Lesson l/Learning Event 11
d. Automatic switching networks.
These networks include automatic
voice (AUTOVON), automatic digital networks (AUTODIN) and automatic secure
voice communications (AUTOSEVOCOM).
e. Circuit requirements. The requirements of circuit quality vary with
the type of service. The least stringent requirements are imposed on voice
(telephone) circuits, and the most stringent requirements being placed on
the automatic switching networks.
f. Quality requirement categories.
The required categories of
parameters for circuit quality generally include frequency response,
envelope delay, noise, crosstalk and harmonics.
Some parameters are more
important to voice communications while some are more important to data
communications.
Especially important to data communications are the
parameters of frequency response and envelope delay.
Equalization is
therefore one of the first operations a technical controller will perform to
condition a voice circuit for high-speed data communications. Equalization
may not be critical in voice circuits such as ringdown and telephone, but it
is always essential in automatic switching networks passing over voice
facilities and usually require equalization.
Learning Event 12: STANDARDS FOR NOISE MEASUREMENT.
a. Noise weighting. Noise weighting is a commercially developed rating
system to identify the characteristics of the type of a receiving element in
a telephone handset (subset).
Each type of receiving element has a
characteristic curve that relates the interfering effect of noise on a
received voice signal.
Data for these curves were arrived at through
statistical analysis of listener's reports of voice understandability in the
presence of known values of noise amplitude and frequency. Noise weightings
therefore refer to specific handset (subset) types and for this reason must
carry identification of the receiving element in the handset.
Noise-
measuring test sets are calibrated in weightings similar to those given the
telephone
receiver.
When tests
are
made
for
other
than voice
understandability (data, for example), the test set must indicate the limits
of the frequency band over which the weighting is specified.
(1) F1A-Line weighting. F1A represents the class of weighting used
in a noise-measuring set to measure noise on a line that is terminated by a
302-type or similar subset.
(2) HAl-Receiving weighting.
HAl-receiver weighting is used in a
noise measuring set to measure noise across the HAl receiver of a 302-type
or similar subset.
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