a. Bandwidth. The bandwidth of each of the four channels in the telephone carrier terminal is
approximately 3,200 hertz (Hz), extending from approximately 300 to 3,500 Hz. In military communications,
for convenience, this channel extending from 300 to 3,500 Hz is referred to as a 4-kHz voice channel. Any
signal below 300 Hz or above 3,500 Hz cannot pass over the system. This bandwidth is usually sufficient to
carry all components of understandable speech. It is evident from Figure 1-1 that all four telegraph channels
can be placed within the bandwidth of one telephone carrier channel. In some installations, as many as 16
narrowband telegraph carrier channels have been multiplexed into one telephone carrier channel.
b. Power distribution. The amount of power in any one channel signal within the 16-channel composite
signal is far less than the power of one channel of a 4-channel system. As long as the signal-to-noise ratio (S/N)
remains high, all 16 channels can be expected to print satisfactorily. However, when conditions are not
satisfactory, it is sometimes desirable to reduce the number of channels from 16. The same principle holds true
for the total number of telephone carrier channels in use at any time. Reducing the number of channels in use
also reduces the composite signal bandwidth. This reduction of signal bandwidth also improves the S/N
because the radio receiver operator can then reduce the bandwidth of the intermediate frequency (IF)
transformers in the same proportion.
c. Signal amplitude. The signal amplitude in each channel of the telephone carrier terminal tends to vary
over wide limits. Each channel, therefore, is normally provided with a peak limiter to prevent the loud talkers
from overloading the system. It is always important that the telegraph carrier composite signal level be
maintained at a level below the point where the peak limiter goes into operation. If the peak level exceeds this
value, it could cause crosstalk and interchannel interference. Further, if the composite signal level from the
telephone terminal overloads the radio facilities in any manner, telephone crosstalk may result, along with
telegraph tone crosstalk. Under this operating condition, the first circuit casualty will probably be a telegraph
carrier channel. Good system operation demands adherence to the established level of signal for each
subsystem channel, as well as for each subsystem composite line signal.
d. Facsimile channel. Since the facsimile signal commonly uses amplitude-modulated 1,800 Hz carrier,
the sideband intelligence and the carrier fit into the 3,200-Hz bandwidth of a telephone carrier channel. Unlike
telephone conversations, however, facsimile reproduction is very sensitive to distortion, because what appears
to the ear as minor annoyance in a telephone message shows up in the reproduced facsimile image as streaks.
Also, any loss of the synchronizing pulse causes the reproducer to lose synchronism, resulting in reproduction
of a "torn" picture. Facsimile needs a circuit that has a high S/N.
LEARNING EVENT 9: DIGITIZED COMMUNICATIONS SYSTEM
1. Vocoder use. Because of its space and weight limitations, the satellite has limited power output. The very
weak signal received at the ground terminal is sometimes masked by a curtain of noise. One method of
improving the S/N is to use a vocoder,
SS0031
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