Rane DEQ 60L Manual page 13

There is irony in knowing that improving Rane's
much praised constant-Q technology required switch-
ing to variable-Q technology to perfect the response vs.
slider position problem. The popularity of Q-terminol-
ogy is unfortunate since what is meant is bandwidth.
In hindsight, naming the complementary technologies
"constant bandwidth" and "proportional bandwidth"
would have been better choices, because these terms
identify the solutions more accurately.
Rane championed constant-Q designs beginning in
1982 as a better solution to the problem of slider-based
graphic equalizers. Constant-Q gave a more honest
front panel representation than proportional-Q. It min-
imized what Rane called "equalizing the equalizer," i.e.,
having to go back and readjust adjacent sliders to coun-
teract the problem of interaction between bands. This
is the phenomena where adjusting one band causes
similar, but reduced, adjustment to adjacent (and even
further out) bands. For example, if you boosted 800 Hz
by a couple of dB, you would inadvertently boost the
energy centered at 630 Hz and 1000 Hz. Constant-Q
interacted less than proportional-Q and now Perfect-Q
eliminates this problem.
Perfect-Q Advantages
The advantages of the Perfect-Q design go far beyond
yielding a more accurate picture; it provides a degree of
adjustment never before possible. Crucial subtle refine-
ments of frequency response are for the first time pos-
sible, allowing for an unequaled ease of operation and
clarity of sound reproduction. Changing a 1/3-octave
setting changes only that setting. This is unlike any other
graphic EQ available (i.e., one providing real mechani-
cal front panel slide controls as of January, 2003).
Perfect-Q-2
DSP Provides the Solution
DSP allows more flexible processing than analog and
permits delaying final filter parameters until the actual
user settings are known – something not possible with
analog. This gives the power to build an EQ that has an
ideal response. The idea driving development of Per-
fect-Q is the same as constant-Q: constant bandwidth
for each EQ band no matter what the setting, but DSP
allows doing things that aren't practical (or in some
cases even possible) in analog circuits, producing an
even better outcome as demonstrated by these Perfect-
Q characteristics:
• What you see is (really) what you get.
• Constant bandwidth for all slider settings.
• Adjusting one band does not change neighboring
bands.
• Improved phase response due to eliminated
interactions.
• No band interaction overload problems.
Graphic Details
Early EQs used passive analog networks resulting in
a proportional-Q (also known as variable-Q; "Q" is
inversely proportional to filter bandwidth) response,
that is, the filter bandwidth became wider or narrower
depending upon the slider setting. While producing
smooth alteration of frequency response, proportional-
Q designs have significant interaction between adjacent
bands. For certain applications this interaction results
in a "sound" some listeners grew to appreciate, even at
the expense of poor correlation between overall re-
sponse and slider position. Figure 1 shows two adjacent
sliders boosted 6 dB, with the resultant proportional-
Q response. As shown the proportional-Q graphic
equalizer's front panel is a poor representation of the
true frequency response curve. Front panel says +6 dB,
but the real output is +9 dB.