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Operator Adjustable Equalizers: an Overview

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Operator Adjustable Equalizers: an Overview RaneNote OPERATOR ADJUSTABLE EQUALIZERS: AN OVERVIEW Operator Adjustable Introduction Equalizers: An Overview This paper presents an overview of operator adjust- able equalizers in the professional audio industry. The • Equalizer History term “operator adjustable equalizers” is no doubt a bit vague and cumbersome. For this, the author apolo- • Industry Choices gizes. Needed was a term to differentiate between fixed equalizers and variable equalizers. • Terminology & Definitions Fixed equalizers, such as pre-emphasis and de-em- phasis circuits, phono RIAA and tape NAB circuits, • Active & Passive and others, are subject matter unto themselves, but not the concern of this survey. Variable equalizers, how- • Graphics & Parametrics ever, such as graphics and parametrics are very much the subject of this paper, hence the term, “operator • Constant-Q & Proportional-Q adjustable equalizers.” That is what they are—equal- izers adjustable by operators—as opposed to built-in, • Interpolating & Combining non-adjustable, fixed circuits. Without belaboring the point too much, it is impor- • Phase Shift Examples tant in the beginning to clarify and use precise termi- nology. Much confusion surrounds users of variable • References equalizers due to poorly understood terminology. What types of variable equalizers exist? Why so many? Which one is best? What type of circuits pre- vail? What kind of filters? Who makes what? Hopefully, the answers lie within these pages, but first, a little history. Dennis Bohn Rane Corporation RaneNote 122 © 1990 Rane Corporation Reprinted with permission from the Audio Engineering Society from The Proceedings of the AES 6th Interna- tional Conference: Sound Reinforcement, 1988 May 5-8, Nashville. Equalizers-1 A Little History band signal loss and then reducing the loss on a band- No really big histories exist regarding variable equal- by-band basis. For example, flat might represent 16 dB izer use. Good short histories appear in [1]-[3]. An loss while a 6 dB boost represented only 10 dB loss. It expanded short history follows. was all a matter of reference point.) Hurrah for Hollywood. Mother Nature and Hol- Another innovative feature of the 7080 was the first lywood spawned the first use of variable equalizers use of staggered mixing amps to aid in smooth com- for sound improvement. Motion pictures with sound bining of the equalized audio signal. Cinema Engineer- brought audio playback systems into theaters for the ing designed 3 mixing amplifiers for 6 bands. Using first time. Soon, some people's attention focused on this approach, no amplifier mixed adjacent bands. The just how bad these reproduction systems sounded. center frequencies were 80 Hz, 200 Hz, 500 Hz, 1.25 John Volkman was one of these people. It was the '30s kHz (labeled 1.3 kHz), 3.2 kHz (labeled 3 kHz), and 8 and Volkman worked for RCA. Credit John with being kHz. The amplifiers mixed 80 Hz + 1250 Hz, 200 Hz the first person to use a variable equalizer to improve + 3200 Hz, and 500 Hz + 8 kHz respectively. Using reproduced sound. He applied this new tool to equalize separate amplifiers to mix signals spaced 4 octaves a motion picture theater playback system. apart, resulted in seamless recombination at the out- While Bell Labs used fixed equalizers earlier than put. (Later Art Davis would use a similar technique this for correcting audio transmission losses [4], Volk- in the design of the first Altec-Lansing active graphic man represents one of the first uses of an external vari- equalizers.) able equalizer as an added component to an installed Not much happened during the '40s and early '50s system. Telephone applications involved integrating due to World War II and its aftermath. Most appli- equalization as part of the receiving electronics, as op- cations of variable equalizers involved post-produc- posed to thinking of the equalizer as a separate entity. tion work. No serious success at room equalization is During the same period Volkman experimented known. Then in 1958, Wayne Rudmose (a professor at with equalizers for reproduced sound, Hollywood Southern Methodist University, Dallas, Texas) suc- found uses for them in producing sound. Langevin, cessfully applied new theories about acoustic equal- Cinema Engineering, and others [4], created outboard ization to the Dallas Love Field Airport. Dr. Rudmose operator adjustable equalizers for post-production published his monumental work [5] and sound system sound effects and speech enhancement. Langevin equalization was born. Model EQ-251A represents very early use of slide con- In 1962, Texas made another major contribution to trols. While not a graphic equalizer in today's sense, variable equalizer history. This time it was the Univer- it was the forerunner. The EQ-251A featured two slide sity of Texas (Austin) and a physics professor named controls, each with switched frequency points. One C.P. Boner. Dr.s Boner and Rudmose were contempo- slider controlled a bass shelving network with two raries and friends, having co-authored a paper 23 years corner frequency choices, while the other provided earlier [6]. Boner, acknowledged by many, as the father peaking boost/cut with four switchable center frequen- of acoustical equalization, built organs as a hobby. cies. This passive unit looked and performed equal to From his organ/room tuning experiences and acousti- anything manufactured today. cal physics knowledge grew a profoundly simple theory. Art Davis's company, Cinema Engineering, de- Boner reasoned that when feedback occurs, it did so at veloped the first recognizable graphic equalizer [4]. one precise frequency, and to stop it all you had to do Known as the type 7080 Graphic Equalizer, it featured was install a very narrow notch filter at that frequency. 6 bands with boost/cut range of 8 dB, adjustable in 1 He went to one of his former students whose company dB steps. (After Art Davis moved to Altec, he designed made precision filters for instrumentation and asked a 7 band successor to the 7080 known as the Model him to design a narrow band audio filter. Gifford White 9062A. A hugely successful graphic equalizer selling agreed, and launched White Instruments into the new into the '70s.) Being an active design, the 7080 allowed field of acoustic equalization. signal boosting without loss—a nice feature. (With pas- Armed with White equalizers, Boner established sive units, boosting of signals requires an initial broad the foundation theory for acoustic feedback, room- Equalizers-2 ring modes, and room-sound system equalizing tech- Choices, Choices, Choices niques [7]-[10]. Expanding Boner's work was a student Figure 1 shows the breadth of operator adjustable of Wayne Rudmose named William Conner. In 1967, equalizers. And this covers only the manually adjust- Conner published a concise paper [11] still considered able analog units—microprocessor-controlled and among the best to describe the theory and methodol- full-digital designs are omitted. Such are your choices ogy of sound system equalization. as a user. Also in 1967, Art Davis, along with Jim Noble and Estimates suggest only 25% of the equalizers sold Don Davis (not related) developed the industry's first find their way into serious permanent sound systems. 1/3-octave variable notch filter set (passive) for Altec- Uses for the remaining 75%, split between program Lansing. Don Davis presented the paper to the Audio enhancement and sound reinforcement. Engineering Society in October, 1967 [12]. Dubbed Program enhancement primarily appears in live per- the “Acousta-Voice” system, it ushered in the modern formance, recording studio, broadcast, and post-pro- age of sound system equalization and represented the duction marketplaces. Within these markets equalizers ultimate in speed and convenience. The Acousta-Voice do everything from simple band limiting to complex system proved another path existed for the control of sound manipulation. room-ring modes. As an alternative to Boner's narrow- Sound reinforcement uses equalizers everywhere band notching technique, 1/3-octave “broad-band” from small lounge acts to large touring companies. filters produced the same results. Most applications are for compensating ragged loud- The rest, as they say, is history. A 20 year history that speaker power responses rather than attempting any witnessed an explosion of variable equalizer develop- sort of serious room equalization. This is true for ments. Among the most noteworthy being the 1/3- monitor loudspeaker systems as well as mains. Yet, the octave graphic equalizer, the parametric equalizer, equalizer is the crucial link in vastly improving the use of integrated circuits, development of the gyrator system's sound. (synthetic inductor), active LC and RC designs, devel- With such diverse applications it is not surprising to opment of constant-Q (bandwidth) graphic equalizers, find so many choices. To understand the choices, how- and the application of microprocessors for control and ever, is first to understand the terminology. memory. All of these developments, in this author's opinion, fall into the category of improvements—albeit, Terminology very important improvements—rather than qualifying Equalizer terminology deserves better positioning than as new concepts applied to variable equalizers. Recent- the back of the book. So instead of a complete glossary ly, however, two categorically new concepts appeared. at the end, an abbreviated glossary appears now. To The first is transversal equalizers: In 1984, Industrial confuse and make sure
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