4-2. AMPLITUDE-FREQUENCY EQUALIZER
a. Characteristics. An ideal telephone pair is a "flat-loss line." That
is, the loss is constant over all frequencies in the voice range. The graph of
such a telephone pair is characteristically a horizontal line. In figure 4-1
it is labeled "desired flat loss." Unfortunately, a long telephone pair will
seldom exhibit uniform loss characteristics. Normally, the loss increases with
a rise in frequency as indicated on the line-loss curve.
Figure 4-1.
Characteristics of a voice-frequency equalizer.
(1) To obtain the desired flat loss, it is necessary to counteract the
effect of the line constants which produce the line-loss curve.
This is theoretically accomplished by inserting the equivalent
lumped reactive constants which will produce the exact opposite
equalizer-loss curve. If the mirror image is complete, the result
is a flat-loss line.
(2) The desired flat loss is always greater than either the line or the
equalizer loss, since the flat line loss results from the sum of
both line and equalizer loss. An amplifier must either precede or
follow the equalizer to compensate for the total loss.
(3) The amplifier used to raise the signal level also has its own gain
characteristic. Combining the equalizer and amplifier may cause the
characteristic to lack uniform response. The recommended procedure
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