Illustration by Mike Cruz
|A close friend of mine once told me he couldn't
understand why recording engineers felt they needed a split-band
compressor in their arsenal of studio gear. He held that a split-band
compressor (also known as a multiband compressor) was a complex
solution for a simple task — controlling audio dynamics —
and therefore unnecessary. But he was dead wrong.
With the twist of a few knobs or virtual controls, a split-band
compressor can pull off amazing feats that a single-band (aka
broadband) compressor — or any other type of processor —
couldn't accomplish in a million years. For example, a split-band
compressor can easily prevent an acoustic guitar, cello, or upright
bass from sounding boomy on low notes without thinning out warm
formants when only high notes are present. Have a vocal track that's
too shrill on several isolated phrases? Taming the sirens is child's
play for a split-band compressor, and it won't eliminate
intelligibility and detail in the process or force you to park one hand
on your EQ knob for the entire mix. Split-band compressors can also
make the bottom end of a mix sound huge while simultaneously increasing
headroom. Try doing that with an equalizer!
FIG. 1: The Arboretum Systems Ionizer applies dynamics processing across 512 discrete frequency bands.
What enables a split-band compressor to perform such heroics is its
multiband nature. Unlike a broadband compressor, which squeezes the
dynamic range of its audio-input signal across its entire frequency
spectrum, a split-band compressor uses a divide-and-conquer strategy to
independently treat discrete frequency bands. It splits the audio
signal into multiple bands, compresses each band independently of the
others as needed, and then recombines the outputs of the bands into a
single (mono or stereo) broadband signal. Because a different set of
controls is dedicated to each band, you can, for example, slam the bass
while leaving the highs untouched or, conversely, de-ess the highs
without squashing the low end.
You can do a lot more with split-band compressors than that. This
article will describe several applications and look at a representative
sampling of the split-band compressor products — both hardware
and software — that are currently available on the market.
But before I explore those subjects, an overview of key features and
controls of split-band compressors is in order. I'll forgo introductory
explanations of how threshold, ratio, attack, release, and makeup-gain
controls work; you can read about the fundamentals of compression in my
article “The Big Squeeze” in the February 2001 issue of
EM, available online at www.emusician.com.
Because of their complexity and their ability to destroy pristine
audio if used indiscriminately, most split-band compressors are poorly
suited for beginning recordists who don't have a firm understanding of
the basics. But if you're comfortable with using broadband compressors
and want to take dynamics control to a much higher level, don't wait
— let's split!
Most currently available split-band compressors offer from three to
five independent bands per channel and handle both mono and stereo
audio. I find that three or four bands are plenty for most applications
in which individual tracks are treated. For mixing and mastering work
— which often combines a broad palette of instruments, a wide
range of spectra, and percussive and nonpercussive elements — I
like to work with as many as five independent bands for each
A good split-band compressor gives you broad control over the width
of each of its bands. Typically, you'll adjust the bandwidth by
specifying the crossover frequencies at the lower and upper ends of the
band. For example, you might choose crossover frequencies of 250 Hz and
2.5 kHz when working with a 3-band compressor. In that case, the first
band would range from 250 Hz down to the lower limit of the
compressor's frequency response (typically 20 Hz in analog split-band
compressors, and as low as DC in digital units). The second band would
range from 250 Hz up to 2.5 kHz, and the third band would cover
frequencies from 2.5 kHz to the upper frequency-response limit
(generally 20 kHz or higher). Using that setup, you could separately
compress the bass frequencies (below 250 Hz), midrange (from 250 Hz to
2.5 kHz), and highs.
FIG. 2: The BSS DPR-901 II can be configured for either single-channel, 4-band operation or 2-channel, 2-band processing.
For optimal flexibility, try to find a split-band compressor with
crossover-frequency choices that overlap to a high degree. That might
allow you to, for example, assign two bands to the bass frequencies,
one to deal with the lowest frequencies and the other to deal with the
A user-friendly and powerful split-band compressor should offer
separate threshold, attack-time, release-time, makeup-gain, and ratio
or range controls for each band. A range control is similar to a ratio
control in that each adjusts the intensity of compression applied to a
signal above threshold. However, a range control also limits the
maximum amount of gain adjustment that can occur as a result of
dynamics processing in its associated band. So although a high ratio
can cause an ever-increasing amount of gain reduction as the input
signal rises farther above the threshold, a range control specifies the
maximum amount of gain reduction that can take place, even with a high
I'M IN THE BAND
To illustrate why separate in-band (as opposed to global)
controls are so useful, imagine a situation in which you're applying
split-band compression to a problematic mix that has a generally weak
bottom end, specific bass-guitar notes that are too loud, and
occasionally severe vocal sibilance.
To make the bottom end of your hypothetical mix sound full without
accentuating the louder bass-guitar notes, set a fairly low threshold
and a high ratio or range for the split-band compressor's bass band
(treating, say, all frequencies below 250 Hz). By setting the threshold
slightly higher than the level of the generally weak bass energy, you
allow the weak bottom end to pass through the compressor untreated.
However, loud bass notes would trigger the bass band's compressor and
be reduced dramatically in level, thanks to the high ratio or range
control setting. You'd probably (but not necessarily) want fairly slow
attack and release times for the bass band, in order to avoid potential
pumping when the compressor kicks in and distortion when it releases.
Now that you've tamed all the spikes in bass-energy level, you can
boost the bass band's makeup gain to raise the overall level of the
bottom end and achieve a beefy mix.
FIG. 3: MOTU's MasterWorks Compressor offers three independent bands of processing and a comprehensive feature set.
After you've dealt with the thin and erratic bottom end of a
problematic mix, you turn your attention to the searing vocal sibilance
that remains. Applying a static EQ cut to the sibilant frequencies is a
poor solution because that would also dull the crack of the snare drum
and decrease the intelligibility of the vocals during phrases that
aren't sibilant. Split-band compression is the answer here as well. A
suitably high threshold in the high-frequency band (with the highest
crossover frequency set to roughly 5 kHz) allows loud vowel sounds to
pass untreated but causes sibilant consonants to trigger the in-band
compressor. You want a high ratio or range setting and very fast attack
and release times for this application so the compressor will kick in
quickly, extinguish the sibilance, and cease gain reduction before any
nontransient material in the high-frequency band is audibly affected.
Since you don't want to boost the highs at all in this example, keep
the makeup-gain setting at 0 dB (unity gain).
Such a treatment of your hypothetical mix clearly illustrates the
power of split-band compression. Aside from the high ratio or range
settings used in both the low and the high band, the settings for the
two treated bands are as different as night and day. That is, each band
was processed independently and in dissimilar fashion, and the midrange
band was left completely alone. That power and flexibility make a
split-band compressor one of the ultimate tools for engineers.
In addition to the above-mentioned controls, a split-band compressor
should ideally have independent bypass controls for each band.
Bypassing and then activating each band in turn allows you to hear the
effect that compression is having in each band, while preserving your
painstakingly wrought settings. In-band bypass controls also allow you
to prevent unintentional compression in any band (such as the midrange
band in our example above) without having to assign a 1:1 ratio or a 0
dB range setting. Audio fidelity will also be best preserved if the
bypass is the hardwire type (which routes the input directly to the
output in hardware units).
FIG. 4: The creamy-sounding VintageWarmer from PSPaudioware provides both broadband and split-band compression modes.
Knee adjustment is another useful feature for split-band
compressors. The knee adjustment (typically a global control that
affects all bands simultaneously) might offer a choice of different
fixed settings, or it might be continuously variable.
Some split-band compressors offer a toggle switch (whether real or,
in software products, virtual) that alternates the response of the
compressor's attack and release times between your manual settings and
program-dependent or program-sensitive time constants. A
program-dependent mode ignores your manual adjustments and adjusts
attack and release times in response to the characteristics of the
input signal. A program-sensitive function, on the other hand,
modifies your manual settings as needed to better handle the
current program material without pumping or causing distortion.
The most user-friendly split-band compressors offer a variety of
meters to help you fine-tune your settings. At the very least, I like
to see a main output-level meter that shows the summed signal from all
bands, as well as separate gain-reduction meters for each band. If the
compressor also features global clipping meters for each channel and
metering to indicate how in-band energy compares to the threshold
level, I'm in hog heaven.
SHOW ME THE TOYS
Now that I've covered the key features and controls of split-band
compressors, I'll take a brief look at a sampling of both hardware- and
software-based split-band compressors currently available. This is
neither a roundup of every multiband compressor in existence nor a
shootout of the products mentioned. However, I will point out
especially noteworthy attributes of each product.
Most of the split-band compressors I'll discuss actually fall into
the broader category of multiband processors. That's because
they perform other types of multiband processing — such as gating
or expansion — in addition to compression. Most multiband
compressors can also perform multiband limiting, simply by increasing
the ratio, attack time, and knee hardness. Except where noted, all of
the offerings listed accommodate both single- and dual-channel
FIG. 5: Most parameter values for the TC Electronic Finalizer Express are determined by the unit's 25 presets.
One last point: split-band compressors are sometimes referred to as
dynamic equalizers. There really is no practical difference
between a split-band compressor and an equalizer that dynamically
boosts or cuts the energy in a given band in response to varying levels
in that band. A dynamic equalizer might use center-frequency and
bandwidth controls to accomplish the same thing that split-band
compressors achieve with crossovers, but what distinguishes the two
product categories from each other is mostly a matter of semantics.
Much more than just a split-band compressor, the software-based
Arboretum Systems Ionizer ($499) also performs multiband noise
reduction (downward expansion), upward expansion, limiting,
equalization, and frequency morphing (applying one sound's frequency
characteristics to another sound, for creating vocoding and other
effects). Ionizer is available in AudioSuite, MAS, Premiere, and
standalone versions for the Mac, and as a DirectX plug-in for the
Ionizer's split-band-compression function sounds great, but the
graphical user interface lacks certain controls and metering, making
compression applications quite unwieldy (see Fig. 1). The
interface generates red, blue, and black graphical curves alongside an
x-y plot of your audio's frequency response. Your subsequent
placements of those colored curves determine the threshold, ratio, and
range of processing across 512 discrete frequency bands. You determine
the crossovers for each band by creating and dragging breakpoints along
each curve. Because Ionizer provides so many bands to work with,
in-band makeup gain and bypass controls are necessarily omitted. Also,
attack and release controls are global, affecting all bands at once.
The only metering provided is an imprecise color bar that dynamically
alters its color and lightness to indicate the relative amount of
processing across the frequency spectrum.
BSS DPR-901 II DYNAMIC
The BSS DPR-901 II ($1,549) is a 1U rackmountable processor that can
operate as a single-channel, 4-band unit (for mono applications) or a
2-channel, 2-band device (see Fig. 2). It provides a per-band
maximum 30 dB of compression or 16 dB of expansion.
The DPR-901 II approaches split-band compression in the guise of an
equalizer, using center-frequency and bandwidth controls in lieu of
crossover controls to determine each channel's bandwidth. Each band
sports its own control for adjusting the intensity of processing, along
with an independent threshold control and a bypass switch.
FIG. 6: The 3-band Tube-Tech SMC 2B is the world's only all-tube split-band compressor.
Comprehensive multisegment LED meters for each band show gain
reduction or expansion on an arbitrary scale rather than in decibels. A
Threshold LED meter shows signal level above and below the threshold
point in decibels. Manual and program-dependent time constants are
offered. The filters for bands 1 and 4 can be switched between
bell-curve and shelving response. Bands 2 and 3 can be switched to
provide either narrow- or wide-band operation, allowing — when
used in conjunction with the other bands — simultaneous
split-band and broadband compression and expansion in one unit.
I've worked with the original DPR 901 only, and found it to be an
extremely transparent unit. So transparent, in fact, that I often found
myself slamming the meters in order to achieve more than a subtle
MOTU offers the MasterWorks Compressor as a standard MAS plug-in
with Digital Performer (Mac, $795; reduced pricing is available for
upgrades and crossgrades). This split-band compressor offers three
discrete bands with adjustable crossovers (see Fig. 3). The
crossovers can be set over an exceedingly wide range, with one notable
exception: the lowest crossover can be adjusted only down to 125 Hz,
which is a tad high for some bass-guitar, kick-drum, and mastering
applications. The plug-in instantiates in mono-to-mono or
Each of the MasterWorks Compressor's bands provides controls for
independently adjusting attack and release times, threshold, ratio, and
makeup gain. Separate bypass and solo controls also serve each band.
The solo function allows you to hear the compression effect in one band
while the other bands are muted, which can be quite helpful. Each band
also sports input, output, and gain-reduction meters and a compression
graph (depicting the in-band compression slope and certain parameter
values). Master input- and output-level controls, with associated
meters and peak-clear function, round out the MasterWorks Compressor's
comprehensive feature set.
PSPaudioware's VintageWarmer (Mac/Win, $149) delivers creamy,
analog-sounding split-band and broadband compression modes at a
bargain-basement price. This downloadable plug-in is available in VST
and RTAS versions for the Mac (including OS X) and in VST and DirectX
formats for Windows.
Now the bad news: VintageWarmer has a very quirky control interface,
which is split between two alternate windows (see Fig. 4). Each
of VintageWarmer's three bands has its own threshold and release-time
controls, but only the low and high bands offer in-band boost/cut
controls. (There is an unwieldy work-around for adjusting mid-band
gain.) The gain controls are placed before dynamics, forcing you to
move threshold controls higher whenever you boost in-band gain
(assuming you want the degree of compression to remain the same).
Master threshold and Speed (ganged attack and release) controls
complicate operation by interacting with the above-mentioned in-band
threshold and release controls. A global knee control is offered in
lieu of in-band ratio knobs. You can't solo or bypass individual bands,
nor can you adjust the attack time for each band independently. In-band
meters (including threshold indicators) are also absent. The owner's
manual is vague and confusing, screen redraws are slow, and save
routines (for storing custom presets) are unfriendly. On the positive
side, you get master input- and output-level controls, excellent global
metering (with clipping indicators and switchable ballistics), and a
low-band crossover that can be adjusted as low as 25 Hz.
Is it quirky? Yes. Is it awesome-sounding? Yes! Don't be put off by
VintageWarmer's interface. You'll quickly become accustomed to it and
get incredible results.
For recordists who want to use split-band compression on their
projects but don't want to sweat the learning curve, TC Electronic's
Finalizer Express Studio Mastering Processor ($1,595) is the answer
(see Fig. 5). This stereo digital multiband processor provides
25 presets, each with different fixed attack, release, threshold, and
ratio values for its 3-band compressor and its limiter. Its crossovers
use linear-phase filters fixed at 315 Hz and 3.15 kHz. Makeup gain is
applied automatically, but you can independently adjust the output
level of each band downstream of the processing block. Defeatable
in-band emphasis controls add additional compression and gain in their
respective bands by lowering the thresholds and kicking in more
automatic makeup gain.
FIG. 7: Waves C4 can apply compression, expansion, limiting, and EQ in any combination (one process per band) to its independent bands.
A global input-level normalizer, an output-stage soft clipper, and
an output fader add to the 1U rackmountable unit's value, as does a
fairly generous offering of analog and digital I/O. High-resolution
LED-ladder metering keeps you apprised of gain-reduction levels for
each band, master I/O levels, and soft-clipper action. The Finalizer
Express can operate at 44.1 or 48 kHz nominal sampling frequency.
Although the Finalizer Express is a simple solution for quick demo
production tasks, I'd be wary of using a preset split-band compressor
for critical mastering applications. If you want to work at higher
sampling rates or you require much more manual control, the TC
Electronic Finalizer 96 kHz ($2,995) offers 88.2 kHz and 96 kHz
operation, upsampling from and downsampling to lower rates, additional
multiband processing options, and in-band manual controls for attack,
release, threshold, ratio, and crossover frequencies.
The world's only all-tube split-band compressor, the dual-channel
Tube-Tech SMC 2B ($4,999), incorporates six optical compressors (see
Fig. 6). The unit's two crossovers are continuously variable,
with the low crossover adjustable down to 60 Hz. In-band threshold,
ratio, attack, release, and makeup gain controls (all continuously
variable) are joined by 11-segment LED displays that show gain
reduction for each band. Global controls include master gain, L/R
balance, and bypass. Unfortunately, individual bands cannot be
Yes, it's extremely pricey. Nonetheless, the SMC 2B is simply the
sweetest-sounding multiband compressor I've heard.
C4 MULTIBAND PARAMETRIC
The Waves C4 plug-in (Mac/Win; native version, $400; TDM version,
$800) supports TDM, RTAS, AudioSuite, and VST formats for both the Mac
and Windows (see Fig. 7). A MAS version for the Mac and a
DirectX version for Windows are also available.
C4 offers four independent bands of compression, expansion,
limiting, and equalization in any combination. You can use band-limited
compression for bass frequencies, for example, coupled with expansion
for the highs only. The slope of the C4's widely adjustable crossover
filters can be varied globally. In-band controls include threshold,
makeup gain, range, attack, and release. Each band can be soloed or
bypassed. Master threshold, makeup gain, range, attack, and release
controls each enable you to increase or decrease associated in-band
parameter values as a group, maintaining interband offsets — a
A wide-ranging knee control, toggling release-characteristic (Opto
or Electro) function, master output-level fader, and Waves' outstanding
ARC program-sensitive release-time mode complete this plug-in's
comprehensive set of master controls. Metering is excellent, providing
a threshold indicator for each band, master L/R output meters and
clipping indicators, and an ingenious graphical display that
dynamically shows gain reduction (or potential boost, for processes
other than compression) in each band.
LINEAR PHASE MULTIBAND
Linear Phase Multiband is sold as part of the Waves Masters Bundle
(Mac/Win; native versions, $900; TDM version, $1800). The Masters
Bundle is available in TDM, RTAS, AudioSuite, and VST formats for both
the Mac and Windows. Waves also offers a MAS version for the Mac and a
DirectX version for Windows.
Essentially an improved and expanded version of the company's C4
Multiband Parametric Processor, the Linear Phase Multiband offers five
independent bands with linear-phase crossovers (see Fig. 8).
Enhancements include a trim control that optimizes headroom at the
plug-in's output, dither, an automatic makeup-gain function, and Waves'
pioneering Adaptive Threshold function, which automatically varies the
threshold of a band to counteract frequency-masking effects. The only
downside to the Linear Phase Multiband is that it's a voracious CPU
The Waves Linear Phase Multiband sounds extremely transparent and
offers more control than any other multiband plug-in I'm aware of. For
DAW-based multiband mastering applications, it can't be beat.
Now that you've lusted over numerous split-band compressors, it's
time to put away your drool bib and get to work. Here's a look at some
real-world applications for split-band compressors, beginning with
Vocal tracks can present a number of challenges at mixdown. One
hurdle is the varying proximity effect caused by a singer moving closer
to and farther away from the mic during a performance. Simply rolling
off bass frequencies with a static EQ setting will cause high vocal
phrases to sound thin. A split-band compressor, however, will put a lid
on the proximity effect without reducing fullness when the singer
reaches for high notes.
FIG. 8: Waves Linear Phase Multiband offers five independent bands of processing, ultratransparent sound, and a feature-rich interface.
A typical approach might be to set your lowest crossover frequency
to somewhere between 200 and 300 Hz, depending on the mic's response at
close distances to the source and other factors. Set your low-band
threshold so that when vocal phrases that aren't bass-heavy occur, they
barely trigger low-band compression; then raise the threshold slightly
higher so that those passages then pass through the compressor
untreated. Any vocal phrases that are even slightly boomy will then
exceed the low-band threshold and will be compressed according to your
ratio (or range), attack, release, and other settings. By the way, you
can use similar settings and adjustments to take the boom out of an
It's easy to de-ess a vocal track with a split-band compressor. Set
your highest compressor crossover to 5 kHz for treating a male singer
or to 6 or 7 kHz for a female vocalist. Then set the high-band
threshold so that only sibilant phrases trigger in-band compression.
De-essing usually requires a high ratio (10:1 to 50:1 is often called
for), a high range (as much as 20 dB of gain reduction for extreme
cases), or both, though much lighter treatment should be used for
dealing with subtle problems. Set your high-band attack time as fast as
possible (50 µs works great) and set the release time to between
50 and 60 ms.
Most of the time, you can also perform light de-essing on an entire
mix without noticeably affecting the sound of most of the included
instruments; cymbals, triangles, and other sources that produce
prominent upper partials are notable exceptions. Whereas standard
de-essing techniques using a broadband compressor will usually punch
“holes” in a mix (causing noticeable dips and quick
recoveries in level for the entire mix), carefully de-essing a mix with
a split-band compressor will fully preserve the dynamics of bass and
midrange content and avoid audible pumping.
TAMING OF THE SHRILL
A vocal track that sounds shrill on select phrases can be tamed by
setting two of your split-band compressor's crossovers to roughly 3 kHz
and 5 kHz. Set your threshold in this band so that only the shrill
passages trigger in-band compression. A light to moderate ratio or
range setting is usually all that's required to take the edge off
everything but the most obnoxious banshees. On the other hand, heavy
compression of low highs will dull the vocal track, so use
You can also lightly compress low highs on a guitar- or
keyboard-heavy mix to prevent fatiguing edginess from rearing its ugly
head during the loudest sections of your program material. But tread
lightly, or your mix's detail and clarity will suffer.
Perhaps the most common use of split-band compression in modern
recordings is to simultaneously fatten and tighten up the bottom end of
a mix. Here, your ratio setting should be set according to how much the
bottom end varies in intensity; widely varying bass energy requires a
higher ratio than is called for when dealing with more constant levels.
You'll have to use your ears to adjust the attack and release times for
your particular mix, but bass frequencies are often best compressed
using moderate or slow time constants. Lower your threshold until the
bass frequencies sound tight, so that they don't bloom or dip too much
at any point in the mix. Then increase the makeup gain in the bass band
to the point where the bottom end sounds huge but does not overwhelm
other elements and frequencies in the mix.
Adjusted properly, you'll often find that the mix's peak bass energy
will now be lower than before you compressed it, which translates into
overall greater headroom for the entire mix. That is the best of both
worlds: the bottom end will sound tighter and you can make your
mix louder if you want to.
TIME TO SPLIT
Split-band compressors have many more applications, but they all
begin with careful listening and evaluation of your program material.
Split-band compressors should be used only when undesirable, excessive,
and dynamic variances in spectral energy are evident (that is,
when static EQ won't fix the problem). Routine application of
split-band compression on material that doesn't need it — or
heavy-handed use on material that needs only small amounts — will
make a dynamic and compelling performance sound lifeless and impotent,
or worse. It's all too easy to go overboard with split-band compression
and ruin a good thing. However, deliberate and deft use of split-band
compression will give your tracks and mixes a polished sound that you
can't achieve any other way.
EM contributing editor Michael Cooper is the
owner of Michael Cooper Recording, located in beautiful Sisters,
Oregon. Cooper's studio offers recording, mixing, and mastering
Arboretum Systems tel. (800) 700-7390; e-mail email@example.com; Web www.arboretum.com
BSS Audio tel. (615) 360-0277; e-mail firstname.lastname@example.org; Web www.bss.co.uk
Mark of the Unicorn (MOTU) tel. (617) 576-2760; e-mail email@example.com; Web www.motu.com
PSPaudioware.com s.c. tel. 48-601-963-173; e-mail firstname.lastname@example.org; Web www.pspaudioware.com
TC Electronic tel. (805) 373-1828; e-mail email@example.com; Web www.tcelectronic.com
Tube-Tech/TC Electronic (distributor) tel. (805) 373-1828;
e-mail firstname.lastname@example.org; Web www.tube-tech.com
Waves tel. (865) 546-6115; e-mail email@example.com; Web www.waves.com