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Average
The loudness of a waveform is closely related to the RMS value.
Loudness meters have ballistic properties that emulate the human
auditory system. The original volume indicator (VI) is such a meter.
It has a 35 uS rise time and a time constant of about 300 ms, both
deter mined by exhaustive testing of human subjects in the early
days of audio. The display of the VI is a good indication of both
the loudness of and power generated by an audio waveform. It is
important to note that because of its ballistic properties, VI’s
do not accurately display peaks in the program material, so it is
possible for clipping to occur even though the meter does not indicate
it.
| The Volume Indicator |
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Pictured at left are both analog and LED volume indicators.
These so-called “VU” meters have ballistic properties
that emulate the human auditory system. Since the meter does
not track peaks in the program material, a “safety margin”
must be left above meter zero to assure that clipping does
not occur. Since these meters display the upper region of
the device’s dynamic range, if the meter does not move
during use of the system, the signal level is unnecessarily
close to the thermal noise floor The meter should read near
“zero” under normal operation. Both of these meters
have similar ballistic properties. Note the “peak”
light on the analog meter face. |
So Which Do I Use?
A manufacturer may choose to display any of the previously described
parameters on the meter of a component. Most indicators are either
peak or average. Special meters can be constructed to read the true
RMS value of the waveform, but the average value is an adequate
approximation for most applications.
The peak light (usually and LED or light-emitting diode) is the
most straight forward. It lights at or near the clipping point of
a component. Most peak lights actually flash when voltage is 3 to
6 dB below clipping, but not always. It is important to determine
the threshold of a peak light by test or consulting the documentation.
A peak light is usually all that is necessary on a component that
is “down stream” from the mixer, since it is desirous
to drive such devices close to their clipping point to fully utilize
their available dynamic range. In most cases, it is normal for a
peak light to flash occasionally during use.
| The Meter Olympics
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| This waveform consists of a steady sine wave that has
had one cycle each second amplified by A true peak-reading
meter will show a 20 dB deflection. A loudness meter will
typically deflect 3 or 4 dB. Such waveforms are easily generated
by PC-based wave editors. They can be used to test the various
properties of an unknown meter.
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The meter on a mixer has a different task than the peak light.
It’s indication should correlate with signal loudness, providing
visual feedback to the system operator to assist the listening process.
Volume indicators are calibrated in decibels. Unfortunately, “meter
zero” can be somewhat arbitrary between brands of equipment.
On most modem devices, it will be +4 dB relative to 0.775 volts.
On other devices, it may indicate the optimum range of operation,
usually 12 -20 dB below clipping.
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The meters of many consumer recording devices have an
average response, but are marked like a PPM.
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The signal LED at right lights at 45 dB below clipping.
The overload LED lights at 4 dB below clipping. The OL (overload)
LED should flash occasionally under normal use..
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The IOC light on this power amplifier lights when the
output waveform is distorted relative to the input waveform.
It is a “true” clipping indicator and should not
flash under normal use. Many power amplifiers do not include
a “safety region” built into the clip light, probably
because the occasional clip is not very audible. Even so,
even occasional clipping can stress the loudspeaker and cause
damage.
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Peak reading meters are particularly useful for recording. The
PPM or Peak Program Meter has a very short response time that allows
it to track the peaks in the program material, assuring that none
of them overload the medium that is storing the information. Because
of their fast response time, the indications of PPMs do not effectively
correlate with the loudness of the signal.
When operating an audio device, it is essential to know what is
actually being indicated by the metering. This may require some
experimentation on the part of the user, since information about
the metering is often not included in the product’s documentation.
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