Recording - January 2017 - 36

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Part 6: Filters and Equalizers-Watching Your Slopes and Qs
In this edition of our series in demystifying the spec sheet, we'll take a little break
from scads of numbers (don't worry-
there'll be some numbers) to address the
terminology used in describing equalizers
and filters, both with respect to the functions they perform and how they work.
The basic terms
While we want the flattest frequency
response for our preamps and recorders,
sometimes things sound better with certain frequencies boosted or attenuated. A
filter or equalizer is the tool for that job.
A filter does one job: it cuts the level of a
specific range of frequencies-highs,
lows, or mids. An equalizer is usually
composed of several filters and can
boost or cut multiple frequency ranges.
Thanks to The Telephone Company,
from where much audio technology
derives, we got the equalizer, a device
originally designed to correct the irregular frequency response of a long transmission cable. I'll be using the terms filter and equalizer somewhat interchangeably here-keep the application in mind
and you'll keep it straight.
Implementations-analog, digital,
passive, and active
A passive analog filter is built from
capacitors, inductors, and resistors;
hence it has no gain, only loss. A
capacitor-inductor circuit can create a
boost at its resonant frequency, but without amplification, there's no real gain.
The sound of a passive filter, particularly
one that uses inductors, is loved by
some due to a tendency to "ring." This
adds frequencies to transients that aren't
present in the source, producing distortion that's usually described as "fat."
Typically, an amplifier in the same box
will follow a passive filter circuit in order
to make up for the signal loss. Even
though it has a power cord, it's usually
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RECORDING JANUARY 2017

considered to be a passive filter since the
amplifier doesn't affect the frequency
response. The Pultec EQP-1 is an example.
An active filter uses the same passive
components to shape the frequency
response, but places them in the feedback path of an amplifier to modify its
frequency response. In this instance, it's
the active component (the amplifier) that
actually does the work.
A digital equalizer emulates passive
components mathematically and can create any passband shape. A DAW equalizer is probably the most common example.
We'll discuss these in more detail later.
Now, let's have a look at the frequency response of filters and the various
parameters associated with a filter's
action. For simplicity, we'll examine filters that cut, but the same characteristics
apply to filters that boost.
1a

Shape, slope, bandwidth, and Q
The three basic filter types are highpass, lowpass, and bandpass, so named
for the frequencies that they pass without
attenuation. Sometimes they're named for
what they cut rather than pass, as in lowcut or highcut. I tend to use the name that
describes the job to be done; I call a mic
preamp's 40 Hz filter "lowcut" since that's
what it's intended to do. "Highpass" is
equally correct, however. Figure 1a
shows the frequency response curves for
low- and highcut filters, and Figure 1b
shows that of a bandpass filter made
from putting the two together end-to-end.
The slope of the curve describing how
fast the signal amplitude drops off as the
frequency goes above or below the filter's
design frequency is specified in dB per
octave. The slope is a function of the circuit design and is often stated as the order
of the filter. A first order filter has a slope
of 6 dB per octave. A second order filter,
which can be made by cascading two
first order filters, has a slope of 12 dB per
octave, etc. (Figure 2) The "brick wall" filter doesn't exist in practice, but modern
digital filter designs can get pretty close.
You may run across the terms passband and stopband. The stopband is the
frequency range where the filter is filtering, the passband is the frequency
range that's unaffected by the filter. The
band edges are defined by the filter's
design frequency, or frequencies in the
case of a bandpass filter.
The filter's design frequency, sometimes
called the "rolloff" or "cutoff" frequency, is
the frequency at which the amplitude of the
filter's output changes from the reference
(no filtering) frequency by 3 dB. A practical filter doesn't change slope instantly at
the design frequency, there's always a
"knee" before the theoretical slope is
reached. Figure 3 is a plot of the measured frequency response plot of a firstorder 1 kHz highpass filter.

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