1-6. Maximum Change in Audio Frequency.
When a signal passes through a telephone communications system that
includes frequency-division-multiplex (FDM) telephone terminals, the audio
signal is subject to frequency translation errors caused by instabilities in
the carrier frequencies used in the FDM terminals. Modern carrier terminals
in which one master oscillator synchronizes the frequencies of all carriers
in the system effectively eliminate the problem of audio-frequency change.
The problem of audio-frequency change arises when one or more older type
terminals using nonsynchronous carriers are included as part of the
communications system. A slight audio-frequency change is not important in
voice communications because a slight change does not significantly reduce
understandability of the sound.
However, in voice-frequency channels
handling digital traffic, a shift of only a few cycles can cause the
development of bias distortion, resulting in message errors.
In the test
for maximum change in audio-frequency, a stable reference audio signal is
injected into a voice channel at the sending terminal, and any deviation of
signal frequency from its original value is observed at the receiving
terminal.
Measurements should be made, if possible, in a voice frequency
channel at the high-frequency end of the translated waveband because
frequency variation will be most noticeable at the highest frequency.
a. Test arrangement. A block diagram of a typical test arrangement for
maximum change in audio frequency is shown in figure 1-10.
Equipment
includes the oscillator and voltmeter sections of Telephone Test Set AN/USM-
181 (appendix A IV, figure 2) and the frequency counter portion of Envelope
Delay Test Model 340B (appendix A, section V, figure 9). Tests are made in
both directions of transmission.
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