What does a compressor pedal do and what does each control do? (output, attack level, sensitivity)

To use a compressor properly, you need to know what it's doing.

Most guitar players will answer the question this way: "A compressor turns up the gain on a note as it decays, making it sustain longer." That's kind of like answering the question "How does that magician do that?" by saying, "He makes a rabbit appear in the hat and pulls it out." You are describing the effect, but not how it gets done.

What a compressor really does is this: it REDUCES the gain of an incoming signal by a certain percentage IF the signal is greater than a certain threshold.

I'll explain this, but keep in mind that most guitar compressors use alternate terms in place of these standard terms (a big reason why guitar players have been widely misinformed about these devices).

Let's say you have your guitar plugged into a compressor, and it is set at a threshold of -18db, and at a ratio of 4:1. So, what the compressor will do is this: if the signal from your guitar (input) comes into the unit at a level higher than -18db, then the unit will reduce the gain of the signal going to the amp (output) by a factor of 4, that is, to 25%. It only reduces the amount of gain above -18db by this factor.... hence, compressing the signal.

So, if your signal comes it at -14db, the compressor will reduce the 4db above the threshold at a ratio of 4:1, and the output will be reduced to -17 db. (4db above threshold / 4:1 = 1db above threshold at output.)

If you're with me so far, you're now ready to understand how a compressor does it's "sustain" magic.

All compressors also have an overall output gain knob. Because you are reducing the output gain of the highest peaks in the signal, you can increase the overall output gain from the compressor. This means that the entire signal below the threshold is louder... and, as the sginal decays below the threshold and the compressor releases the signal, you can hear it longer, creating the illusion that the compressor is "turning up the volume" as the signal decays.

A studio compressor has controls for threshold, ratio, output gain, attack time (how fast the compressor reacts to the input) and release time (how fast the compressor "lets go" of the signal as it approaches threshold. But in a guitar compressor, most of these values are pre-set in the circuit (for example, attack and release are almost always pre-set.. a two-knob Dyna-Comp, for example gives you control over threshold and output gain, and that's it).

IN your example, I would guess that "attack level" is controlling attack speed (with release pre-set), and "sensitivity" means threshold. In this case, the ratio of compression would be pre-set. The other possibility is that attack and release are both pre-set, and what the manufacturer means by "attack level" is actually ratio.

Even though every maker of a stomp-box compressor seems to label the device in their own way, you will get much more from your compressor if you understand how it really works. Some Web searching may reveal the specific values being controlled by the knobs on your compressor.

A great post SlapChop! Thanks for breaking it down in layman's terms.

Wow, really informative, thanks!

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My Baby, 2008 Standard American Jazz Bass

This is truly, an outstanding explanation!!

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You dig?

uhh... yeah... i knew that ;)

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What are some common words that take the place of "threshold" and "ratio" on a compression pedal?

Thanks for the kind words, ya'll. Over the years I've explained how a compressor works os many times... like I say, guitar players always imagine that it's doing something it isn't doing, which is why so often they get a compressor and end up saying, "Man, compression sucks! Why does anybody use one?" Or, more often, buying studio gear and end up scratchiong their heads over their first rack mount compressor.

guitarmanjt, like I say, it's all over the map. Not many pedals give you control over threshold (AFAIK), but those that do might call it "Level" or "Attack" (because raising the threshold allows for more of the normal pick attack to come through). Pedals that give you ration control (which seems to be more common) usually just call it "Compression."

now, I'm not intimately familiar with every pedal on the market, so YMMV. One I don't know about is the venerable Dyna-Comp which has two controls: "Output" and "Sensitivity." Does that mean threshold or ratio? You got me. If I had to guess without looking it up, I'd say threshold.

Finally, there's a control you might see on a couple of boutique compressors - like the Barber Tone Press (I finally caved and bought one and it's schweeeet) and the Wampler Ego - that you won't see on a studio compressor. It's a "Blend" control," and what it does is allow you to choose a phase-corrected blend of the original, unaffected signal and the compressed signal. When this is properly implemented as it is in the Tone Press, you can retain virtually all of the guitar's natural pick attack and still dial in long, ultra-compressed sustain.

Is a compressor generally supposed to add gain when using it for distorted tones? (classic-rock levels of distortion)

guitarmanjt, a compressor shouldn't be distorting on it's own... that is, you shouldn't be using it as a distortion pedal, if that's what you mean. You can use it like a boost to overdrive pre-amp stages, I guess, but there are better choices for that.

Also, overdriven guitar tones are naturally compressed. The sound that everybody loves from a tube amp is the sound of tubes pushed to the point of compressing the incoming signal all by themselves. I'm not saying you can't use a compressor with distortion, but there's not a lot of point to it, especially really high gain tones.

My compressor works best with clean tones and with that softly clipping lead tone, the one that's just bubbling under the edge of break-up... it's not clean, but it's not really overdriven... that Larry Carlton / Robben Ford kind of thing.

BigJay, I think I can sorta do that:

Gain is an increase of electrical energy in a circuit created by an amplifier stage. So gain and volume are related, of course. But the difference is: gain can be added or subtracted at many points along the signal chain... between the guitar and the preamp, in the preamp but ahead of the power amp, in the power amp itself. The amount of gain generated in each stage affects how the next stage is going to sound. But it's not "volume" until it gets out into the air. The loudness of the entire signal chain - the actual sound in the room - is volume. WHen recording, the goal is usually to keep gain even and non-distorting at each stage of the chain: the mic, the preamp, the channel fader, the outboard gear, the channel output. Controlling the gain at each stage is called, logically enough, "gain staging." So when you hear someone tell you to pay attention to the gain staging, that's what they mean.

So you can add crazy amounts of gain between the guitar and the preamp with an overdrive pedal and drive the pre-amp into a fizzy frenzy, but still control the volume of the amp with the knob that controls the output of the power amp stage.

A "presence" control is usually an additive EQ, boosting a narrow band of frequencies around a given notch in the upper mids (the mid-point frequency is probably different for each brand of amp). Boosting upper mids improves the intelligibility of the human voice and accents the attack of guitars, basses, etc. Remembering that 2K is the "fingernails on a blackboard" frequency, adding a little of these upper mids can improve that mysterious quality guitar players like to call "cut."

In the studio, cutting muddy frequencies in the low-mids is usually preferable to boosting upper mids.

Yep, you are. A presence control is a pre-set additive EQ.

Is there such a thing as a "presence" stomp box? I thought it was always something built into an amp. But yeah, if you were considering doing something to add a presence control to a rig, I'd just buy a good EQ, which would let you cut mud frequencies and increase overall amp volume.... a better-sounding way to get to the desired goal.

A slight cut at 300hz or so and turn up the amp a bit... clean, louder, real present. You could still boost a bit at 4K or so....