6.
Be able
to
distinguish
between
reference
and
reference-return
frequencies.
7.
Be able to calculate envelope-delay distortion, given the total delay
and the reference delay.
8.
Be able to distinguish among characteristics of test sets designed to
measure envelope-delay distortion.
ATTACHED MEMORANDUM
2-1. Envelope-Delay Distortion.
The difference between the maximum and
minimum transit time of different frequencies within a specified bandwidth
through a circuit is called delay distortion or, more simply, delay. When
transit time varies with frequency, each frequency arrives at a slightly
different time or phase relationship. This causes the shape of the received
modulation envelope to be different from that of the transmitted signal, and
the effect is called envelope-delay distortion.
Since delay cannot be
decreased, equalization or corrective network must always add delay so that
the characteristics of a line with envelope-delay distortion together with
equalizer networks yield a near-constant-delay time at all frequencies
within the band of interest.
The resulting constant-delay time is nearly
always greater than the maximum observed delay before equalization.
One
type of test set used to measure envelope-delay distortion of a telephone
line is Envelope-Delay Distortion Measuring Set TS-2669/GCM, shown in figure
8 of appendix A, section V.
The test results are used to plot a delay-
frequency curve which the circuit conditioner compares with designated
parameters.
The TS-2669/GCM consists of a transmitter and a receiver
section.
We will refer to these two sections as the transmitting or
receiving TS-2669/GCM depending on which function it is serving.
The
technician at the transmitting TS-2669/GCM is responsible for performing the
test.
a. Test arrangement.
The preferred method of making envelope-delay
distortion measurements is end-to-end with reference-return path shown in B
of figure 2-1.
This method simplifies the operation because all readings
are taken by one technician. However, this method can only be used when a
good quality auxiliary line is available. If no suitable auxiliary line is
available, the end-to-end method, without reference return, will be used as
shown in A of figure 2-1.
In either method, two TS-2669/GCM's are
necessary, along with two order wire telephones.
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