used.
Equalizers give technicians tools to correct for
distortions
that
communication circuits introduce into the telephone signals.
a. Analog Signal Distortions. A voice signal is a form of analog signal.
Analog signals vary in amplitude and frequency, and theoretically have no
discontinuity (break) in the signal waveform.
(1) Harmonic distortion.
Harmonic distortion is minimized by the
assurance of correct operation of amplifiers and other circuit
constants within their amplitude capabilities.
In other words,
technicians must make every effort to see that repeaters and
amplifiers in carrier terminals are not overloaded. Equalizers can
do nothing to minimize harmonic distortion once it is produced.
Harmonic distortion produces additional frequencies not present in
the original signal.
(2) Frequency distortion. Frequency distortion of an analog signal is
produced mostly in equipment items which modulate a carrier with
sound energy. Frequency distortion is minimized by the assurance of
identical carrier frequencies for modulation and demodulation
wherever possible.
Equalizers cannot correct for frequency
distortion.
Frequency distortion causes a shift in all the
frequencies in a received signal simultaneously.
(3) Amplitude-frequency distortion. When an analog voice signal leaves
the output of a terminating device, such as a telephone or a line
amplifier, there is a relationship between the amplitude and
frequency of the signal, which should be maintained throughout the
circuit.
Any variation in the received signal between the
amplitude-frequency relationship is amplitude-frequency distortion.
This is the type of distortion that line equalizers were originally
designed to minimize.
b. Digital Signal Distortions.
Most digital communication circuits use
modems; these are combination modulators and demodulators. Digital signals at
the sending end enter the modulator as dc pulses and leave the demodulator as
dc pulses.
The line signals between the modems usually carry tone signals.
One task of the technician is to maintain the quality of these line signals
throughout the length of the circuit.
The intelligence within the digital
signals is carried in the transitions. Every effort is made to minimize the
effect of the circuit on the received signal transitions, and thus minimize the
effect of distortion. It is therefore desirable that all received frequencies
in the signal maintain the same amplitude, frequency, and phase relationships-
as when transmitted.
(1) Amplitude distortion. The voice-frequency (VF) signals that carry
the intelligence contain enough of the transitional frequencies to
allow the demodulator to correctly reproduce the digital pulses,
with a minimum of error and distortion. Maintaining the amplitude
of these frequencies is not a difficult technical matter.
Line
amplifiers overcome line loss.
Finally, a combination of
amplifiers, limiters, and rectifiers in the demodulator reproduces
the digital pulses to their original form.
333 L4
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