Lesson 2/Learning Event 5
can pass any inferior type of signal. However, conditioning all circuits to
AUTOSEVOCOM requirements seems to be a waste of money and time.
On the
other hand, if circuits are to be switched between different types of
service, it may be advantageous to condition each circuit to the highest
requirement, namely AUTOSEVOCOM. Table 5 shows that one schedule may satisfy
more than one type of service.
An example is circuit parameter code D2,
which is specified in items 1P, 2A and 5C for data, voice-frequency
telegraph (VFCT), and AUTODIN, respectively. The regional control office of
DCA selects the appropriate technical schedule and forwards the information
to station personnel.
g. Upon receipt of a request for specified service by a subscriber, DCA
prepares a Telecommunication Service Order (TSO) and forwards copies to the
various facilities involved in providing the service.
The TSO includes,
among other items, a specification of parameters taken from the DCS
technical schedules (appendix).
The circuit conditioner then clears the
circuit with the technical controller, and proceeds to condition it. When
the circuit meets requirements of the parameter code indicated in the TSO,
it is returned to the technical control.
h. Circuit
parameter
codes
carrying
letters of
identification,
including V, D, S, N, and Z. These generally signify voice, data, special
grade, DC teletypewriter, and secure voice, respectively.
The order of
ascendancy is likewise V, D, S, and Z.
A circuit conditioned for a D
parameter code can serve for either D or V, but a V code cannot serve for D.
Of course, any circuit conditioned for Z can serve any lower parameter,
parameter code N is usually handled as a separate matter.
i. Frequency response (insertion loss).
The most important circuit
parameters in voice communications to the circuit conditioner are those
parameters covering insertion loss and envelope delay. Where voice signals
are transmitted, the insertion loss characteristic assumes the greater
importance of the two.
Where data signals are transmitted, the envelope
delay characteristic is more important, but for now we will keep the
discussion with frequency response.
(a) Channels having poor frequency response generally discriminate
against frequencies toward the low and high ends of the channel bandwidth
while favoring the frequencies near the midband. The resulting distortion
of the composite signals are known as frequency-versus-amplitude distortion.
Frequency-versus-amplitude
distortion
is
caused
by
the
electrical
characteristics of communication system components.
(b) The human ear is fairly tolerant of frequency-versus-amplitude
distortion; however, excessive distortion can degrade
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