desirable to have uniform delays across the voice-frequency band. This ideal
condition is indicated by the corrected circuit curve. Notice that the "flat"
characteristic here means uniform delays at each of the various test
frequencies. Keep in mind that circuit parameters, stated in terms of envelope
delay, relate delay time of the carrier modulation envelope to various test
frequencies in the spectrum.
In order to make all delays uniform, it is
necessary to insert a network.
The sum of delays of the line and inserted
network is nearly constant. However, the difference between delays is also a
constant, but is never zero. Another name for the inserted network is a delay
equalizer.
(1) Uniform delay is achieved when the inserted network delay curve is
an exact mirror image of the delay curve of the uncorrected circuit.
In practice, this desirable condition is difficult to achieve, but
the circuit conditioner should strive for it.
(2) Since variation of amplitude-frequency response and delay are inter-
dependent, the circuit conditioner usually must compromise both
conditions to give the best combination of characteristics.
(3) In practice, the technician does not measure delay in the inserted
network (equalizer). His first operation is to plot the envelope-
delay characteristics of the uncorrected circuit from readings taken
at
several
selected
frequencies.
He
then
inspects
the
characteristic curve to determine whether parameters for that
circuit are satisfied.
If parameters are satisfied, the line or
circuit needs no correction, and he needs no delay equalizing, even
though amplitude-frequency equalizing may still be necessary.
If
delay equalization is required, he inserts estimated network
segments in the circuit so that the total delays of circuit and
network approach similar values.
Further adjustment makes the
delays at all selected frequencies nearly uniform.
The ideal
condition is reached when all delays are identical, in which case
the characteristic becomes a straight line across the graph.
(4) The following examples are based on figure 4-3.
Assume that the
frequency range is from 800 Hz to 2.1 kHz, and that maximum delay is
3.5 milliseconds (ms).
Example 1. What is the delay of the uncorrected circuit at 1 kHz?
Solution: The uncorrected circuit curve intersects
the
1-kHz
frequency line at approximately 2.9 ms.
Example 2. How much delay is inserted at 1 kHz to provide the
corrected circuit delay of 3.5 ms?
Solution: Inserted delay = corrected delay - uncorrected delay
= 3.5 - 2.9 = 0.6 ms.
Example 3. What are the delays at 800 Hz?
Solution: Uncorrected delay (point A) = 3.2 ms.
Inserted delay (point B) = 0.3 ms.
Corrected delay (point E) = 3.5 ms.
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