Of all the processes used in modern music production, compression isperhaps the least understood. One reason is compression’s sonicresults are often subtle and thus hard to hear—especially forbudding engineers. Another hurdle is presented by the various anddiffering compressor control parameters; those, too, are typicallysubtle in their individual sonic effects, and they work togetherinteractively, further complicating the stew. Then there’s theconfusion that lies in the bewildering array of product types andmodels the engineer must choose from before even reaching for a controlknob. For example, for a given application, should you select aVCA-based compressor or one controlled by an opto-electrical element? Asolid-state or tube design (or a hybrid of the two)? Analog or digitalcompression? A hardware compressor or one that is software based? Andso on.
With so many variables, it’s no wonder compressors andcompression remain a mystery for many users. Yet, if you want to masterthe arts of recording and mixing, learning compression’sintricacies is imperative. After all, the production processes for mostof today’s popular music forms—with the notable exceptionsof classical and some jazz—rely heavily on compression. Simplyput, if you’re not compressing properly, you’re not gettingthe best sounds possible.
This article will guide you through the maze of compressor optionsand explain practical compression applications in plain English.I’ll start with the basics of compression, citing examples ofvarious production techniques and the theories behind them. I’llalso tell you which features to look for in a compressor and whythey’re important. Finally, I will survey specific types anddesigns of compressors, describe some models, and offer opinions aboutwhich models do the best jobs on which instruments.
Compression falls under the broader category of dynamics processing.The term “dynamics” refers to changes in loudness level, sodynamic range is the difference between the softest and loudest soundsthat a source produces, or that a track contains. A dynamicsprocessor’s purpose is simply to increase or decrease asignal’s dynamic range, which alters how the levels fluctuatewithin that range. Types of dynamics processors include gates,expanders, limiters, levelers, and compressors.
A compressor is a type of dynamics processor that“squeezes” a signal’s dynamic range—that is, itreduces the difference in volume, or level, between the loudest andsoftest parts of a performance. The process of reducing volume iscalled gain reduction. Properly applied, gain reduction makes aperformance sound more consistent from beginning to end. For thatreason, compression is a great remedy for a performance in which thelevels fluctuate too widely.
By reducing dynamic range, a compressor also allows for theprocessed signal’s overall level to be raised—that is,become “hotter”—resulting in increased loudnesswithout pushing the signal’s loudest parts into distortion.Bringing up the overall level has the additional benefit of makinglower-level sounds louder than they were before compression. The resultis that subtle nuances such as mouth sounds and ghosted notes—aswell as burps, string buzzes, and snare rattles—are louder,clearer, and easier to hear.
Of course, you may not want to make burps, string buzzes, and otherincidental performance sounds more audible. Therefore, applycompression only when musically appropriate—when the end resultwill sound better than what you started with.
You can always add compression after a track is recorded (duringmixdown), but sometimes it is desirable to use compression during therecording process. That approach has several potential benefits. Forone, a compressor makes it easier to capture usable tracks whenrecording an instrument with a wide dynamic range. Moreover, solvinglevel-fluctuation problems during tracking frees you from having tosolve them at mixdown. That, in turn, leaves more time and brainpower—not to mention gear—for focusing on the mix’screative aspects.
For those recording to any digital medium, using a compressor duringtracking ensures that sounds are encoded at a higher level. Becausemore bits are used, better bit resolution results. Furthermore, byputting a lid on peaks, the compressor also helps avoid digitalclipping on extraloud notes. For those recording to analog tape,compressing during tracking allows the signal level to be raised higherabove the noise floor, which results in an improved signal-to-noiseratio.
Tricks of the Trade
In addition to problem solving—smoothing out roughperformances, improving digital resolution and signal-to-noise ratio,avoiding digital clipping, and the like—you can also employcompressors in numerous creative applications. For example, acompressor can dramatically change the envelope of a sound in much thesame way an envelope generator works in a synthesizer. That and othercompression tricks can give a vicious attack to a lackluster snaredrum, add crunchy edge and sustain to a mild-mannered electric guitar,make a lead vocal sound so urgent that listeners will dial 911, or pumpup an entire mix until the band sounds like it’s exploding out ofthe speakers.
In simplest terms, think of a compressor as an automatic volumecontroller. Indeed, before compressors were invented, engineerstypically had to “ride gain” on a channel to maintainconsistent volume levels. (Then again, many engineers still ride gain,even when using compressors.) However, a compressor controls levelswith a speed and accuracy that is impossible to achievemanually—sort of like a magic genie adjusting the track’sfader with lightning-fast reflexes. The compressor’s controlsettings determine when and how much that fader moves.
Depending on how its controls are set, a compressor reduces eithertransient peaks—the short-lived, attack portions of asound—or the average-level portions of the sound, and sometimesboth. Examples of transient peaks include the stick strike on a drumhead and guitar-string plucks. A sound’s average-level portionsinclude a snare drum shell’s ringing and the sustain of a guitarnote after it is plucked. Certain instruments—a wood block, forinstance—produce mostly transients and very little sustain.Others, such as vocals and organs, typically produce mild transientsthat barely peak above their average levels.
The number of controls on compressors varies greatly, depending ondesign, cost, and other factors. Units that employ voltage-controlamplifiers (VCAs), for example, typically have at least five controls:threshold, ratio, attack time, release time, and output level.Full-featured VCA models may offer more than twice that many controls,whereas some expensive opto-electrical compressors may provide only twocontrol knobs.
Note that units with fewer controls are not necessarily lesscapable; rather, they typically provide automatic control of parameterssuch as attack and release time, or they “gang” twoparameters (threshold and ratio, for example) on to one knob.I’ll discuss those types of compressors in more detail later.First, I’ll analyze the five controls common to most VCA-basedcompressors.
Threshold is the level at which compression kicks in and starts toreduce the signal’s level, or gain; the threshold control letsyou set that level. With threshold at 0 dB, for example, all signals ator above 0 dB get compressed, while those that fall below 0 dB areunaffected. Therefore, to control peaks, set the threshold to a levelbelow the level of the peaks but above the average level of the signal.That way, peaks that exceed the threshold get attenuated while theaverage levels pass unaffected through the unit. Clearly, a properthreshold setting is critical to a compressor’s performance: ifthe threshold is set too high, the unit will not process any of thesignal; if the threshold is set too low, the unit will reactto—that is, attenuate—every portion of the signal.
Ratio expresses the difference between signal increases (volume) atthe compressor’s input and increases at its output; the number onthe left refers to input and the right to output. Therefore, the ratiocontrol determines how much the signal will be attenuated once itexceeds the threshold. For example, a 2:1 ratio will let a signalincrease in level only 1 dB for every 2 dB it exceeds the threshold(see Fig. 1). Likewise, if the signal exceeds the threshold by 6 dB ata 2:1 ratio, the compressor attenuates the signal by 3 dB, a net gainincrease of only 3 dB. In that case, the compressor’sgain-reduction meter (if it has one) will show 3 dB of gainreduction.
Typically, different instruments and performances call for differentcompression ratios. For example, to compress a ballad’snear-perfect vocal track, a mild 2:1 ratio would probably suffice; atthat ratio, and with the appropriate threshold dialed in, thecompressor tightens up the performance enough to ensure quiet phrasesare not lost in the mix and higher levels are not overbearing. At theother extreme, a bass guitar track that alternates between mellowfinger-pad technique and aggressive pop ’n’ slap can easilyhave a huge dynamic range. To yield consistent levels from that type ofperformance, a higher ratio such as 10:1 may be in order.
Note that threshold and ratio work together to affect asignal’s output level. The lower the ratio, the less control thecompressor has on the signal; the lower the threshold, the lower thesignal level subject to compression. The relationship between the twocontrols affords flexibility and sonic variation. There are, forexample, two different-sounding ways to get the same amount of gainreduction out of a compressor—low threshold and low ratio or highthreshold and high ratio.
Attack time is how long it takes—measured in milliseconds (ms)or microseconds (µ)—for the compressor to kick in once thesignal exceeds the threshold. A slow attack time lets inherently fasttransient signals pass threshold before compressing the rest of thesignal; a fast attack catches transients, but may diminishhigh-frequency content.
One thing worth noting is that manufacturers sometimes measureattack times differently. Some specify attack time as the time it takesfor the compressor to react after the threshold is exceeded, and othersspecify attack time as how long it takes for the compressor to reach,say, 67 or 90 percent of the maximum gain-reduction level it willultimately achieve. Fortunately, the exact definition is of littleimportance, as typically attack time is set by ear. Depending on whatkind of effect you’re going for, simply decrease the attack timeuntil unruly peaks are tamed or increase it until average levels arelowered and desirable peaks get through unscathed. If you’rehaving trouble hearing your settings’ effect, watching adownstream peak-level meter (that is, one that monitors the levelsafter the process—the compressor’s output-level meter, forexample) will let you visually confirm what portion of the sound isattenuated.
Release time is how long—measured in seconds or hundredths ofa second—it takes for the compressor to return the signal tounity gain (its unprocessed state) after the signal falls back belowthreshold. That is, once the release time passes, the compressor letsthe signal pass through unaffected. In general, slower release timesresult in a more natural sound.
In general, set fast attack and release times when you want thecompressor to do its job and get out of the way quickly—forinstance, when you want to put a lid on transient guitar plucks butallow the ringing notes to pass through unaffected. Conversely, amoderate attack time coupled with a long release is perfect for thoseDavid Gilmour–esque guitar solos in which you want notes tosustain forever. At two seconds or longer, the extended release timecauses the compressor to slowly restore compressed levels to theiroriginal (higher) gain, just as the sustained notes start to naturallydie off, which counteracts the decay and makes the tails of the noteslouder.
A compressor’s last control stage is its output level. Thatcontrol is also known as make-up gain because it is used to make up forthe gain reduction caused by the compressor. The usual approach is toincrease the processed signal’s output level so it matches theunprocessed signal’s level. That creates unity gain between thetwo signals, which makes it easier to compare them using the bypassswitch and ensures appropriate levels when recording or mixing.
Path Not Taken
A compressor can degrade or ruin an audio signal as well as enhanceit; therefore, one of the most useful features on any compressor is thebypass switch, which lets you compare processed and unprocessedsignals. After using the output-level control to balance the levels ofthe processed and unprocessed signals—a critical step becauselouder signals sound brighter and fuller—you can judge whetheryour control settings actually improve the sound by switching thecompressor in and out of the circuit.
Fortunately, most compressors provide a bypass (a notable exceptionis the Universal Audio Teletronix LA-2A). Typically, this is a switchthat disables the compressor circuitry; ideally, it also disables theinput- and output-level controls. A hardwire bypass is usually the bestdesign because it routes the input directly to the output and bypassesall compressor circuitry, such as input and output amplifiers andgain-control devices.
Actually, on some high-end units the signal is kept in circuit evenin bypass mode—a viable design as long as the signal path ispristine enough not to color the original signal. The advantage of thatdesign is it avoids the use of bypass relays and audio-pathswitches—elements that inevitably degrade during a period of timeand compromise audio quality. The bypass on the Millennia TCL-2 TwincomOpto Compressor/Limiter ($2,995), for example, defeats the unit’ssidechain control only, thus preventing compression from taking place.(I’ll discuss sidechains later.)
Don’t Need Your Input
Many compressors also provide an input-level control, but those areoften superfluous—if not undesirable. For one thing, a compressorwith a wide-ranging threshold control can handle almost any inputlevel. So an input-level control is necessary only if the thresholdrange is too high or too low to act on the input signal as is.
For example, if the threshold’s highest setting is +2 dBV andyou’re feeding the compressor +12 dBV levels, you’llcompress most or all of the time unless you somehow lower the level atthe compressor’s input. That’s one instance in which aninput-level control comes in handy. Conversely, if the thresholdrange’s minimum setting doesn’t go down very low, thecompressor may not kick in when fed low levels. In that case, aninput-level control is necessary to boost the input to an appropriatelevel.
The reason an input-level control can be thought of as undesirableis it adds yet another amplification/attenuation stage to thecircuitry, thus degrading signal quality. For that reason, high-end,minimalist compressors, such as the Millennia TCL-2 Twincom, typicallyomit the input control to maintain a pristine signal path.
In addition to the controls and parameters already discussed,several more-subtle parameters and design features often figureprominently into a compressor’s performance or sound. One suchparameter is the knee, which is related to the compressor’sthreshold control. The knee determines how quickly and smoothly thecompressor will transition from no action to the full ratio of gainreduction set on the unit once the signal passes threshold. Generally,a compressor’s knee is hard or soft, though some units—theAphex 661 tube compressor/limiter ($749), for example—provideswitchable hard- and soft-knee compression.
In hard-knee compression, the unit processes the audio signal at theselected ratio once the input signal passes the threshold. Althoughuseful for applications such as peak limiting and de-essing (discussedlater), a hard knee can sound abrupt, especially with higherratios.
A soft-knee compressor, or one set to soft-knee compression, beginsto compress as the signal approaches the threshold level and graduallyincreases the ratio until the signal attains threshold, at which pointit equals the selected ratio value. The gentler, logarithmic increaseof soft-knee processing tends to sound more transparent (lessnoticeable) than hard-knee compression, and thus is usually preferablefor most vocals and instruments.
In addition to manual controls for attack and release times, somecompressors offer an automatic mode, called auto mode, that does someof the tweaking for you. That is often referred to as program-dependentor adaptive processing. In auto mode, the compressor’s detectorcircuitry analyzes the program content (the audio-input signal) anddynamically adjusts the attack and release times accordingly. Forexample, if a guitarist starts picking harder, the unit automaticallydecreases and therefore quickens its attack time to catch the increasedpeaks. On the other hand, an increase in average levels typicallyprompts longer release times to avoid pumping while the compressorreturns to unity gain.
Auto mode’s main benefit is it precludes the need to tweakattack and release settings on performances in which the dynamicschange radically. It also lets you set up quickly yet still get goodresults when the pressure is on. The downside is you lose some controlover the sound. For example, you may like those peaks when theguitarist picks harder—in which case you probably would not wantto use auto mode.
Some compressors—such as the MindPrint T-Comp Stereo TubeCompressor ($1,099)—offer a semiautomatic mode of operation. Asthe name suggests, semiautomatic mode lets the attack and releasesettings exert some influence on the adaptive processing.
Opto-electrical compressors may or may not offer an auto mode;however, even without one, those units provide something akin toautomatic processing in that attack and release times—manuallyset or not—fluctuate based on program content. That is due to theinherent nature of opto-electrical compressors, which in general areslower and less exacting than VCA-based designs. Because the attack andrelease controls on optical compressors provide only approximateresponse times, many manufacturers simply put “fast” and“slow” on either side of the knob, rather than hash marksindicating exact times. (More on optical compressors in a bit.)
Most dual-channel compressors offer stereo linking, a feature thatlets you run two channels—for example, stereo acoustic guitar oreven an entire mix—through the compressor and have each channelbe attenuated the same amount. That keeps one side’s level fromdipping more than the other, which would throw the stereo image out ofwhack.
True stereo linking works by having the channel that exhibits themost gain reduction determine the gain reduction for the other channel.Another form of linking establishes a master/slave relationship betweenthe two channels in which one side (typically the left) is thepredetermined master and the other follows its attenuation pattern.
It is commonly said that compression becomes limiting at ratios of10:1 and higher, but that is not the entire story. Actually, thedetector circuits in compressors and true limiters differ by design. Acompressor’s detector circuit is usually designed to detect RMS,or average, levels rather than transient peaks. Therefore, transientpeaks almost always overshoot a compressor’s threshold level, nomatter how high the ratio and how fast the attack time is set. A truepeak limiter, on the other hand, employs a detector circuit thatresponds to peak energy levels and thus reacts faster.
Whereas all true compressors use RMS-sensing detector circuits,detectors for different models can differ substantially in theirreaction times. That means two different compressors set to the sameattack, release, threshold, and ratio values may nevertheless respondquite differently to the same signal. (That is one of the many reasonsit is difficult to recommend specific control settings for compressingvarious instruments.)
Chain, Chain, Chain
Every compressor has a sidechain detector circuit that“sees” when the threshold has been exceeded and tells thecompressor’s gain-control element or amplifier to attenuate thesignal. The sidechain is not in the audio path; it’s merely atraffic cop that tells the compressor when to attenuate the signal. Thecircuits for threshold, ratio, attack, and release are also found inthe sidechain.
Full-featured compressors typically provide sidechain inserts ontheir rear panels. Think of a sidechain insert as an effects loop thatpatches into a compressor directly before the detector; like the restof the sidechain, it is not in the audio path, so its effectisn’t directly heard. Sidechain inserts therefore let you processthe compressor’s input signal before it reaches the detector.That permits de-essing and other frequency-conscious applications.Here’s an example of how to perform de-essing.
To de-ess a vocal, first patch the send and receive from thecompressor’s insert into an equalizer’s input and output,respectively. Next, boost the equalizer’s high frequencies andcut its lows and mids. That causes the compressor’s detector tohear the vocal as having excessive highs. Whenever the whistling soundof sibilance raises its ugly head, the sensitized detector circuithears it much louder than it really is, causing the circuit tovigorously reduce gain in the audio path. With attack time set toaround 50 µ and release time between 50 and 60 ms, the compressorcan be made to quickly attenuate the sibilance and get out so the restof the vocal is left unchanged. Of course, the compressor’sthreshold must also be set properly—above the vocal’saverage levels—for that to work.
You can also use a sidechain insert to make the detector react to asignal entirely unrelated to the audio-input signal. The classicexample here is “ducking”: a sidechain application in whichan announcer’s voice is set to trigger a music bed’sattenuation. To set up this type of ducker, play stereo music tracksthrough a dual-channel compressor and patch the voice-over track (orchannel) into the sidechain insert’s receive jack. Next, set thecompressor threshold low enough that it responds to every vocalutterance. When the announcer speaks, the detector hears the voice andinstructs the compressor to lower the music bed. You can also use thattechnique to automatically lower, say, guitar levels whenever a leadvocal comes back in. To accomplish this, patch a mult of the vocal intothe insert receive jack of the guitar’s compressor channel.
The misconception that splitband compression is the same asfrequency-conscious compression is common. A splitband compressorsplits the audio signal into two or more frequency bands so each bandcan be processed by its own independent compressor circuitry (each withits own controls). That lets you compress, for example, aguitar’s bass frequencies differently from the highs.
A compressor that offers—or is set up toprovide—frequency-conscious compression is still a full-banddevice acting on the entire signal. The difference between it andnormal compression is simply that the detector is set to be called intoaction by the prevalence of specific, user-selected frequencies.Frequency-conscious compression has dozens of useful applications, butspace limitations dictate I save that vast subject for a futurearticle.
Down and Dirty
Now I’ll discuss various types of compressors on the marketand which designs and models are best for different recording andmixing applications. I’ll start with analog compressors, whichcan be subdivided into four categories based on the type ofgain-control element they use: opto-electrical, Variable-Mu, FET, orVCA. Each design has benefits and drawbacks.
To increase the usefulness of this article, I tested a sampling ofcompressors chosen to represent the various design and feature setsavailable. Bear in mind that this sampling is not meant to becomprehensive, nor is it intended to overlook or denigrate units notmentioned here.
An opto-electrical compressor (opto, for short), uses a specialopto-electrical cell in its sidechain that consists of either anelectro-luminescent panel—basically a night-light—or an LEDthat shines on a light-dependent resistor (LDR). In simple terms, thelight panel, or LED, shines with increasing intensity on the LDR as theaudio-input signal gets louder, and the LDR causes a correspondingincrease in compression of the audio-input signal. Because the LDR hasan inherent memory effect, it releases slower when the light isbrighter or has been shining for a while. In practical terms, thatmeans heavy or near-continuous compression results in longer releasetimes.
Vintage-style optos generally have only two control knobs, typicallylabeled gain reduction and gain. Turning up the gain-reduction knobfeeds more signal to the opto cell, effectively lowering the thresholdand causing more compression. The gain knob sets post-compressionoutput level, or make-up gain.
As mentioned earlier, opto elements have a natural lag time in theirattack response; indeed, all the time constants are inherentlyadaptive. Optos also have, by nature, a soft knee. For those reasons,opto compressors tend to have a natural-sounding attack and release.The downside is they are usually not quick enough to catch fasttransients, so substantial overshoot is not uncommon. Typicalapplications for opto compressors include vocals, bass, and electricguitar. However, one thing I’ve learned is not to try topigeonhole equipment based on general design characteristics.
A case in point is the Joemeek C2 stereo opto compressor ($399). Asolid-state unit, the C2 sounds great on snare drum, serving up anoutstanding power-pop snare tone with a dark yet explosive attack (seethe sidebar “Dialing in Hot Sounds”). The ability to get adefined but warm edge also makes the C2 a good choice for creatingcrunchy electric-guitar sounds. The C2 also does a surprisingly goodjob of smoothing out levels on inconsistent kick drum tracks. Whereassome optos respond to such transient material with inconsistent attackand release responses, the C2 is rock steady. You get a bit oftransient overshoot, but overall the control is excellent. Overall, theJoemeek C2 is—at least for certain applications—the bestcompressor I’ve heard in its price class.
Another dual-channel opto compressor that does a good job of reiningin kick drum tracks is the Bellari RP583 ($650). This hybrid unitemploys a tube circuit for the internal processing and solid-state opamps for the input and output circuits.
An even better-sounding unit on kick drum is Joemeek’s SC2.2stereo optical compressor ($799). The SC2.2 offers outstanding levelcontrol yet doesn’t thin out the drum sound very much—auniversal problem when heavily compressing that instrument. It is oneof the best compressors I’ve heard on kick drum.
In general, opto compressors are not my first choice for compressingarpeggiated acoustic guitar tracks. That’s because optos tend topump in this application; you can clearly hear the level dipping andthen recovering. Pumping occurs when the bass frequencies in a pluckednote trigger heavy compression, which in turn attenuates highlynoticeable mid and high frequencies. That said, the SC2.2 delivers asurprisingly transparent sound on acoustic guitars.
The C2 and SC2.2 also sound smooth, natural, and beautifully warm onvocal tracks. But the most venerable of vocal compressors is thesingle-channel Universal Audio Teletronix LA-2A Leveling Amplifier($3,495), a faithful reproduction of the highly coveted, ’60s-eraLA-2A tube compressor. The LA-2A has an uncanny ability to warm up apiercing or thin vocal, and it smooths the most unruly vocal dynamicswith a transparency hard to match in other compressors. The LA-2A alsosounds great on bass guitar, kick drum, and snare drum.
Another unit that sounds absolutely gorgeous on vocals is theaforementioned Millennia TCL-2 Twincom, a dual-channel unit withswitchable tube and solid-state audio paths for each channel. Ifyou’re looking for a compressor with stunning clarity, nuance,and depth, look no further than the TCL-2.
The Anthony DeMaria Labs ADL 1500 ($2,995) is another greatopto/tube compressor for recording and processing vocals. It’s adual-channel, vintage-style (two-knob) opto with a fat sound and a softtop end.
Although out of production, the Groove Tubes CL1S dual-channelopto/tube compressor is worth mentioning in case you can find one used(the original price was $2,295). The CL1S is one of the mostlush-sounding compressors ever made; it imparts rich yet presenttextures to everything it touches. The unit’s only downside is itis quite finicky; it pumps readily if not set up with painstaking care.But for dialing in huge, velvety vocals, thundering bass, orslammin’ electric guitar, it’s the compressor toget—if you can find one. The CL1S also sounds great on kick andsnare.
Continent of Mu
The first compressors ever made incorporated a Variable-Mu design.Those use a vacuum tube—such as a dual triode orpentode—for the gain-control element. Technically orientedreaders will be interested to know that this is a fully differential,push-pull design in which one side of the tube handles the positivewaveform phase and the other side the negative phase.
Variable-Mu compressors do not offer an adjustable ratio control.What people love about the Variable-Mu compressors is that theycontinuously increase their ratio the harder they’re pushed (thehigher above threshold that the input signal rises), resulting in anincreasing desensification of sound. Though Vari-Mu compressors offerfaster attack and release times than optos, they are not as fast as VCAdesigns, and therefore they’re not as effective at handling peaksas VCA-based units. Also, as a class, Vari-Mu compressors cannotproduce as much gain reduction as other types of compressors becausethe employed tube typically runs out of dynamic range sooner than othertypes of gain-control elements. A Vari-Mu usually gets 12 to 15 dB ofgain reduction and sometimes considerably more.
Few Variable-Mu models are currently on the market. I am aware ofhigh-end units made by boutique manufacturers such as Manley Labs andPendulum Audio. Because of design differences, Pendulum Audio’sVariable-Mu compressors typically offer faster attack times thanManley’s.
The Manley Stereo Variable-Mu Limiter/Compressor ($4,000) soundsawesome on bass and electric guitar; it imparts a fat, lush tone withplenty of presence and clarity. When set to limit mode, the Manley alsosounds great on snare drum. However, the unit is intended primarily asa stereo-bus compressor for processing an entire mix. In thisapplication, I had to keep the attack time near its slowest setting toavoid pumping. But properly dialed in, the Manley Stereo Variable-Mucan really fatten up a mix.
FET in the Cap
FET-based compressors are scarcely available now. They use an FET(Field Effect Transistor) as the gain-control element, which hasadvantages and disadvantages. On the plus side, FETs offer attack andrelease times that are much faster than optos can provide, and evenfaster than many VCAs. The downside is FETs have a somewhat limiteddynamic range. In traditional FET designs, very hot levels can causeamplitude-modulation artifacts (for example, pumping) and waveformdistortion.
Crane Song’s single-channel Trakker ($2,550) and dual-channelSTC-8 ($4,450) compressors use a unique Pulse Width Modulator (PWM) FETdesign to solve the distortion problem. Unlike traditional FET designs,a PWM FET does not modulate the gain reduction, so there is lessdistortion at high gain. In technical terms, the PWM is essentially aswitch that turns the audio signal on and off at a 1.1 MHz frequency,letting only a percentage of its energy through at any given time tocontrol the gain. A filter rids the signal of switching artifacts.Because the signal is either on or off, it doesn’t modulate thegain-control element’s resistance or resulting gain reduction.The result is that the distortion content is linear as gain reductionincreases. In addition, attack time can be as fast as a fewmicroseconds. Unfortunately, I have not worked with any Crane Songcompressors in my studio.
Universal Audio’s reissue of the vintage (late ’60sera), solid-state 1176LN Limiting Amplifier ($2,295) is one of the bestcompressors I’ve heard on kick drum, electric guitar, andespecially snare drum. Many of the explosive snare sounds heard on pastand present hit records were processed with an 1176LN. Thesingle-channel unit also dishes up crystalline vocals and a burpybass-guitar tone. Whereas the LA-2A shines on thin, piercing vocals,the 1176LN sounds particularly flattering on woolly vocals by improvingclarity and intelligibility as it reins in levels.
Gimme a V, Gimme a C . . .
VCA-based compressors can be lightning fast and therefore generallyoffer better peaks control than opto and Variable-Mu units. They canalso attain absurdly high gain-reduction levels—useful when youreally need to smash down a signal. The downside of VCA designs is thetendency of lesser-quality units to dull high frequencies at highgain-reduction levels. In addition, some engineers don’t like thecoloration VCAs can impart to the audio path. However, VCA-basedcompressors’ quality ranges widely, and some units sound farbetter than others.
I frequently use a pair of stereo-strapped Aphex Expressors (thesolid-state Model 651s, which have been discontinued) for compressingan entire mix because, simply put, they are more transparent than anyother full-band (as opposed to splitband) analog compressors I’veheard. By transparent I mean that, at reasonable settings, theExpressors exhibit virtually no discernible amplitude-modulationartifacts (pumping and breathing, for example).
The acid test for any compressor is how well it can handlebroadband, percussive material—that is, material with a lot ofsharp transients and extreme bass and high frequencies mixed together.A good example of broadband, percussive material is a full-ensemblestereo mix complete with drum set, bass, and arpeggiated acousticguitar. Most full-band compressors on the market do a mediocre if notterrible job handling that kind of material. When transients or heavybass content cause abrupt compression, full-band compressors tend topump, sucking down easily audible mid and high frequencies.
A pair of stereo-linked Expressors can really beef up a mix withoutpumping. The Expressor is also extremely stable on acoustic guitar, andit is the best compressor I’ve heard on kick drum. A serviceableworkhorse on bass, vocals, and electric guitar, the Expressor’sonly downside is it can be a tad noisy and thin sounding. Becausecompressing a full mix usually makes bass content perceptually louder,the latter consideration is not much of an issue—I’ve heardtube compressors that offer a much fatter sound but are not nearly asstable. (If you’re interested in the Expressor note that theoriginal, solid-state model 651 was replaced by the Aphex 661 tubecompressor/limiter, which adds a tube in the audio path and an automode. The 661 features the same ultratransparent VCA 1001 gain-controlelement, but the audio path sounds a bit veiled compared to thatoffered in the original solid-state Expressor.)
Another really great VCA-based compressor is the Empirical LabsDistressor ($1,499 for the single-channel version, $2,899 for thedual-channel). The Distressor is a digitally controlled analogcompressor featuring a custom Class A VCA and standard Class A/B opamps in the signal path. The digital circuitry actually switches theunit between four different and independent solid-statecompressors—all in one box—making for one of the mostversatile compressors on the market.
If you buy a Distressor, order the new British mode option, whichcosts an additional $100. Simply put, British mode “kills.”With the right settings, you can make a Distressor in British modesound a lot like a vintage 1176LN; it serves up unbelievably savagepower-pop snare sounds, crunchy guitars with beautifully long sustains,and in-your-face vocals with crystalline highs. (See the sidebar“Dialing in Hot Sounds” for sample control settings.) TheDistressor also delivers fat, burpy electric bass-guitar tracks (withBritish mode turned off) that sound quite similar to what a great tubecompressor would produce.
The MindPrint T-Comp Stereo Tube Compressor ($1,099) is a tube- andsolid-state hybrid, soft-knee compressor that performs transparently onstereo mixes and acoustic guitar. However, it’s quite noisy andlacks the headroom to handle really hot pro levels. Just the same, theT-Comp is a good workhorse compressor for vocals, bass, and snare-drumtracks.
At the bargain end of the VCA price scale is the PreSonus BluemaxSmart Compressor/Limiter ($199). This fixed-stereo unit providesnumerous compression presets, and it also offers a manual setting withwhich you can dial in attack and release times to taste. Despite itslow cost, the Bluemax is one of the best compressors I’ve heardon kick drum. It also provides excellent control of acoustic guitartracks.
Many engineers lust after the warm, round tones that great tube gearcan deliver. Tubes—especially when driven hard—generallyproduce more even-order harmonics than solid-state devices and theyalso tend to saturate in a more gradual and pleasing way. On the otherhand, the most pristine solid-state designs tend to offer a morefocused sound and slightly better transient response (detail). Bothtopologies have their place.
Quite a few hybrid devices are available for purchase. Hybriddesigns employ both tube and solid-state devices in their audio paths.The dual-topology Millennia TCL-2 Twincom lets you switch betweencompletely independent all-tube and all-solid-state audio paths in thesame box.
It’s helpful to know a compressor can be marketed as a tubeprocessor and yet have a solid-state device—for example, a VCA oropto cell—in its audio path. Some purists insist a compressor isnot “all tube” unless the gain control element is alsotube-based (as in Variable-Mu designs), but that is not a practicalconcern. The gain-control element affects the compression curve’senvelope, which in turn can affect the sound’s timbre. But awell-designed tube output amplification stage can certainly give thelush timbral coloration and depth most folks are looking for in a tubedevice. For example, the Groove Tubes CL1S combines an opto elementwith a tube output stage, and that box has incredible warmth anddepth.
On the other hand, just because a compressor has a tube in itdoesn’t mean it’s going to give a fat sound. Just bylistening to the Bellari RP583, for example, I would never know it wasa tube compressor. Also, I’ve heard other tube units that soundmore like distortion pedals than pieces of studio equipment. Generallyspeaking, you get what you pay for—and the best-sounding tubegear tends to cost a lot.
Interestingly, there are also quite a few solid-state compressorsthat offer wonderfully warm tones (the Empirical Labs Distressor andJoemeek units immediately come to mind). My advice is to judge eachcompressor, tube or solid state, on its sonic merits and try not to getcaught up in the hype.
Ones and Zeros
One advantage of digital compressors is most of them offer“look ahead” circuitry. (Interestingly, dbx also offers ananalog compressor with that feature.) Because the compression algorithmis in software, the compressor can analyze what it is about to processand place the attack time right at the onset of—or evenbefore—the sound, resulting in a zero attack time. However, whilea super-quick (or zero) attack time is great for catching transients,it doesn’t always sound the best. Therefore, use such powerjudiciously; the crack of a snare drum without any attack justdoesn’t sound right.
In addition, digital compressors usually offer incremental controlof every parameter imaginable, as well as the ability to store settingsfor later recall. Perhaps the biggest benefit of working with digitalcompressors is the ability to stay in the digital domain. Ifyou’re working with a digital audio workstation or digital mixer,there are strong arguments for not re-entering the analog circuits.Most importantly, by staying in the digital domain, you avoid thesignal degradation and distortion caused by multiple conversions.
If you’re considering buying a hardware digital compressor,make sure it has great-sounding A/D and D/A converters. It’s alsohelpful if the software is upgradeable through user-installable EPROM,CD-ROM, or some other user-friendly method. In addition, you shouldinsist on a box with a word clock input. Without word clock inputs, youwill be limited to using only one digital compressor at a time.
The main reasons to buy a software digital compressor are easyupgrade capability and less hassles with dither and jitter—keyconsiderations in yet another vast and controversial subject that Idon’t have space here to discuss. Suffice it to say that workingwithin a self-contained digital environment (such as a DAW)—asopposed to patching together peripheral digital devices—cansimplify things a great deal.
The biggest concern with all digital compressors, ofcourse—both software and hardware based—is audio quality.Quite frankly, until just a few years ago, most of the digitalcompressors on the market sounded terrible. They would just suck theair out of any audio they processed and then spit out dull, lifelesstracks with no top end or depth. That, however, is changing rapidly.Although there are still relatively few digital compressors to raveabout, some truly singular ones stand out.
Because I usually work with digital compressors only when mastering,I can speak authoritatively in that context only. The compressors inthe Alesis Masterlink ML-9600 Master Disk Recorder ($1,699) soundoutstanding. In DAW, the Waves Renaissance Compressor (sold as part ofthe Renaissance Collection bundle—$300 for the Native version and$600 for TDM) and Arboretum Systems Ionizer ($499 for the MAS version)are extremely transparent compressors. Ionizer offers great splitbandcompression and other dynamics processing, as well as EQ and noisereduction.
Clearly, it’s important to choose the right compressor for thejob at hand. One thing I learned from years of working with variouscompressors is it’s not so much the design but the execution ofthe design that makes a compressor good or bad for a specificapplication. Be wary of any generalizations about compressors.We’ve all heard, for example, that opto-electrical compressorsprovide transparent and natural-sounding compression—as if thatwere a given. But the fact is, some optos do and some don’t. Asalways in audio, it’s the sound that counts, not the hype.
Hopefully, this article not only opened your eyes to the wide,wonderful world of compression but also has helped you steer an easiercourse through the labyrinth of compressor types, features,applications, and models. Deft and artful compression takes time andexperience to master, of course. Naturally, as with any otherdiscipline, practice makes perfect. So go forth and squeeze!
Michael Cooperis the owner of Michael Cooper Recording,located outside the beautiful resort town of Sisters at the base of theOregon Cascades.
Starting From Scratch
Here’s a quick guide for setting a compressor’sparameters. First, make sure the compressor is switched on and set tosoft-knee mode. If processing a mono track with a dual-channel unit,make sure the stereo link or “slave” switch is turned off.Also, disable or bypass any other special functions such astube-saturation circuitry, expansion, and so forth.
Next, set the compressor’s ratio to its minimum value, usually1:1, and the threshold to its highest value. Those settings render thecompressor inactive but still in the signal path.
Now, set up the compressor for unity gain throughput. Most unitshave hash marks—typically labeled 0 dB—screened around theinput and output control knobs. If your unit provides those referencemarks, set both knobs at 0 dB for unity gain. If no marks are provided,you’ll either need to call the manufacturer to find the unitygain for each knob or use a tone generator in conjunction with theunit’s input and output meters to determine unity settings. Ifthe compressor has no input meter, you’ll have to rely on themanufacturer’s word.
To determine unity with a tone generator (the one in your consolewill do), feed a 1 kHz tone to the compressor’s input and set theinput-control knob so the compressor’s input meter reads the samelevel as the tone generator’s output. Then switch thecompressor’s meters to show output levels and adjust thecompressor’s output control knob for the same reading. It’snot a bad idea to mark unity gain settings for future reference.
At this point, the compressor is set so that what goes in comes outunchanged in level. You’re now ready to make ballpark settingsfor processing the signal.
Set the attack and release time controls to an average value,usually close to the twelve o’clock position, and the ratio toroughly 2:1 or 3:1. Those mild settings reduce the risk that you willovercompress the signal. Switch the compressor’s meters to showgain reduction and lower the threshold until approximately 4 to 6 dB ofgain reduction is attained on peaks. It is most important here that thelowest signal levels do not exceed the threshold and trigger thecompressor. In other words, make sure the gain-reduction meters do notkick in during soft passages.
Once you’ve set the threshold, it’s time to startvarying the ratio, attack, and release time controls and beginlistening to the results. If you want more compression, increase theratio; if you want less, reduce it. Use fast attack and release timesfor compressing only the peaks. Use slow attack and release times tomake a signal sound more dense. Most importantly, let your ears be theguide.
After finding settings that provide the results you want, adjust theoutput control to make up the gain that was lost to gain reduction. Ofcourse, you can add more or less than that amount if youwish—just make sure you’re paying attention to proper gainstaging with regard to any downstream gear. That is, don’t boostthe compressor’s output if doing so requires you to lower theinput on the next device below its unity gain setting.
These general rules will get you started and prevent most processingmistakes. Once you have some experience, you can tweak settings formore extreme processing. Just remember: the rules are meant to bebroken!
Dialing in Hot Sounds
Ask any experienced engineer for suggestions on compressor settingsfor various instruments and you’re likely to be met with a blankstare. He or she is not trying to be evasive. The best settings dependon a host of variables: the type of compressor used, the unit’sdetector-circuitry response, the amount of peak versus average energyin the track you want to process, the dynamics of the performance, whatkind of envelope shape or sound, the outboard gear’s noise floor,and on and on. “Use your ears” is a tired phrase, but earsare still the best tools to contextually evaluate the sound of dynamicsprocessing.
Just the same, here are a few ballpark settings for getting greatsounds from some of the compressors mentioned in this article. Asalways, use your ears to make additional adjustments if the initialsound is not to your liking.
Joemeek C2 power-pop snare sound: Set the attack and releasetime knobs fully counterclockwise (CCW) and the compression knob fullyclockwise (CW). Boost the input almost to the point of clipping, aimingfor 16 dB of gain reduction on peaks.
Empirical Labs Distressor on electric bass guitar: Use the“Distortion 3” setting with a ratio of 6:1, attack time setto 2.2, and release set to 2. Tweak the input for 6 to 8 dB of gainreduction on peaks. Those settings preserve some of theinstrument’s attack, but tame it enough to allow a hotter levelin the mix for a fat-sounding bottom end.
Stereo-linked Aphex Expressors on a stereo rock mix: Here,optimal settings depend greatly on the mix’s spectral balance,the type of music, instrumentation, how much individual tracks werecompressed during the recording process, and more. To pump up thelevels and add thickness, set both units to soft-knee processing and nolow cut in the sidechain, and then dial in a 2:1 ratio, about 10 msattack time, and roughly 0.2-second release time. Set the threshold toachieve 4 to 6 dB of gain reduction and apply enough make-up gain toprint as hot as possible without clipping. If drum hits get softenedtoo much, you can increase (lengthen) the attack time or shorten therelease time.
Dress to Compress
Here is a quick reference guide for some of the compressorsmentioned in this article. Compressors that belong to a software bundleor recording device (the Alesis Masterlink, for example) areomitted.
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*out of production
The Squeeze Is On: Compressor Manufacturers