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Types of Compressors

September 25, 2013

Compressor sound wavesHi folks.  Welcome back to The Corner.  This week we’re going to discuss types of compression.  Compressors are an oft misunderstood and misused effect.  Hopefully this article will enlighten those that aren’t quite sure what a compressor is.  We’re going to start off with describing what compression is, some basic terminology, and artifacts of compression.  Then we’ll go through the major types of compression.   

What is a Compressor?

The most basic explanation of an audio compressor is a specialized amplifier that reduces the gap between the loudest part (peak) and quietest part of a signal.  The peak-to-average proportion of an audio signal (difference in volume between the peak level and average level) is called the dynamic range.  A compressor reduces dynamic range.  For example, the dynamic range of a guitar is roughly 20db.  A compressor has the ability to reduce the difference in order for the quiet notes to be louder and the peak notes to be quieter, in effect producing a more even sounding signal.  This can also be boosted in order to raise the average loudness of the signal.  This is common in broadcast; the dynamics of a recording are reduced in order to raise the average loudness.  This helps compete with the assumed noisy environment associated with driving a car. 

Now that we have a basic definition of a compressor, let’s talk about some common terminology associated with compressors.

Ratio:  The ratio controls the amount of dynamic range reduction the compressor is initiating.  For example, a ratio of 4:1 means that it takes an increase of 4db to the input signal in order to raise the output of the compressor by 1db.  4:1, 8:1, 12:1, and 20:1 are common, preset ratios with anything over 8:1 considered by most to be the line at which a compressor becomes a limiter.

Threshold:  The threshold is the level of signal required to “open” the compressor.  In other words, you would set the threshold to a certain db level then only the parts of the incoming signal that crossed that threshold would be affected.  Any part of the signal below that threshold would not be affected by the compressor but any signal above it would be “compressed” down to the chosen ratio. 

Knee:  The knee is the exact moment the compressor begins reducing gain.  “Hard Knee” is a sudden shift from unaffected to compressed, there is no transition just instant compression the moment the threshold is crossed.  “Soft Knee” broadens the range of the threshold and provides a less noticeable shift into compression. 

Attack Time:  This refers to the amount of time that elapses between the signal crossing the threshold and the compressor kicking in.  Measured in milliseconds, typical attack times range from 1ms to over 100ms.  This has an effect on the overall perceived brightness of the tone.  The front end transient information conveys brightness character.  Using fast attack times immediately reduces the transient and can result in a duller sound, especially with percussive sounds.

Release Time:  This is the amount of time the compressor uses to return to unity gain once the signal falls below the threshold.  Typical release times range from 5ms to over 5 seconds.  The release time can have a pretty major effect on the sound.  Very short release times can distort low frequencies.  Very long release times can result in “pumping”, a sudden gain reduction that is usually unwanted or “breathing”, a slow return to unity level with a noticeable increase in the noise floor. 

Compression and Sustain

Compressors are commonly used in the guitar world as sustainers.  There is a common misconception that sustain is what a compressor was intended for but the truth is, sustain is an aftereffect of compression.  The job of a compressor is the keep the output level more-or-less the same even while the input level is decreasing.  This is what causes the compressor to increase sustain. 

One note I would like to make about compressors in general before we talk about types.  A compressor reduces the gap between the softest note and the loudest note.  It does this by reducing the output level of the loudest notes.  When we adjust the volume of an amplifier, we do so with the loudest notes in mind.  When those loudest notes are compressed down to a level closer to average, some sort of gain make-up is required in order to retain the original level.  This is usually done with the Output (volume) control of the compressor.  Now, we just reduced the loudest notes in volume which means the average volume being output to the amplifier is going to seem like the quietest notes are being amplified.  This includes the noise floor.  This means that any sort of noise in the signal is ultimately going to sound amplified at the output of the compressor.  It isn’t actually amplified but since the levels are brought closer together and the quiet notes aren’t as far away from the loud notes, the background noise at the lowest levels will also be closer to the loudest notes, therefore standing out more.  This is not completely understood by people that have little or no experience with compressors and often times this natural artifact of the compressor is mistaken for the compressor being “noisy”.  If a compressor sounds noisy, often times it is the result of noise induced somewhere in the signal chain that is unnoticeable until the signal is compressed.  In my experience this is usually caused by an overdrive or fuzz pedal, bad guitar wiring, or bad patch cable.

Types of Compressors

There are five major types of compression that are based on the electrical circuitry used to attain gain reduction. Ross Compressor

Optical

These use an optical isolator circuit made up of a light bulb or LED and a photocell.  The light source glows brighter or dimmer depending on the input level.  The photocell reads the varying brightness of the light source and changes gain accordingly.  The response time of the optical compressor is slower than other types but I’ve found them to be very natural sounding with a smooth attack and release that can be hardly noticeable unless they are pushed hard.  The Diamond Compressor and the EHX White Finger are two examples of optical compressors.

VCA

Probably the most common type of compressor is the VCA (Voltage Controlled Amplifier).  These are super versatile and can respond to the toughest, most specific conditions needed.  A VCA compressor uses an integrated circuit (IC) to give precise control over gain reduction.  These usually are less colored and have very little distortion making them one of the most popular compressor types on the market.  I believe the Ross and Dyna-Comp compressors belong to this category (if not, please correct us on this one).

FET

These use Field Effect Transistors to vary gain.  FET compressors use transistors to emulate tubes with more reliability.  They tend to have a unique sound that is fast and clean.  There really aren’t many of them out there due to the extra circuitry needed.     

Valve

Valve compressors are sort of a misnomer in that they do not use a tube for compression.  Most valve compressors are based on one of the above circuits with a tube or tubes in the signal path for a warmer, creamier tone.  The Electro-Harmonix Black Finger is an optical-based valve compressor.

Multi-Band

While not normally associated with pedal-based compressors, the multi-band is nevertheless worth mentioning.  A multi-band compressor is exactly what it sounds like, a compressor that works differently on different frequency bands.  This allows one to dial in more or less compression on the lows than the highs or vice versa, by frequency.  The advantage is that compression artifacts on certain frequencies can be dialed out by reducing or increasing the amount of compression in that frequency band.  They work by splitting the input through several filters.  Each signal is then fed through its own compressor then recombined at the end.  These will mostly be found in mastering suites and DAW plug-ins.

 

Wampler Ego Compressor

 

Parallel

Parallel compression has become more popular in the guitar effects industry over the last few years.  Parallel compression involves the dry signal being run in parallel with the compressed signal.  The two signals are then blended back together via a Blend knob.  This allows the user to dial in the amount of compression wanted and then balance it with the dry signal eliminating unwanted artifacts from the compression.  A great application of this is when a guitarist wants to increase sustain.  Since the amount of compression needed to get large increases in sustain is pretty large in itself, the resulting artifact can be “squashed” attack where the initial attack of the note is noticeably lower in level than the sustained note.  By blending in the dry signal, the initial attack can be retained with all the sweet, singing sustain coming in behind it and blending perfectly.  The Barber Tone Press is a very popular parallel compressor.

So there you have it.  Compression in a nutshell.  For those of you who were in the dark, we hope this article sheds some light on it.  Compression isn’t tonal “fairy dust” and for guitar related applications it can be downright tonally degrading if not used properly.  When used properly, compression can add nuances that are more felt than heard.  One basic rule of thumb with compression that a lot of studio guys use is this…If you can hear it, you’re using too much.  Thanks for reading folks.  We’ll see you next time, in The Corner.

 

Comments

  1. kidmag says:

    I guess understanding what a compressor does is the first step in learning to use one.  But I’d hardly call compression a “vital” pedal.  I’ve yet to find compression useful for my styles of music, now a sustain pedal… that is useful!

    posted on September 25, 2013 at 6:37 am
  2. Xopher says:

    I never thought compression was “vital” either, until I got a Wamper Ego. Now it is vital, and almost always on.

    posted on September 25, 2013 at 6:52 am
  3. Apostolis Hadoulis says:

    “vital” pedal would surely be true with bass players like myself. While compression is not essential, it can smooth out and add punch to your tone and control the very wild frequencies of a bass guitar. I only add a small amount of compression to my sound, but truly value what it does add to my tone. Actually I would love to see more Bass-centric pedals and gear information. I know there are not as many of us bass players out there, but we sure do exisit.

    posted on September 25, 2013 at 6:52 am
  4. Jay T. says:

    I find a lot of people who confuse audio signal compression with digital file compression. Very different animals. Good explanation, PGS.

    posted on September 25, 2013 at 7:17 am
  5. Jonas says:

    I believe a lot of guitarists not using a compressor may not see how that can be a vital effect. But once they’ve tried one, in most cases there’s no going back. Running an amp clean, or wanting something to ‘work against’ a compressor is irreplacable.

    It can often be used as a sustaining effect and while all increase sustain to some degree, some were not designed with that purpose in mind.

    I’m looking into a compressor right this minute, so could this possibly be a sign of sorts? I still have a challange to fully grasp what the knobs really do, but the clean tone of my rig is asking for some level of compression as it lacks finesse and some level of uniformity, being so sensitive to the touch.

    posted on September 25, 2013 at 7:32 am
  6. micaiah flores says:

    for someone that wants a very controlled clean tone like myself (even out of a 13 excelsior ran at 9_11 o clock) is a crucial pedal in my chain. to run my pedals clear ans well as my amp more so. its always on, and i love the color my block logo dyna comp gives off and adds to my tone

    posted on September 25, 2013 at 7:38 am
  7. Rick says:

    RACK- I use an old DBX266. what i find nice is “Threshold” has 3 lights, Below/OVEREASY/Above. when you can get that yellow light Over easy to light or in the Zone, that thing sounds good !.A pedal would be alot simpler to me..but its fun..I like the break down of terms in the artical.& to be honest comps still got me confused….

    posted on September 25, 2013 at 8:12 am
  8. Michael says:

    I am 58 now,been playin since I was 12. After many years and boxes and boxes of pedals,the search ended for me. I am very content now. First on my board, the jangle box. Love it.

    posted on September 25, 2013 at 8:17 am
  9. Jim A says:

    I was pretty much first on line for the original Orange Squeezer.
    For many years I was hooked on the Boss CS-1
    I have a DBX MC6 (overlooked gem) and several 163X’s.
    I am now using a Belcat cmp-509.
    It is quite nice.
    Compression is a must have for me.

    posted on September 25, 2013 at 8:54 am
  10. John Hunter says:

    Yes, I do own a compressor. But normally I run my amps and pedal board is compressor free. This post, however, causes me to think that maybe, just maybe, there’s something new that I can apply, learn, or even add to my signal chain. So thanks for the tip, and have a great day.

    posted on September 25, 2013 at 8:56 am
  11. will says:

    Great explanation of of the Comp box. But why not a ‘5 most overrated comp boxes” list to fuel the brand fan-boys ire? Or hopefully, this is a trend towards technical themes again. Go with the technical stuff and leave the “fan-boys wetting their panties topics” to the forums that deserve it.

    posted on September 25, 2013 at 9:00 am
  12. Abbacus says:

    Ross for rockin’ and Orange Squeeze for pretty.

    posted on September 25, 2013 at 9:51 am
  13. travis says:

    How about some ideas for practical applications of compression, beyond sustain? That would have been helpful. In what situations is it most useful? What situations is it not suited for?

    posted on September 25, 2013 at 10:43 am
  14. Zampuuji says:

    I regularly play a 12 string Rickenbacker in our sets. I use a Janglbox compressor for the 12,and a Philosopers Tone for a Strat. I always have one or the other on.
    If you play a Rick 12 the JangleBox will give you the exact McGuinn 12 string tone.

    posted on September 25, 2013 at 11:29 am
  15. Atomic says:

      I like my Empress compressor.  Yeah, never thought of a compressor as “vital” until I used one.

    posted on September 25, 2013 at 12:19 pm
  16. Bjorn says:

    I found that most compressors on the market squash my tone way too much and it took me a long while to find one that I can have on all the time without it removing all dynamics in my playing.
    The Cali76 is a huge beast but it sounds amazing and I find that my guitar now sounds more like it does on record (where the signal from the mic is almost always compressed) I also have to mention the Joe Meek FloorQ as a slightly smaller, slightly cheaper option.

    posted on September 25, 2013 at 2:15 pm
  17. JP says:

    The best analog yet transparent compressor, imho : http://www.effectsdatabase.com/model/guyatone/box/ps103

    Once it has been moded true bypass, it’s perfect

    JP

    posted on September 25, 2013 at 5:26 pm
  18. Richie says:

    Good explanation to an often misunderstood gadget. I used my Boss Compressor Sustainer for years as a kind of booster for solos until I read somewhere what they are really for and set the controls for the heart of the sound. I now have it on all the time and it really enriches my whole sound.  For lead sounds I use a Boss Distortion. Boss rules, yo.

    posted on September 25, 2013 at 5:36 pm
  19. MoJoToJo says:

    I find compressors a very useful pedal,especially with a Strat :)

    posted on September 25, 2013 at 5:42 pm
  20. chris says:

    Sob… I left my Keely comp in taxi.

    posted on September 25, 2013 at 6:28 pm
  21. Marcus says:

    In my experience,
    Analogman Bi-Comp here, with the Ross side used more for creating layered / sustained sounds and the Orange Squeezer used for punchy, snappy rhythm work.

    posted on September 25, 2013 at 6:51 pm
  22. Scott says:

    I’ve always heard that if you use fairly high gain, a compressor is a useless effect. Is this true? I’ve tried the comp models on my Line 6 M9, but have never really liked the sound of any of them. Am I missing something?

    posted on September 25, 2013 at 10:10 pm
  23. Ron says:

    Of course, a compressor is a “vital” pedal if you are selling pedals!  Just kidding.  Good article especially for guys like me trying to understand different effects.

    posted on September 25, 2013 at 11:35 pm
  24. Jon Patton says:

    Hoo boy.

    Re: VCA compressors.

    Almost all pedal compressors are VCA. This just means that the pedal uses a control voltage to affect a variable resistance element. This is done in almost all situations by rectifying the guitar signal with an envelope circuit, which converts the audio signal to direct current.

    In the Ross compressors, the control voltage affects a transistor, which in turn affects the bias resistance on an OTA chip. In the orange squeezer and other “FET compressors” the control voltage affects a FET (a type of transistor) instead (in the Orange Squeezer, it changes the resistance to ground and forms one leg of a voltage divider, lowering the input signal volume; in the Rothwell Lovesqueeze, it affects the bias of an op amp and drives down the gain). In optical compressors, the control voltage lights up an LED, the light from which reduces the resistance on a photocell.

    This is rarely what people are referring to when they mention a VCA compressor, however; those are usually THAT chip-based compressors, and are often rack units. They aren’t the best rack units, but the rack units are usually more involved than anything you find in a typical pedal. The humble and common Boss CS-3, however, is a VCA comp in pedal form.

    You seem to be confusing “OTA” with “VCA” when you say that you think most Ross Comps are in this category. OTA compressors, which form the bulk of boutique compressors, are, to be charitable, much more limited, much less sophisticated, and certainly are nothing like your description of being “uncolored” and “very little distortion.” OTA chips have awful headroom and noise problems. And most OTA-based compressors continue to use the 40-year-old design that sticks the noisy, low-headroom chip right in the audio path as the primary amplifier even though there are other ways to take advantage of it. When people say OTA compressors are “noisy,” they aren’t just talking about the pedal bringing up the noise floor. The chip itself produces noise, which it amplifies and which is very prominent when it’s idling at max gain.

    Also, and I say this as a long-time reader of the corner and with the best intentions, very little research would have been necessary to learn what a VCA compressor was, nor did you need to guess about what type of compressor a Ross is. There are plenty of electronics forums where you could have solicited the input of people who are very knowledgeable on the subject; you guys run an effects store and you could have simply called up a good pedal builder; or you could have used the vast resources of the internet and perhaps stumbled across www.ovnilab.com, which provides reviews of nearly every compressor on the market and has a fantastic FAQ. At the VERY least, Wikipedia exists: http://en.wikipedia.org/wiki/Variable-gain_amplifier. You don’t have to be an electrical engineer (I’m certainly not one) to do research on this stuff.

    Re: FET comps

    You absolutely do not need much extra circuitry for a “fet compressor.” I really don’t know where that idea comes. A FET forms a variable resistor between two pins when voltage is applied at the third pin. The circuitry for a FET comp can be as simple as you like—the orange squeezer and Lovesqueeze, which I’ve already mentioned, are two of the lowest parts count compressors out there. There are *also* some famous compressors that use FETs that happen to also be rack units, like the 1176. But guess what? If you look at the schematic, the extra stuff has nothing to do with using a FET for the variable resistance element, and almost all the amplification devices in them are regular bipolar junction transistors!

    posted on September 26, 2013 at 12:36 am
  25. David Fellows says:

    If you play a strat type guitar you might believe that a compressor is a “vital” pedal I use to use mine as kind of a nice clean boost but I mostly just play my sg and my pedal board is just collecting dust. with the exception of my MXR script 90 and carbon copy for my acoustic I’ve become an amp player so I don’t believe any pedal is “vital” or awesome.That said I have about 15-20 of them but most are in draws somewhere.

    posted on September 26, 2013 at 1:46 am
  26. Clint says:

    Using a compressor really has a lot to do with your amp settings, too.  Someone who runs an amp cranked (especially a tube rectified one) will already have so much tube compression that a compressor pedal on the front end may not even be noticeable.  However, cleaner amps that have a little room to be pushed can really be sweetened up by using a compressor, while maintaining the clean sound.  Vital part of my rig.  Especially with single coils.

    posted on September 26, 2013 at 2:00 am
  27. JP says:

    I usually use a compressor to give more presence to my Strat “crunchy sound” for lead parts.

    A real booster pedal wouldn’t do the job cause it would transform the crunch into some kind of overdrive, and I want the boosted sound to remain clean-crunchy while making it a little bit warmer with a little more sustain and increased volume.

    Many compressors I tried are not suitable for this kind of use ( ie : the MXR kills basses). I stick to this old Guyatone Drivin’ box, which has clarity, headroom, and no bass lost. But there may be others. Probably the “parallel” ones would make a great job imho (btw : if the Guyatone wasn’t a collector , I would mod it with a 500k blend pot to try the parallel thing :) )

    posted on September 26, 2013 at 2:36 am
  28. Laurens says:

    The need of a compressor is defined by the amp. If you use very sensitive amps, you will find a little bit of compression smooths the stuff out you can’t do yourself just by playing great.

    Especially the breaking up sound of an amp, with a single coil pickup tends to lose sustain. The crunchy ‘pushed preamp’ tone I use cuts through the mix, where the cleans tend to get lost. A comp smooths out that sweet spot.

    If you already use a heavily compressed lead tone on your amp with humbuckers, a comp would be overkill. But for a saturated sensitive tube tone you really love, a compressor just makes the difference.

    Everybody buy an Ego-comp at PGS!            ——> do I get a free one now?

    posted on September 26, 2013 at 5:06 am
  29. MarcAustin says:

    I want a germanium Philosopher’s Tone

    posted on September 27, 2013 at 6:12 am
  30. thad cornett says:

    Thanks…it’s about time someone explained what a compressor really is.I have talked with guitar tech’s for touring musicians that never get it right.I have never used a compressor in my effects chain but after knowing what they are really capable of i just may consider it.Thanks Andy,you are a great help and awesome player.

    posted on September 27, 2013 at 10:44 am
  31. Richie says:

    Does anybody have the faintest idea what the hell John Patton is on about?

    posted on October 2, 2013 at 5:31 pm
  32. Stefaun says:

      I’m a tube line driver and compressor fanatic !!!!!!  Can’t live without them !!!!!  Always on !!!!

    posted on October 4, 2013 at 2:44 am
  33. MarcAustin says:

    ^^Stefaun - pray tell what is this ‘tube line driver’ you speak of ?? Links for more info Tks for your time

    posted on October 4, 2013 at 2:26 pm
  34. Stefaun says:

    To MarcAustin :  I have different types of line drivers to choose of before the compressor of choice. One type is on my instrument strap.  I use Analysis Plus Studio Oval and Bass Oval instrument cables only, cut to the shortest correct length needed for the purpose. The tube line driver here in front of me is a Fryette Valvulator. It can use any 9-pin regular preamp tube in it. I use JJ ECC83S tubes in it for bass, and JJ ECC83S gold pin tubes for guitar. The first tube has a very round lower end sound for bass, and the second tube has a little less round lower, and better high frequencies. You can also use a 5751 tube to slightly lower your instrument gain. This unit has two outputs so you can send to to two places. Once you use this, and a tube or other compressor, you will never be satisfied with your sound without it !!!!!  I have three of these Fryettes. Use G&H 1/4 ” plugs on the ends of the Anal.Plus cords that I shorten. If you have any more questions, please ask !!  Have fun with your tone !!!!

    posted on October 4, 2013 at 8:55 pm
  35. Bill says:

    Boss CS1 Was great untill I put a Fromel Mod in it, Now my tone has changed so much for the better that I will not do without it. Tone is so much fatter that without it any amp I have tried seems thin.

    posted on October 5, 2013 at 2:48 pm
  36. matt says:

    let me have a diamond compresser for free

    posted on October 10, 2013 at 8:06 am
  37. Kevin Maxfield says:

    Compression is an effect that I’ve only really come to use very recently.  I’ve had a Boss CS-3 for the better part of two decades but used it only occasionally for sustain.  Now I’ve come to use a Dyna Comp fairly regularly on more vintage style “singing” leads and a Guyatone “Mighty Micro” compressor for things like evening out arpeggios or giving clean rhythm playing a smooth, sparkly sheen.  I think I’m still in the process of getting to know this effect, but I’m liking what it does…as long as it stays subtle!

    posted on October 15, 2013 at 5:31 pm
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