Moving Toward A New Standard
It became obvious to Dorrough engineering that both standardized
metering approaches were "acceptable" when properly
applied and interpreted. It was even more apparent that modern
technology, guided by a unique insight into the studio environment,
would be capable of eliminating the need to choose, and even
the need to interpret. The new Dorrough meter concept is based
on the actual relationship of PEAK to QUASI-AVERAGE composite
waveforms. In other words, this revolutionary instrument is modeled
on the complex "picture of sound" displayed on an audio
oscilloscope. Because the scope is impractical for real-time
sound monitoring, the data is interpreted via micro-computer
and presented in an instantly comprehensible form.
Looking back, the first step in the
creation of the new metering standard and display instrument was
to question the outdated assumptions and limitations imposed by
the old standards. Audio systems, recording techniques and program
material have evolved beyond the VU standard's "10 dB over
reference level" ceiling. It was also necessary to dispose
of the PPM's assumption that peak-to-average relationships are
relatively constant. Once unshackled from this older technology
we could finally move audio-level measurement toward the demands
of the next century.
In moving toward this new standard
we could not forget the valuable contributions of those who developed
and refined the VU and PPM systems. For instance the VU experience
informed us that the new system should be more than a simple "peak
meter", and that the ballistics of the new meter would correlate
with the sound's cadence. Combining the VU and PPM on a single
scale could theoretically provide a way for us to begin to examine
the relationship between RMS and PEAKS. The composite-meter brings
to mind that old yarn about the two blindfolded men at opposite
ends of an elephant. The man touching the trunk thinks the beast
is a snake, while the man holding the tail is absolutely certain
that the animal is a mouse! The problem with the basic concept
of the composite-meter is that its two components, VU and PPM,
were developed independently as "stand-alone" measurement
systems. Exacerbating the problem is the fact that both standards
have been distorted and compromised over half a century in an effort
to keep up with the times. The mating of two indicators with such
differing rise and decay characteristics, on the same scale, results
in an unhappy marriage.
This brings us back to the model
of the audio oscilloscope. A long persistence CRT is capable of
displaying the full complexity of the audio signal. The peak excursions
are shown in real-time with recurrent amplitudes building density
toward the center of the screen. These factors, PEAK, AVERAGE,
and their relative positions on the scale, are the touchstones
of the first "dimensional" sound measurement system.
Recent breakthroughs in digital technology make it possible for
Dorrough to compress the "essential perceptions" of the
scope into a practical size and format. This device can also analyze
and then translate the data into a streamlined, instantly comprehensible
The New "User-Interface"
Dorrough quickly determines that a "non-mechanical" display,
free of physical recoil and backswing is instrumental in making
this concept most effective. Rather than having LEDs merely mimic
inherently, non-linear mechanical movements, the new device fully
exploits the unique characteristics of LEDs. For example, peaks
are tough to monitor via the typical sluggish mechanical movement.
LEDs don't suffer from inertia. The PEAK LED, can ride ahead of
the field of AVERAGE LEDs at the breakneck speeds of an oscilloscope
trace! Unlike the fleeting oscilloscope trace, the "persistence
of vision" (lasting impression on the eye/brain) of the LED
allows the user to comprehend the reading.
LEDs can be configured by color to
provide the user an instantaneous impression of what an automobile
racer would call, "redline". This frees the harried sound
technician from having to read a specific "dB" number.
A back-lit scale provides the numbers for those requiring more
precise information. This display is the first of its type to offer
high resolution, assigning an LED to each dB. The viewer can read
the scale accurately from any angle thanks to the aforementioned
lack of the "parallax" so problematic in mechanical displays.
This feature is greatly appreciated by users of larger mixing boards.
In the Dorrough meter a continuous row of AVERAGE LEDs represents
the math of the scope's Quasi-Average field, with the superimposed
single PEAK LED riding ahead.
The Patented Ballistics Behind The
As stated earlier, the new meter's PEAK ballistic is roughly analogous
to the fleeting peak scope trace. This works out to an Integration
Time about 100 times faster than PPM. Subtle damping is incorporated
to avoid oscillation. Also, the lasting retinal impression of the
LED makes this a usable speed. The PEAK ballistic is represented
by a single LED usually riding ahead of the pack. Because both
are ultimately linked to an analysis of the same waveform, "pure
tone" will cause the PEAK ballistic to merge with the AVERAGE
This brings us to the next step in
the development of the Dorrough metering system, the best Integration
Time for the continuous AVERAGE display. Getting back to our story,
if we think of the sound as the proverbial "elephant",
the power or LOUDNESS is in the "body" of the beast.
PPM ballistics obscure average data, while the VU is all but oblivious
to rapid, sharp peaks and transients. Determining the ideal Average
Integration Time for the new meter required extensive research
and experimentation. Relieving the AVERAGE display circuit of the
burden of having to make some concessions to peaks allowed the
development of an un-compromised average power algorithm. The result
is a true reading of energy in the complex waveform. The VU is
an example of a simple "averaging" type voltmeter with
a scale calibrated in power. The new meter's AVERAGE or "Persistence" bar-graph
display works out to have an integration time about twice the duration
of VU. The reference level for AVERAGE is set at 65% of full scale,
with the individual PEAK reading LED referenced at full-scale.
This metering device is the first to indicate both the peak and
quasi-average value of the composite waveform, relative to the
effective loudness of the program material.
The third factor bringing true "scope-like
dimension" to the new metering system is the meaningful relationship
between the two ballistics. As the "power density" of
the audio signal builds, the gap between the PEAK and AVERAGE ballistics
narrows. If only the sound density increases, as with processing,
the maximum peak LED reading remains relatively stable as the AVERAGE
closes in behind it, effectively quantifying the degree of compression.
The user is presented with more information, so well integrated
that the data is more quickly perceived and more easily understood.
Rather than having to choose between the VU's mission of insuring
consistent levels and the PPM's mission of protecting peaks, the
Dorrough Loudness Monitor accomplishes both, simultaneously!
© Copyright 1997 by Dorrough Electronics