c. Data Terminals. The data terminals in A of figure 4-6 are necessarily
low-speed because their speed is limited primarily by the bandwidth of the
filter in each telephone channel.
High-speed data terminal operation can be
secured by use of the full bandwidth capability occupied by four channels, as
shown in B of figure 4-6.
By doing this, we sacrifice the four independent
channels to gain the benefit of one wideband facility.
Under such an
arrangement, a suggested method of operation is to place the data terminal near
the telephone carrier terminal so that the same receive line equalizer serves
both the telephone terminal and the data terminal for amplitude-frequency
correction.
With this system we avoid the necessity for modulation and use
only amplifiers to pass the signal directly through the wideband channel
facility.
However, if envelope delay still exists, a combination equalizer
will have to be used to compensate for the delay.
4-6. FUTURE DEVELOPMENTS
Conditioning telephone lines to pass data signals is relatively new.
Equalizers now in use require considerable skill and training to operate, as
well as much time for the adjustment procedure.
Commercial services are
developing highly sophisticated equalizing devices. One such device is semi-
automatic in operation in that the operator first plots uncorrected circuit
responses after which he sets controls in the exact opposite positions to
selected points on the response curves. This procedure largely eliminates the
time-consuming manual method now being used.
Another new type of equalizer
uses entirely automatic compensation under control of a computer.
This
combination
makes
possible
self-analysis
and
correction
of
line
characteristics.
One disadvantage of the fully automatic system is that
control is taken out of the hands of the operator. He is sometimes unable to
take corrective action on the line because his attempt may cause the automatic
features to readjust the equalizer away from the desired parameters.
STUDY EXERCISES
In each of the following exercises, select the ONE answer that BEST
completes the statement or answers the question.
Indicate your solution by
circling the letter opposite the correct answer in the subcourse booklet.
1. The original purpose of an equalizer was to equalize a telephone circuit
so as to minimize
a. phase-frequency distortion.
b. amplitude-phase distortion.
c. harmonic-frequency distortion.
d. amplitude-frequency distortion.
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