Lesson l/Learning Event 9
transitions is related to the speed of transmission. Likewise, the expanse
of sidebands developed in the modulation process are related to the speed of
transmission.
Since each channel contains a send filter to limit the
expanse of sidebands, the bandwidth of this filter limits the speed of
transmission that the channel can carry. The higher the transmission speed,
the wider must be the channel bandwidth. Increasing the transmission speed
beyond the channel capability is almost certain to result in excessive
telegraph distortion.
b. Data-transmission. The use of data signals is rapidly increasing in
military communications.
Since data pulses are much shorter than normal
telegraph pulses, more pulses are used in the code and the keying rate is
faster.
Existing
telegraph
terminals
designed
for
teletypewriter
transmission cannot be used directly for data, but changes can be made to
accommodate the higher speed data signals. Four methods that can be used to
accommodate the data signals in a communication system using telegraph
terminals are:
(1) Use telegraph carrier equipment having wide-band channels.
(2) Slow down the rate of keying to the equivalent rate of telegraph
transmission so that the signals may pass over a narrowband channel of
existing terminal equipment.
(3) Replace existing telegraph
carrier
terminals
with
terminals
having enlarged bandwidth capabilities.
(4) Use serial-parallel transmission.
c. Serial-Parallel transmission.
Operating principles of serial-
parallel transmission are illustrated in figure 1. This is a system to make
use of low-speed narrow-band channels of a multichannel frequency-division
multiplexing (FDM) terminal for the purpose of transmitting high-speed data
signals.
(1) Figure 1 illustrates one-way transmission only, using a two-wire
line.
When four-wire transmission is used, a full-duplex circuit can be
established, each two-wire line being used for one direction of
transmission.
(2) Each station has a compatible serial-parallel converter.
In
minimizing distortion it is most important that the converter at the east
station be synchronized to the incoming signal so that the switching occurs
in the same sequence as the received signal pulse.
As a signal pulse
arrives in the DC receive loop of a given channel, the converter is
sequentially switched to sample the output of that channel.
Failure to
maintain synchronization causes distortion of that received signal.
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