Audio Processing
A retrospective


Frank Foti

Chief Engineer. Cutting Edge Technologies/Omnia Audio

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 Thus, if there are items missed or events that might be out of date, we apologize.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.

Enter FM
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 with.

The Optimod
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 overshoot

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!

Other contributions
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 and Texar

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.

Composite clippers
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 broadcasters.

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 well.

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.

Unity 2000i
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.

Enter DSP
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, 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 necessary.

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.
Frank Foti