(2) All three curves are plotted so that 0 db at 1,000 Hz is the minimum
attenuation each of the three telephone receivers introduces. A set
of sensitivity curves would indicate C-message having the highest
sensitivity, F1A the next, and 144 the least. C-message weighting
showing the widest bandwidth and highest sensitivity will pass both
signal and noise with less attenuation than the other two
weightings. The improvement in the telephone receiver voice quality
requires a similar reduction in circuit noise. Noise measurements
are usually taken with respect to 1,000 Hz because all three
weighting curves show minimum loss (maximum output) at near that
frequency. It is apparent from these curves that a listener will
hear maximum sound from the handset receiver in the neighborhood of
1,000 Hz. It is also apparent that C-message weighting delivers a
range of frequencies with more uniform response than either F1A or
144 weightings.
1-3. AMPLITUDE-FREQUENCY DISTORTION
Unequal attenuation is called amplitude-frequency distortion because the
amplitude of the received signal changes as the frequency of the transmitted
signal is changed. The measurement technique requires a technician to measure
and record received signal levels at specified frequencies. The transmitting
station sends the specified frequencies at constant level.
The difference
between the received signal and the constant level indicates the amount of
amplitude-frequency distortion present in the line or circuit.
Amplitude-
frequency distortion is caused by the phase shift between current and voltage
through a reactive circuit. Values of L and C in the line, together with the
frequency of the signal, determine the effective value of reactance that exists
at any particular time.
a. True and Apparent Power.
(1) True power. True power is the power in a circuit when the load is
purely resistive. This condition is obtained when the phase angle
between current and voltage is zero.
At this time, power is the
product of voltage and current.
(2) Apparent power. Apparent power is the power in a circuit when the
load is reactive.
Reactance due to inductance and capacitance in
the load causes a shift in the phase angle between current and
voltage.
A change in reactance occurs with each change of
frequency, causing the power in the load to vary with frequency.
Thus, when we measure the output level from a telephone line as the
transmitted signal frequency is varied, we will notice a change in
measured power output.
b. Reactance Variation.
Inductive and capacitive reactances in a
telephone line do not change in the same proportion to a given change in
frequency. When loading coils are inserted in the line, this effect is even
more pronounced.
When a loaded telephone line transmits frequencies higher
than cutoff, distortion will greatly increase beyond that point.
If
frequencies higher than the cutoff must be transmitted over this line, better
results will be achieved by removing the loading coils and making correction in
a suitable terminating device such as an equalizer.
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