Designer Notebook: The Quest For True Graphic EQ
The back-story on the evolution of equalization
and the Rane DEQ 60
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One day out of the blue, Rane Senior Design Engineer Rick Jeffs approached Senior Software Engineer Ray Miller with an idea: “We need to design an analog-controlled, digital graphic equalizer with unsurpassed accuracy.” |
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Having studied this problem before, Miller replied that he thought he could reduce adjacent band interaction a little, but Jeffs shot back, “That’s not good enough we need to eliminate it.” |
Five years earlier, Miller had investigated a primary Achilles’ heel of graphic equalizers, namely, that the front panel controls do not accurately show the output response. However, he’d resigned himself to the problem of adjacent band interaction being nonlinear, which prevented a simple and effective solution.
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With the subject in front of him again, Miller found himself interested enough that it started occupying his spare time on weekends. If only it could be treated as a linear system, he mused. Finally he hit on an idea what if he could make it linear? Miller knew that a filter’s Q value (a measure of selectivity) affected the level away from the center of the band of frequencies being changed, so it occurred to him that Q could possibly be adjusted to make the problem linear (that is, a change of x-dB to the center would always produce a predictable change of y-dB at the skirts). |
A linear solution was important because linear problems are straightforward
to solve mathematically.
BIRTH OF AN ALGORITHM
A nonlinear problem forces compromise by either ignoring the non-linearity
or finding a solution using approximations that converge toward a solution;
however, that requires lots of computing time. He spent weeks trying to
use standard linear approximation techniques and other methods before
realizing that they were not going to get the job done, and that he was
going to have to create his own solution which he did on a Seattle rainy
weekend. Thus was born the Perfect-Q algorithm. (The term “Perfect-Q”
was coined by fellow engineer Michael Rollins, who combined “perfect filter
response” and “adjustable Q.”)
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Similar attempts to solve this non-linearity problem relied on
trial-and-error iterative methods, where adjustments are made, the
error analyzed, then adjustments are made again, and so forth, until
the error is sufficiently small. This is what a person who could
see the amplitude vs. frequency response result would do. Although
a computer does it much faster, this equalizing-the-equalizer procedure
still results in an undesirable time lag between changing settings
and the desired response. |
Miller’s method for linearizing the filter band interaction uses Variable-Q
techniques that effectively allow a real-time solution. He proceeded to
write the code. The tricky part was writing it in Motorola 56000 DSP assembly
language so that it would run extremely fast, resulting in no perceptible
delay in control adjustment. Sure enough, it worked.
GRAPHIC REALITIES
The advantages of a true graphic equalizer go far beyond yielding a more
accurate picture; it provides a very high degree of adjustment. In fact,
to our knowledge, this level of adjustment has never been possible before.
Crucial subtle refinements of frequency response are also unique, allowing
for an extreme ease of operation and clarity of sound reproduction. Changing
a one-third-octave setting alters only that setting, and this is most
certainly unlike any other graphic EQ available.
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True graphic equalizer adjustments produce tighter, more defined
sound that is predictable, clearer and more precise with no surprising
characteristics. A true graphic equalizer lets the sound professional
modify only what they want, without offensive side effects. |
Ever since RCA’s John Volkman pioneered the use of equalizers for improving reproduced sound in the 1930s, graphic equalizers have been the preferred choice due to their ease of use and convenience. The down side is the limitation of fixed one-third-octave bands and filter response overlap resulting in significant interaction between controls as shown in Figure 1. The display shows the result of boosting two adjacent sliders 6 dB, which produces a combined curve greater than 9 dB.
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It is this adjacent band interaction that makes parametric equalizers
the preferred choice of professional audio users despite their limited
number of bands and difficulty of use. Band interaction makes graphic
equalizers difficult to use for “ringing out a room,” often resulting
in a hollow sounding performance due to gain changes in more than
the desired one-third octave bands. Constant-Q designs, pioneered by Rane in 1981, provided significant improvement as shown in Figure 2. Here it is seen that the combined response is reduced to 7 dB better, but not perfect. |
Throughout the years, Rane continued to study and evolve graphic equalizer
technology, first with Constant-Q, then interpolating Constant-Q, MIDI-programmable
interpolating Constant-Q, THX Home Cinema EQ and first-generation digital
signal processing (DSP) based graphic equalizers. However, none of these
advancements provided a slider-based adjustable equalizer with the flexibility,
accuracy, immediacy and ease of use possible with a true graphic EQ.
DSP finally freed designers from the constraints of analog design. The
tools were in place for the next major step forward. Using Miller’s Perfect-Q
technology, Jeffs designed the first analog-controlled graphic equalizer
to eliminate band interaction in real time, the Rane DEQ 60. It virtually
eliminates band interaction and ripple between bands as shown in Figure
3. The output response precisely matches the slider settings. The
DEQ 60 is the long sought true graphic equalizer.
WHAT YOU SEE...
Figure 3 and Figure 4 demonstrate the main principles of
a true graphic equalizer; namely that what you see is (really) what you
get all slider settings exhibit constant bandwidth behavior, and each
band is independently adjustable from its neighbor. There are no overload
problems caused by band interaction gain increases, and the always narrow-band
correction guarantees minimum phase response.
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In addition to the above response characteristics, the DEQ 60 contains
auxiliary features necessary for a complete professional equalizing
tool. These include adjustable high- and low-cut filters for band-limiting
the overall frequency response, a set of three-band tone controls
with steep roll-off rates and full-off operation for timbre control,
and separate input and output level controls with independent input
and output metering. A novel ability is being able to choose either A or B channel as the control assignment. This handy function allows stereo linking, A/B curve comparison, or even acts as two analog memories. |
And because we live in a complicated world where no one solution fits
every occasion, the DEQ 60 includes choice between Perfect-Q or Proportional-Q
responses, ±12 or ±6 dB range selection, and separate channel bypass switches
(selectable between bypassing just the filter sections or bypassing everything).
Figure 4 demonstrates an example of DEQ 60 slider positions lined
up with the frequency responses corresponding to the Perfect-Q and Proportional-Q
settings. There’s a scoop around 250 Hz to remove some low-mid “woof,”
a few notches around 1 kHz and 2 kHz for feedback control, and a dip in
the 8 kHz region to tame a pesky high-frequency hot spot.
Note the difference between the two curves, especially the interactions
between adjacent bands in the low-mids and the 6 dB offset at 1.25 kHz.
The Perfect-Q response exactly matches the slider setting while the Proportional-Q
response shows as much as 6 dB error. Note that the maximum intended change
in any one band was only 4.5 dB.
CONSTRUCTION DETAILS
DSP provides many benefits beyond creating the desired graphic response.
All analog graphic equalizers suffer from susceptibility to magnetic and
radio frequency interference due to the large number of high impedance
slide controls connected directly to summing amplifiers. Digital designs
isolate the analog signal, mitigating susceptibility problems. Figure
5 compares the internal construction of Rane’s analog GE 60 versus
the new DSP DEQ 60.
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In the analog design the audio signals must run throughout the chassis and all along the front panel, while in the digital design the analog signals are confined to the small PC board at the back of the chassis and to a small area on the digital board. Another important factor is the elimination of the resistors and capacitors required to create the analog filters and their real world value tolerances. |
Even 1 percent resistors and 2 percent capacitors allow great variations
between the same bands on different channels and from unit to unit.
In the DEQ 60 DSP implementation all bands are exactly the same, every
time, in every unit. This is valuable in applications where it is necessary
to set several units with the same curve something very difficult with
analog designs. Lastly there is the obvious reduction in the number of
active parts required to complete a finished unit, and fewer active parts
means greater reliability.
Figure 5 also shows the chassis differences between Rane analog
and digital products. Pro audio products containing microprocessors, DSP
and digital audio sig- nals mandate redesigned chassis. Both are manufactured
from cold-rolled steel, as has always been Rane’s standard for strength
and shielding, but digital circuits mandate a tighter fit than analog.
The new chassis creates a secure shielding environment so that RF (radio
frequency) components generated inside the box do not leak outside, and
simultaneously prevent RF signals outside the box from getting inside.
This is what compliance engineers call satisfying the emission and immunity
requirements.
And an analog-controlled DSP solution like the DEQ 60 presents tougher
shielding problems because all the slide controls cut slots into the front
panel, breaching its shielding integrity. This required adding the steel
pan attached behind the slider controls as shown in Figure 6. And
making the rack mounting ears separate pieces that bolt to the chassis
sides makes for a stronger, more road-worthy touring unit.
The DEQ 60 is manufactured at Rane headquarters in Mukilteo, Washington,
where every unit is tested to rigorous limits using Audio Precision test
equipment and to catch what computers cannot listened to by real humans.
A SINGLE PASS
The advantages of a true graphic equalizer are many. With the elimination
of band interaction, it is possible to adjust a graphic equalizer based
on real time analyzer (RTA) readings in a single pass.
It also is possible to compensate for room-ring modes in several bands
without creating a “hollow” sounding performance. Professionals are able
to “draw” a predictable and accurate response with front panel sliders.
Musicians, live sound and recording engineers are able to make large or
small changes in a single band without affecting neighboring bands.
While they still have their place, many difficult tasks previously reserved
for parametric equalizers are now easily addressed by a new generation
of true graphic equalizers.
Editor’s Note: All techniques and algorithms discussed in this
article are covered by applications filed by its inventor, Ray Miller,
and Rane Corporation with the U.S. Patent and Trademark office and other
international patent agencies.
Rick Jeffs is senior design engineer and Ray Miller is senior software engineer with Rane Corp. Dennis Bohn is a principlal partner and vice president of research and development at Rane. He is author of numerous in-depth technical articles and complied the Rane Pro Audio Reference Book, where he also wrote many chapters. It’s available on-line at www.rane.com/library.html and is also available for purchase in book form.
July 2003 Live Sound International