STUDY GUIDE
Appendix A and Appendix B carry related information. As you study them,
relate the information contained in the two publications.
For example,
paragraphs 25 and 26 in Appendix A and paragraph 5 of Appendix B discuss the
effect of capacitance in a transmission line; however, the latter publication
expands on the information in the former. Follow this procedure when answering
the exercises in this lesson.
ATTACHED MEMORANDUM
1-1. IMPAIRMENTS TO TELEPHONIC COMMUNICATION
Interfering electrical energy which falls within the voice-frequency band
(approximately 200 to 3,000 Hz) impairs the message reception of the listener
in a telephone communication system. Telephone system components, as well as
the end instruments, must have sufficient quality for the voice to be
recognized in normal telephone communications. There is no need to have higher
quality in the system than the telephone receiver can reproduce.
Higher
quality requires the use of a wider passband. An increase in the passband of
the system invites the entry of noise and crosstalk into the signal through the
unused portion of the band. Although the increased bandwidth will not require
additional power for normal voice transmission, usefulness of the received
signal will not be improved because the end instruments limit the bandwidth.
apparatus which will not markedly improve voice quality. The entire emphasis
on maintaining voice quality is therefore centered on minimizing the effects of
impairments in the limited bandwidth required for voice transmission.
a. Responsibilities of Installers and Users.
A telephone communication
system is an extensive combination of wires, apparatus, and people.
Some
people are responsible for installation and maintenance of the system, while
the users are responsible for proper operation over the system.
Correct
installation will minimize the introduction of impairments. Correct operation
will gain for the user the best advantages offered by quality installation.
(1) Installing telephone facilities requires careful work, and testing
after work has been completed. This involves good planning, layout,
and workmanship. Poor workmanship in the installation process makes
the work of the technical controller and circuit conditioner much
more difficult than it need be.
Although they cannot correct
mistakes in installation, circuit controllers must be able to
recognize the effect produced by those mistakes and to report their
observations to higher authorities.
(2) Correct installation requires consideration of the location of power
lines because the power carried over them is so much greater than
the voice signal passing through telephone lines. Induced voltages
from power lines may be so much greater than the voice signal as to
completely overwhelm the weak telephone currents. Telephone lines
should be installed so as to pass under power lines at right angles.
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