Recording - January 2017 - 38

4

amount of phase shift necessary to achieve
the desired filter characteristic. For a minimum
phase filter, the amount of phase shift changes
with frequency, and not linearly. Figure 7
shows the frequency and phase response of a
300 Hz lowcut filter. The phase shift is 180°
for much of the slope up, reaching 90° at the
design frequency, and approaching 0° as the
frequency response levels off. The result is that
not all frequencies passing through the filter
are delayed by the same amount of time. This
can cause "smearing" of complex waveforms.
That's not as bad as it sounds, and is the
sound that we have become accustomed to
hearing when using traditional analog filters.
A name like "minimum phase" implies an
alternative, and indeed there is one. For a linear phase filter (Figure 8), the phase shift
changes linearly with frequency, resulting in a
fixed time delay through the filter for all frequencies. The payoff is that a linear phase filter preserves the shape of a complex waveform more accurately than a minimum phase
filter. The tradeoff is a greater total amount of
phase shift (though in this graph it's normalized to zero), and a substantial amount of
latency.
It's often difficult to hear phase shift on a
single source, but understanding an equalizer's phase characteristics becomes important
when mixing two sources with a lot of content
in common, such as when two mics are used
on the same source. If the same frequency in
the two signal paths is delayed by different
amounts, then when mixing them together,
comb filtering effects will be very different
than if both paths had the identical phase shift
characteristics.
Active filters and equalizers
An active filter has several advantages over
a passive one. It has gain, so it can boost as
well as cut. An active filter can be built from
smaller and less expensive components than a
passive filter, and it can have a wider adjustment range than a passive filter. The disadvantage is that more parts and a source of
power are required, and a little noise from the
active circuitry is unavoidable.
Most active equalizers are based on the "StateVariable" circuit. This uses operational amplifiers
(opamps) with the frequency-determining components placed in the amplifier's feedback path. A
state-variable filter circuit can function as either a
constant slope or shelving low- or highpass filter, a
38

RECORDING JANUARY 2017

bandpass filter, or a notch (band-reject) filter. Capacitors substitute for more expensive
inductors, and variable resistors (pots) allow continuous adjustment of frequency and
gain. A parametric equalizer is a special case of the state-variable circuit that offers
adjustable bandwidth. With today's inexpensive and high quality opamps, the statevariable equalizer is nearly universal in all but the simplest mixing consoles.
Other than the number and range of controls, you'll see few numerical specifications for an equalizer, though a standalone or 500 Series equalizer will
often provide specs for nominal signal levels and noise.
Digital filters
A digital filter is one in which a mathematical algorithm is used to compute
the desired frequency and phase response. One example is an equalizer in
the form of a DAW plug-in. Many modern powered loudspeakers employ a
dedicated programmable chip to provide digital low- and highpass filters for
the crossover network; furthermore, digital filters for correcting a driver's frequency response, phase-aligning the drivers, and even assistance in correcting
room acoustic problems are becoming more common.
Digital filters are usually characterized as IIR (Infinite Impulse Response) and FIR
(Finite Impulse Response). An IIR algorithm involves feedback, around a series of
delays and adders. The "infinite" in its name comes from the fact that when a test
5

6

signal consisting of one full scale sample followed by a bunch of zeros is applied
to the filter input, that pulse re-circulates through the computation many times (not
really an infinite number because that wouldn't make a very good filter) and
appears in the output with the same sort of ringing that follows a transient in some
analog filters. Thus, an IIR filter is good at emulating an analog filter design, and
indeed, that's what most common plug-ins employ. The use of feedback reduces
the complexity of the calculations, so IIR filters tend to be fairly computer-efficient.
An FIR filter has no feedback, therefore there are a finite number of calculations performed between input and output. Computations are complex and
rather brute-force. Because an FIR is computation-intensive, when used in conjunction with a host DAW, it can gobble up CPU power. You may need to be
judicious in how many you apply in a mix. An FIR filter introduces considerably more latency than an IIR, often requiring a few hundred milliseconds of
delay compensation. An FIR filter is strictly digital-although it can model an
analog filter, you can't build one from analog components.
Is one design better than the other? Not really. They sound different. For a minimum phase filter, the simpler IIR design works fine. For a linear phase filter, FIR
is more common, although it uses more computer horsepower.
There's a certain infatuation with FIR equalizers because they can do things
that classic equalizers can't. A linear phase FIR filter has a characteristic known
as "pre-ringing." Because of the way that the delays are interconnected, it's possible for the ringing portion of the filter's initial response to a transient to appear



Table of Contents for the Digital Edition of Recording - January 2017

Contents
Recording - January 2017 - Intro
Recording - January 2017 - Cover1
Recording - January 2017 - Cover2
Recording - January 2017 - 1
Recording - January 2017 - 2
Recording - January 2017 - 3
Recording - January 2017 - 4
Recording - January 2017 - 5
Recording - January 2017 - Contents
Recording - January 2017 - 7
Recording - January 2017 - 8
Recording - January 2017 - 9
Recording - January 2017 - 10
Recording - January 2017 - 11
Recording - January 2017 - 12
Recording - January 2017 - 13
Recording - January 2017 - 14
Recording - January 2017 - 15
Recording - January 2017 - 16
Recording - January 2017 - 17
Recording - January 2017 - 18
Recording - January 2017 - 19
Recording - January 2017 - 20
Recording - January 2017 - 21
Recording - January 2017 - 22
Recording - January 2017 - 23
Recording - January 2017 - 24
Recording - January 2017 - 25
Recording - January 2017 - 26
Recording - January 2017 - 27
Recording - January 2017 - 28
Recording - January 2017 - 29
Recording - January 2017 - 30
Recording - January 2017 - 31
Recording - January 2017 - 32
Recording - January 2017 - 33
Recording - January 2017 - 34
Recording - January 2017 - 35
Recording - January 2017 - 36
Recording - January 2017 - 37
Recording - January 2017 - 38
Recording - January 2017 - 39
Recording - January 2017 - 40
Recording - January 2017 - 41
Recording - January 2017 - 42
Recording - January 2017 - 43
Recording - January 2017 - 44
Recording - January 2017 - 45
Recording - January 2017 - 46
Recording - January 2017 - 47
Recording - January 2017 - 48
Recording - January 2017 - 49
Recording - January 2017 - 50
Recording - January 2017 - 51
Recording - January 2017 - 52
Recording - January 2017 - 53
Recording - January 2017 - 54
Recording - January 2017 - 55
Recording - January 2017 - 56
Recording - January 2017 - 57
Recording - January 2017 - 58
Recording - January 2017 - 59
Recording - January 2017 - 60
Recording - January 2017 - 61
Recording - January 2017 - 62
Recording - January 2017 - 63
Recording - January 2017 - 64
Recording - January 2017 - 65
Recording - January 2017 - 66
Recording - January 2017 - 67
Recording - January 2017 - 68
Recording - January 2017 - 69
Recording - January 2017 - 70
Recording - January 2017 - 71
Recording - January 2017 - 72
Recording - January 2017 - Cover3
Recording - January 2017 - Cover4
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