interconnect with commercial systems. Military equipment
and manpower can be saved by using commercial carrier channels when they are available.