Study Hall

Blast From the Past: An “Automatic” Third-Octave EQ Circa The 1970s

Long ago, the author was inspired to create an equalizer that would use LEDs to let even untrained users adjust its bands for a flat response. Here's how it went...

Editor’s Note: Ethan Winer is the author of the excellent book “The Audio Expert,” available here.

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!

Sine Sweeps & Pink Noise

Modern room measuring tools use a swept sine wave, which has several advantages over pink noise. One big advantage of using a sweep is it offers a better signal to noise ratio. After the sweep is miked and recorded the software analyzes the result, applying a tracking filter as it measures the level at each frequency.

So while it’s assessing the volume at 40 Hz, for example, it ignores all other frequencies. This way midrange noise such as from the ventilation system won’t influence what’s measured at 40 Hz. This is a very powerful feature!

A part of the large main circuit board showing typical parts placement for one of the 27 bands.

Software can also process and analyze a swept sine wave faster than humans can read the level of band-limited pink noise as it plays in real time.

At high frequencies, an SPL meter is stable enough to read quickly. But once you get below 100 Hz the needle dances around, and at very low frequencies it can swing 5 dB or even more over time. So you have to watch the needle carefully for 10 seconds or longer, and mentally note the approximate “average” level displayed. Again, this was what we had to use in years past.

Most audio engineers know that white noise has the same energy per Hz, where pink noise falls off at a rate of 3 dB per octave. So with white noise the band between 100 and 200 Hz has the same amount of energy as the range from 800 to 900 Hz.

Both bands encompass a range of 100 Hz, and so would read the same on a VU meter. But with pink noise, each octave (or fractional octave) contains the same amount of energy. So the octave from 100 to 200 Hz has just as much energy as the octave from 1,000 to 2,000 Hz even though it spans a much narrower range of frequencies.

Winer also designed and built this 16-track recording console in the mid-1970s, along with many of the outboard devices in the rack.

Most noise generator circuits create white noise, which can then be filtered to reduce higher frequencies to obtain pink noise. Early white noise generators used a reverse-biased transistor as the actual noise source.

But that requires testing many transistors to find one that produces “good” noise, then you have to amplify and filter the relatively small signal.

A more modern design uses a series of digital logic shift registers as shown in Figure 1. This clever design (not mine!) creates a very pure white noise that can be filtered as needed. However, one big problem remains: The simplest type of high-cut filter, using one resistor and one capacitor, rolls off high frequencies at a rate of 6 dB per octave.

Figure 1: A modern pink noise generator uses digital logic to create high quality pure white noise, then a complex filter is needed to roll off high frequencies at the non-standard rate of only 3 dB per octave.

A single filter stage rolls off at 6 dB per octave, two stages roll off at 12 dB, and so forth. This is why loudspeaker crossovers always transition at a multiple of 6 dB per octave. So a single filter stage gives twice as much reduction as needed for pink noise, and obviously there’s no way to use half a capacitor.

As you can see in Figure 1, four resistors and four capacitors are needed to obtain half the roll off of only one stage! (The fourth stage uses two capacitors because the necessary capacitance isn’t a standard value, so two must be combined to obtain the capacitance needed.)

As with most electronics projects, we find things to add or change after the initial design. So please excuse the mess!
Study Hall Top Stories