a. Low capacity, 6/12-channel (pulse code modulation (PCM)).
b. Medium capacity, 12/24-channel (PCM).
c. High capacity, 48/96-channel (PCM).
d. Tropospheric scatter, 24-channel (PCM).
2.
Multiplexing.
Before proceeding into the identification of the multichannel radio
system components, we will review the principles of multiplexing. It
is important for you as a signal officer to understand the principles
of time division multiplexing and pulse code modulation.
These are
the
means
by
which
we
accomplish
tactical
multichannel
You also need to know the different components we
use to turn analog voice frequency signals into a digital PCM signal.
Finally, you need to know how these components form assemblages to
accomplish specific missions.
Multiplexing is a technique used to transmit or receive several
channels of voice or data simultaneously over a radio or cable link.
This allows the use of more channels.
There are two types of
multiplexing.
a. Frequency Division Multiplexing (FDM).
FDM allocates a
particular section of the bandwidth for each channel over the entire
transmission period. See figure 1-1.
b. Time Division Multiplexing (TDM).
TDM allocates the entire
transmission bandwidth to each channel for a specific period of time.
See figure 1-2.
This is the method most commonly used for Army
multichannel radio systems.
TDM assigns each channel a time interval in sequence with all
multiplexed channels. These time intervals are short and they repeat
at a high frequency. Samples taken from each channel must convert to
a form suitable for transmission in the selected medium.
The
receiving terminal demodulates the samples and separates them into
their proper channels using a timing signal from the transmitting
terminal.
Figure 1-3 illustrates the TDM principle in a simplified
circuit.
Synchronized switches S1 and S2 are in position A at the same time.
They shift to position B simultaneously. A telephone call on line 1
completes when both switches are in position A.
1-3
SS0016
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