Lesson 1/Learning Event 9
c. Transition delay. In an ideal signal pulse, the change between two
levels occurs instantaneously. Consequently the transition time between the
two limits of the pulse is zero. When that ideal pulse travels through a
device having inductance or capacitance, phase shift causes a delay in
recognition time of that pulse by the receiving device; The time delay from
the start of the transition to the time the receiving device senses the
change is called the transition delay.
Delay compensation is used to
minimize the variation in transition delay; that is, make all delays
approximately equal to the longest one.
In this way the variation in
transition delay is minimal.
It should be apparent that if the beginning
transition has a different length from the ending transition, the pulse has
changed length so far as pulse recognition is concerned.
Whenever
transition delays are unequal, pulse lengths vary and the received message
will suffer.
These principles hold true for telegraph, pulse-code
modulation, or any other form of pulse transmission. Moreover, the effect
is related to the length of the pulses.
The higher the speed of pulse
transmission, the shorter are the pulses, and thus the greater is the effect
a given transition delay has on signal quality.
Transition delay is a
characteristic of a circuit and is therefore an indication of circuit
quality.
The quality of a circuit determines the quality of the signal
passing through it. The term transition delay is normally used in reference
to DC signal pulses, while envelope delay is normally used in reference to
signal pulses derived from modulation of an AC (VF) carrier wave. When a
voice channel is conditioned for a voice circuit, and a digital circuit is
routed over the channel, the channel must be checked for envelope delay.
d. Frequency translation.
When a technical controller attempts to
interface two communication circuits, he sometimes finds signals other than
communication signals. Moreover, these special-duty signals sometimes occur
at different frequencies. If such frequencies are not identical in the two
circuits to be interfaced, frequency translating devices will have to be
incorporated in the interfacing facilities. These special-duty signals are
used for signaling, ringing, switching, or testing.
Learning Event 9: TELEGRAPH SIGNAL QUALITY
Unlike telephone distortion, which you can detect by poor sound
quality, telegraph distortion does not make its presence known until it
becomes so bad as to cause the teletypewriters to misprint. Because of this
fact, one of the major duties of a technical controller is to periodically
measure the distortion in the DC receive telegraph loops. Only in this way
can he determine that signal quality is deteriorating.
If he waits until
misprinting occurs, he has delayed too long; the channel
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