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At the same time, peak levels have remained the same, and this
invariably raises the questions: Is the signal limiting we are applying
deleterious to the signal? Can you hear it in operation? The answer
is mixed; an experienced listener may be able to identify the signal
limiting as such, but it will not sound unnatural if it is properly
done. The limited signal is louder and as such permits an improvement
in intelligibility.
In normal speech applications 12 dB would be about the maximum amount
of signal limiting that would be employed. However, for music applications
it is customary to provide for a higher degree of signal limiting,
plus some degree of compression. Compression and limiting are related
operations, and a combination of both enables level manipulations
to be made over a fairly wide dynamic range.
An example of the need for both limiting and compression would be
a speech reinforcement system in a house of worship where both clergy
and lay persons may be called upon to talk. Both experienced and
inexperienced talkers will present a wide range of levels at the
microphone that can be safely processed by a limiter and compressor
in tandem.
METERING IN AUDIO TRANSMISSION SYSTEMS
Today there are basically two kinds of metering, average and peak.
The common VU meter is an example of an averaging meter and as such
has nominal rise time and fall-back times of about
0.3 second. The meter’s rise time is the time taken for a
steady-state input signal to the meter to reach 63 percent of its
final deflection; the fall-back time is the time taken for the steady-state
signal to return from full deflection to 37 percent deflection.
Rise and fall-back times are known collectively as the ballistics
of the meter.
The original VU meters were passive devices and as such, had ballistic
characteristics of a spring-loaded coil with inertia immersed in
a magnetic field. Since it is basically an average-reading device,
the VU meter has met with continuing success in broadcast work,
inasmuch as its readings correspond to the perceived loudness of
speech signals.
From their inception, peak program meters (PPM) have been electronic
devices an as such can be made to respond very quickly. Typically,
a PPM has a rise time of about 10 milliseconds and a fall-back time
of about 4 seconds. The rapid rise time permits accurate reading
of signals of very short duration, while the slow fall-back time
gives the operating engineer adequate time to observe the signal’s
value.
Figure 11 shows views of the VU meter (A) and
the PPM meter (B). Rise time ballistics of the two types of meters
are shown at C. Relative calibration points on the meter faces for
four kinds of meters are shown in Figure 12. If
both VU and PPM meters are calibrated as shown in Figure 12, normal
speech program will read maximum values of about +2 or +3 VU, while
on the PPM the corresponding readings would be between markers 4
and 6 on the face of the meter, due to the more rapid rise time
of the PPM relative to the VU meter.
Figure 11: The VU meter (A); the
peak program meter (PPM) (B); rise-time ballistics for VU
meter and PPM (C). |
Figure 12: Comparison of European
PPM and American VU meter calibration standards. |
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