Recording - January 2016 - 61

Higher-slope filters exist in lowpass form as well, and like their highpass
brethren they come in Bessel and Butterworth flavors. There's a third variety, too,
called Chebyshev (after yet another mathematician); these feature a small peak
in the response just below the cutoff frequency. These aren't often used in audio
recording (they ring at the frequency of the peak), but they're vital in electronic
music synthesis, where the peak just below cutoff lends flavor to the final sound.
There are highpass Chebyshev filters too, but hardly anyone uses them in
recording. Lowpass filters aren't used much in recording studios either, except
for special effects. (However, jumping ahead to our next topic, they're essential to the digital recording process. And we'll mention them again when we
talk about reverb.)
There are also filters that select a narrow band of frequencies and reject
everything else (like peak-and dip EQ on steroids); logically, these are known
as bandpass filters. They can produce the "telephone" sound that periodically
becomes fashionable in the recording world.

Figure 2

Figure 3

Likewise, there are filters that reject a narrow band of frequencies, and pass
everything else. These are known as bandstop or bandreject filters (another
logical name); these can be useful when you're recording an instrument with,
say, a bad honk. A bandstop filter with a very high Q, which essentially filters
out a single frequency, is called a notch filter. These can be worth their weight
in gold if you're recording electric guitars with single-coil pickups that pick up
60 Hz hum from absolutely everywhere, or if you're working on a live recording where one of the mics is ringing on the edge of feedback (I just fixed one
of those; Figure 3 shows the response of the notch filter I used). "Acoustic
Guitar Amplifiers" (I put the phrase in quotes because it's an oxymoron) often
have sweepable notch filters to minimize feedback during live performances.
Gates and expanders: compression in reverse
If you take the circuit of a compressor and juggle its elements a little,
you'll come up with its sibling in the world of dynamics management, an
expander. In an expander, rather than reducing the gain when the input signal is above a certain threshold, the "little engineer in the box" turns the
volume down when the input signal is below the threshold. So, for example,
it the threshold is -50 dBu, any signal lower than that will have its level
reduced. (If the level is reduced enough to effectively turn the signal off, the
expander has become a gate.)

Why might that be useful? First, for reducing
noise. Electric guitars are great for detecting stray
electrical fields from light dimmers, power cables,
and even their own amplifiers' power transformers. Single-coil pickups are notorious for this, making life difficult for those of us with a decadeslong affection for Teles and Strats. An expander
or a gate can help. By deciding (arbitrarily) that
anything under, say, -50 dBu on the input line
must be noise, and setting the threshold there,
you can effectively squelch the offending buzz.
The second usage of expanders and gates is
dealing with leakage. I normally like leakage,
and use it to create a natural sense of space
(see "Outwitting the Phase Monster, Part 1",
November 2014). But for engineers who closemike drum kits, yearning for total control over
the mix of individual drums, leakage can be a
nightmare. The problem, as you can probably
guess, is phase. When the sound of the snare
is picked up by its own mic, and a few milliseconds later by one of the cymbal or tom mics,
that leakage can cause big problems with
phase cancellation and comb filtering. Add to
that leakage of the cymbals into the snare mics
or tom mics, or the kick into everything, and
you're looking at a real nightmare. When multiple miking of drumkits became common in the
1970s and 1980s, expanders and gates quickly became common tools in studios.
The problem with using expanders and gates to
reduce noise and leakage is that they aren't perfectly transparent; often you can hear them working, particularly as the signal decays; a smooth,
soft decay can turn into an abrupt chopping-off of
the sound or a "chattering" as the gate hovers near
its threshold. There are all sorts of workarounds,
including triggering the expander/gate's action
from an external source via the sidechain. For a
while, it was fashionable to use such "keyable program expanders" (the most common was the
Kepex) on every channel of a multi-miked drum kit
recording, and more on the drums' reverb. (The
sound of "gated reverb" was part of what made
the Eighties sound like the Eighties.)
I've relegated expanders and gates to this
part of this series because I very seldom use
them. I deal with guitar buzzes by orienting the
player, when possible, in such a way that the
guitar doesn't buzz. As for leakage in drumkit
recordings, rather than use multiple gates on
drum mics, I simply avoid multimiking drums.
As you'll have gathered from previous articles
in this series, I prefer simpler drum setups, like
an ORTF pair over the drums and maybe a
close mic on the kick. None of this applied to
the Ottoman Underground session; there wasn't
a drum kit, any leakage I got was all to the
good, and Shlomo's bass doesn't buzz.
Okay, that's got us all the way to the
recorder-in this case, a digital audio workstation program on a computer, or DAW for
short. Next time we'll begin our discussion of
digital audio; see you then!
Paul J. Stamler (stamler@recordingmag.com)
is a recording musician, engineer, educator,
and collector of bizarre vintage recordings,
living in St. Louis.
RECORDING January 2016

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