Lesson 1/Learning Event 9
must be removed from service until the trouble is cleared.
The technical
controller must understand that when the circuit distorts the telegraph
signals, little can be done to remove the distortion. Part of his job is to
report the existence of the distortion and to compensate for its effect
whenever he can. The task of finding and removing the source of distortion
belongs to the equipment technician.
Moreover, removing the cause of
distortion is a far more useful operating technique than compensating for
the effects.
a. Sources of telegraph distortion.
Telegraph signal quality can be
impaired anywhere along the communication system, from message transmission
to message reception. The communication system may include cable circuits,
radio circuits, and carrier channels, as well as the terminating
teletypewriter sets.
Telegraph signal quality can also be impaired by
interfacing communication circuits and signals that are not compatible.
(1) Teletypewriter sets.
The driving motors of teletypewriter sets
must rotate at the same speed to print good copy. Minor speed differences
can be accommodated by the synchronizing scheme of the telegraph signals.
Major differences in motor speed must be corrected before the system can be
turned over to traffic.
Moreover, the selecting mechanism within each
printer must "see" and correctly interpret the information from the received
signal pulses. If system analysis proves that motor speeds are correct and
the teletypewriters are free of signal distortion, the remaining distortion
in the system is created within the facilities that interconnect the
teletypewriters.
(2) Information transfer.
The telegraph signal code used for
teletypewriter communication consists of a series of DC pulses in a
prearranged code. The intelligence conveyed by the code is included within
the space-to-mark and mark-to-space transitions.
As the pulses travel
through a communication facility, the change in pulse waveshape causes the
delays in transitions. Likewise, improper loop current and maladjustment of
equipment items can cause variations in transition time. However, as long
as enough of the pulse arrives to be recognized by the terminating device,
the message should be printed correctly.
Misprinting occurs when the
transitions are displaced in time beyond the capability of the printer to
interpret the code combinations correctly.
(3) Channel bandwidth of telegraph carrier terminal.
When DC
telegraph signals enter a channel of telegraph carrier equipment, the
information contained within the telegraph pulse transitions is impressed on
the carrier. The rate of occurrence of these
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