(d) Envelope delay distortion is the calculated limits () of the
final line delays from the reference delay.
c. Performing the test, end-to-end method.
(1) Preparation.
Remove all circuit conditioning equipment by
strapping it out of the line circuit. Such items of equipment may introduce
delays that will interfere with accuracy of the tests. If single-frequency
(SF) units are used, make sure they are in the off-hook condition (no
transmission of ringing frequency). Connect the TS-2669/GCM's to the line
as shown in A of figure 2-1. The technicians must have a telephone order
wire so they can communicate while performing the tests.
(2) Selection of carrier frequency. The carrier frequency is varied
by means of the CARRIER FREQUENCY knob, and the technician may select any
one of three equal-level amplitude-modulated frequencies (25, 83-1/3, 250
Hz).
The most often used is 83-1/3 Hz because it provides the best
compromise between accuracy and stability.
(3) Function of the TS-2669/GCM. The transmitting test set modulates
a carrier and sends the signal down the line. At the other end of the line
the receiving test set demodulates the signal and extracts the modulation
envelope. In addition, the receiving test set generates a similar modulated
carrier within itself and extracts that modulation envelope.
Through
synchronized logic circuitry, the receiving test set compares the two
modulation envelopes and indicates the delay time between them on the DELAY
meter.
(4) Preliminary circuit test. The transmitting technician varies the
carrier frequency produced by his TS-2669/GCM across the frequency band
specified by parameters, and the receiving technician records initial line
delays at the various frequencies. Using the maximum and minimum observed
delays, they determine the primary delay and the average delay. The primary
delay describes the spread between maximum and minimum initial line delays.
The average delay is determined from these two initial line delays. Further
readings to determine relative values at all other frequencies in the band
of interest are based on the average delay.
The decision to equalize
depends on calculations for relative delays at all frequencies listed in the
parameter.
If the primary delay and relative delays are within parameter
tolerances, no equalization is required.
However, if any primary or
relative delays are out of parameter tolerances, the technician will have to
equalize. If no equalization is necessary, the average
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