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Blast From the Past: An “Automatic” Third-Octave EQ Circa The 70s

When I started recording professionally in the early 1970s, many recording studio control rooms and live venue sound systems were “tuned” using third-octave graphic equalizers.

You’d play pink noise through the monitors, then use a microphone and third-octave Real Time Analyzer (RTA) to measure the system’s frequency response out in the audience, or at the mix position for a recording studio. The RTA’s output would guide you to adjust the equalizer to obtain a flat response.

This was long before dual-channel FFT software like (Rational Acoustics) Smaart was available, and even longer before Room EQ Wizard (REW) – an excellent freeware program popular with home studio owners. These modern programs are vastly superior to the crude tools we used back then because they include many important features such as:

— Much finer frequency resolution, to identify problem frequencies that fall between the standard third-octave centers.

— Time-based properties displayed as a high-resolution waterfall plot, plus RT60 decay times in either octave or third-octave bands.

— Showing the level and arrival times of individual reflections, to help identify the source location of problematic reflections.

The limitations of third-octave analysis and correction are well known, especially in control rooms as opposed to live venues that are much larger. But that was the best we had back then, so that’s what we used. When I built a large professional studio in the late 1970s we hired White Plains, NY acoustic consulting firm Klepper, Marshall, and King, and they used pink noise with a third-octave RTA to verify our control room monitoring, though we didn’t try to “improve” the response with an equalizer, which in hindsight was probably just as well.

Inspired Thinking

Besides performing as a musician and recording professionally in the 1970s and 1980s, I also had a keen interest in analog and digital circuit design. I never got a degree or even went to a trade school for electronics, but I worked as a technician for many years under some fantastic “real” engineers. I asked a lot of questions and learned a lot, and eventually, I managed to design and build equalizers, compressors, several analog synthesizers, and even an entire 16-channel recording console. I also wrote a monthly column for Recording Engineer/Producer (RE/P) magazine, and most of my technical and DIY construction articles are still on my website.

Old school calculations on the filters.

One day in the mid-1970s I got the idea to invent an “automatic” third-octave equalizer that would use LEDs to let even untrained users adjust its bands for a flat response.

Besides avoiding the need to hire a professional, it doesn’t require an expensive RTA either because each band has its own level detector.

You set the EQ to play pink noise through the sound system using its built-in noise generator, then adjust each slider until neither LED is lit.

If the level in the room for a given band is too high, the top LED lights up and you know to lower the slider. And if the level is too low, the bottom LED is lit so you’ll raise the slider. Very easy!

For the most accuracy when adjusting a PA system or studio monitors, you’ll use a calibrated microphone, typically an omnidirectional condenser model with a small or “tiny” diaphragm. But for live use, you could just as well use the same model mics used by the performers.

If the singers are all using (Shure) SM58s, then using an SM58 when adjusting the equalizer will include the mic’s own response errors within the correction. Once I worked out the logic and features needed, I built a prototype, which worked well, and managed to convince New Jersey audio retailer Irv Joel to display it at his New York Audio Engineering Society (AES) convention booth. I even made a simple brochure to hand out so we could see how many people expressed interest.

Winer came up with the idea for the automatic equalizer in 1976, and had a friend and a lawyer confirm the date in case he decided to pursue a patent.

In the end I never bothered to pursue selling this invention. I already owned a very successful recording studio – the largest in Connecticut – and my studio partner brought in his production company. So we were busy doing radio ads and commercial music for many clients, both famous and not so famous.

Related, at the AES show where I exhibited this equalizer, someone from another booth came over to me and complained that his company has a patent on the same idea, so I better not try to sell mine. In hindsight, I suspect he was lying because I never saw anything even close to my EQ until many years later!

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