Chief Engineer. Cutting Edge Technologies/Omnia
One interesting facet
about broadcasting is that many engineers and program directors have long thought of audio
processing as "black magic," using "black box" technology to
accomplish apparently magical things. Well, there is definitely real world technology that
is applied to the task, but it's the proper application thereof-the creative aspects of
the technology-that makes it mystical. If you were to review the history of this art, it
is usually a disjointed collection of stories about what this engineer did, or that
company attempted. So, as a matter of reference, we've tried to map out a history of audio
processing. This may be the first known "public" assemblage. Thus, if there are
items missed or events that might be out of date, we apologize. This is not meant as a
"be all, end all," in-depth research piece, but more as a matter of tracing the
lineage of this interesting technology. Enjoy! Our heartfelt "thanks" to Jim
Somich, of Processing Solutions for contributing this wonderful essay!
Contrary to popular belief, audio processing is not as old as broadcasting itself. If we
consider the introduction of the CBS Audimax I in 1959 to be the birth of modern audio
processing (and many do), then the art is less than forty years old. Before Audimax, there
were various AGC amplifiers and peak limiters, but their primary purpose-and the way they
were marketed-was to either "ride gain" or prevent overmodulation. It wasn't
until CBS Laboratories, working on a research project for the CBS Radio Network,
introduced the Audimax I that true audio processing was born. Even then, the primary
touted advantage of the Audimax was "gain riding." The Audimax was a true audio
processor, but the broadcast community had yet to understand audio processing as an
enhancement rather than a tool for technical housekeeping. CBS Laboratories sold the
products directly and offered a 30-day trial. This was one of the most successful
marketing ploys in processing history, and is still used today.
The early Audimax was based on the GE-6386 remote-mu-controlled tube. A derived DC control
voltage was mixed with the program audio, and set the tube gain. To cancel this DC bias at
the output, the units employed push-pull (balanced) circuitry. The identical DC component
in each side of the push-pull circuit was therefore canceled at the output. This assumed
that the push-pull circuit was well balanced, and there were the omni-present
"thump" suppression controls to accomplish this task.
The range of 6386-based devices in the 50s included the GE-Unilevel series and Gates
Sta-Level, both simple ungated AGC compressors. Feedback control was utilized to develop
the DC control voltage, which was further modified by simple R-C networks to determine
attack and release times. The Gates Level Devil added simple gating and return-to-zero
functions to the Sta-Level circuit.
Before the 6386, it was common practice to employ grid bias as the controlling element in
peak limiters and AGCs. A popular box in the late forties was the Langevin
"ProGar". The ProGar employed push-pull 6J7s driving push-pull 6V6s. The
principle was the same as the units to come later. The ProGar might be considered the
first "true" audio processor, except that it sounded quite bad!
In the era ruled by AM, the prime purpose of non-linear audio processing devices was to
prevent overmodulation. When an AM carrier reaches cutoff (at 100% negative modulation),
distortion increases very rapidly and spurious harmonics are generated en masse! Every AM
station, from the smallest "peanut whistle" to the giant 50kW powerhouse,
employed a peak limiter at the transmitter to avoid the dreaded "cutoff" at 100%
negative modulation. A popular unit in the early 50s was the GE BA-6.This limiter suffered
from thumps if the push-pull stages were not balanced perfectly. In the mid-fifties, GE
came out with a peak limiter that-in an effort to reduce processing
artifacts-amplitude-modulated the audio onto an RF carrier, controlled the carrier
amplitude, and demodulated the audio.
Suffice to say that,
until the introduction of the Audimax, "riding gain" was a profession in the
broadcast station rather than a piece of hardware. Anyone attempting to use a compressor
to ride gain reaped the disadvantage of severe processing artifacts such as pumping,
breathing, thumps, background noise buildup, and clipping distortion.
While FM did not become a reality until the late forties, there were no
processors specifically designed for this new medium until the 70s! Either a station used
a standard peak limiter to prevent overmodulation, or they used no processor or limiter at
all. This was not as unusual as you might think. Average modulation levels were kept low
and careful gain riding was employed. The 75 microsecond pre-emphasis curve played havoc
with these simple processing schemes. I can still remember muted trumpets pinning our
Hewlett-Packard FM modulation monitor in the late fifties!
Those adventuresome engineers who placed the pre-emphasis network before the peak limiter
avoided this problem, but created enormous holes in their program material due to the
broadband peak limiting. Fairchild Recording Equipment Co. introduced the
"Conax" in the late fifties to address this problem. Nothing more than a
pre-emphasized dual band clipper, the results were preferable to wideband pre-emphasized
limiting, but hardly optimal.
The Audimax reigns
The CBS Audimax was "king of the hill" from the late fifties through
the sixties. The Audimax I was followed quickly by the II and III, which were improved
versions. A gating circuit was added in the early sixties, which minimized noise buildup
during pauses. A key to the excellent performance of the Audimax was its use of a
gain-platform. This was nothing more than a dual time-constant release circuit, but it
permitted a much slower time constant unless the program audio fell outside the gain
control "window." Later, in the sixties, CBS introduced a solid-state version of
the Audimax and added the Volumax, a pre-emphasized FM Peak Limiter, in which the gain
control element was simply biased diodes. At the end of the decade-long reign of the
Audimax, there were the new 1-rack unit models, and CBS had added units for AM as well.
The DAP and Mike Dorrough
The reign of the Audimax abruptly came to an end in 1971 with Mike Dorrough's
introduction of the DAP, the Discriminate Audio Processor. Even though we take multiband
processing for granted today, it was a unique idea in the early 70s. Altec-Lansing had
marketed a two-band compressor in the fifties, but it was not aimed toward broadcasters at
all. I knew Mike during those early years, and I can tell you unequivocally that the
processing world did not welcome Mike and his new idea with open arms. The Luddites stood
firmly behind their broadband gated processors. None of the big boys would consider
marketing the DAP ,and Mike wound up building them on his kitchen table in Burbank
California and selling them door-to-door. The rest, as they say, is history. Mike Dorrough
made one of the most important contributions to audio processing. He believed in his then
radical idea, and put his money where his mouth was. The DAP made Mike Dorrough famous.
The DAP ruled the processing world.
The DAP was a three band compressor and peak limiter with rather gentle characteristics.
Individual parameters-attack, release, band splits, etc.-were not user-adjustable. A PWM
gain control scheme was used, similar to those still used by some manufacturers today. In
an era before low-cost, high-performance VCAs, this was a solid engineering design. Final
peaks were caught by a "spongy" clipper across the output stage, which consisted
of several diodes in series.
Mike instinctively knew that sharp band
splits would affect audio quality. The DAPs had very gentle slopes, on the order of
6dB/octave. Phase aligning filters were still in the laboratory stage. Any decent engineer
around during the DAP phase knew that you could radically alter the sound of a DAP with a
"greenie" and a little patience. There were dozens of trim pots inside to play
FM was coming on strong in the mid-seventies and the DAPs did not directly address the 75
microsecond pre-emphasis problem. Sure, you could get a DAP with internal pre-emphasis and
let the three band compressors control your high frequencies, but what about filter
FM Stereo requires very sharp low-pass filters in the audio path to avoid interference to
the stereo pilot at 19 kHz. As the FM stereo art began to mature, observant engineers
noticed that tightly-controlled audio became significantly looser after passing through
these filters. The culprit, as we all know today, is group delay: the propagation of
different frequencies through the filter at different speeds. Indeed, the filters would
add peaks of as much as 6-8dB to the heretofore tightly controlled audio. Some engineers,
in an attempt to be louder without overmodulating, removed the filters entirely from their
stereo generators. As you can imagine, the severe interference with the pilot was a poor
trade-off indeed. Fortunately, modern CDs with their superior high frequency response
(compared with carts and records) were still years away.
Robert Orban theorized that, by combining all processing functions with a stereo
generator, he could better control these overshoots. The Orban 8000, introduced in 1975,
proved him right. His 15kHz low pass filters were non-linear filters without significant
overshoot. The box sounded good. The 8000 employed two bands of processing, thereby
reducing intermodulation substantially. Later, the 8100 continued the approach pioneered
by the 8000 and went on to become the most popular FM processor thus far in history!
No brief history of audio processing can ignore the contributions of several others. While
they may not have been as precedent-setting as CBS, Dorrough or Orban, they are important
contributors nonetheless. These companies include CRL, UREI, Pacific Recorders, Inovonics
CRL pioneered the "phase rotator," which kept the highest peaks of the audio in
the same direction. This allowed AM stations to overmodulate on positive peaks and attain
additional loudness-that is, if their transmitter was capable of producing this additional
power in a linear fashion. A similar strategy was employed in the UREI BL-40 Modulimiter
and the Pacific Recorders Multi-Limiter. Pacific also made a three-band compressor using
optical gain control, called the Multi-Max.
Most FM stations could now be substantially louder than before. Filter overshoots were now
a thing of the past, and integrated processors made it easy to set up the air chain for
maximum performance and minimum artifacts. Those aggressive broadcasters seeking even
greater loudness than their competition began to experiment with composite clippers: hard
clipping of the stereo baseband signal. The early units clipped everything, including the
stereo pilot, and were thus found to be in violation of the FCC rules. In 1982, Eric Small
of Modulation Sciences introduced and patented the CP-803 composite clipper that did not
clip the pilot signal. It was an immediate hit and is still used to this day by gonzo
Check out the audio processing chains in most stations today, and you will still find
products from the golden age of audio processor design: the 1970s. It is a tribute to
these companies that their 20-year-old designs are still on the air and performing very
The next phase
In the 80s, the Audio Prism by Glen Clark's Texar company was a unique four band audio
processor using optical gain control and a "hold" circuit to reduce
"hunting" in the individual bands. It was a hit and is still used by many
stations as a pre-processor. As its name implies, it tended to bring out detail in music
that had been lost by other boxes.
Another contributor was Greg Ogonowski, who had formed the company Gregg Labs. An
excellent design engineer in the analog domain, Greg is credited with developing a
technique to utilize multiband clippers that were coupled with a low pass filter in each
band. This concept reduced clipper-induced IM distortion and allowed for greater perceived
loudness. In the late 80's Greg was known for a custom FM processor that not only utilized
this multiband clipper idea, but had a "killer" bass EQ. Some of the concepts of
this custom processor was later employed in the digital Optimod. Greg should also be given
credit for developing a method for reducing or eliminating modulator/demodulator
overshoots in STLs, exciters, and modulation monitors.
Another innovative product was Frank Foti's Vigilante, which was a modified Aphex
Dominator. Still on the air in many major market stations today, the Vigilante was a force
to contend with in the world of competitive audio processing. This product launched
Cutting Edge, a company behind some new concepts that had been developed at Z-100 of an
New York, when Frank was still working "in the trenches" at a radio station.
Using a completely different approach to pre-emphasized limiting, the Vigilante was able
to generate considerable loudness, yet retain an openness in the presence and high
frequencies that had not been heard before.
Designed around a concept known as "digitally-sampled analog," the Unity 2000i
found a wealth of popularity worldwide. It was the first complete processor to incorporate
a composite clipper within the system. It was also one of the first processors to allow
the user to have complete remote control of the system using a computer as the interface.
Expanding upon the concepts developed in the Vigilante, the Unity 2000i was able to take
advantage of the best features of both digital and analog processing. As Frank Foti has
since stated, it was a great platform to use as the precursor, or stepping stone, to an
all digital processor.
Digital Signal Processing (DSP) is the high-speed real-time manipulation of digital data.
It was understood as early as the late 80s that if audio processing could be performed in
the digital domain, many advantages could be realized. The technology was not in place at
that time to make these dreams a reality. Beginning in the early 90s, however,
inexpensive, high-speed DSP chips became available and made the promise of digital audio
processing a fact of life.
Early DSP processors included the Audio Animation Paragon and the Valley multiband units.
Gentner even took a stab at it, after they acquired the Texar firm in 1988. None of these
efforts achieved fame to any great degree. The old analog boxes still sounded better and
louder. The Orban Optimod 8200 was the first serious DSP design for broadcast and achieved
instant favor due in part to the Orban reputation. In the mid-nineties, CRL introduced
their own DSP-based processor, which exhibited even better performance.
Until the Omnia.fm, all DSP-based processors exhibited artifacts that made them more of a
curiosity than a serious processing tool. What Frank Foti discovered was that a modern DSP
processor did not have to be a clone of old analog designs. The analog designs did what
they did because that was all they had to work with. A fresh look, a new approach was
Omnia audio processors are the culmination of not only all of the DSP knowledge currently
extant, but, in a peculiar way, the culmination of all of the processing knowledge that
has come before-the ProGars, UniLevels, Audimaxes, DAPs and Optimods-but elevated to
levels heretofore unforeseen. The bar has truly been raised by Omnia. It would be tempting
to say that broadcast audio processing has now reached perfection. It has not. But it will
take considerable ingenuity and innovation to better the technology of the Omnia-6.