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Looking back, the first step in the creation of the new metering standard and display instrument was to question the outdated assumptions and limitations imposed by the old standards.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 format.

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 Display
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 ballistic.

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