What's a Compressor For?
I’m a compressor nerd. I know it. In my editing suite for video, in addition to a Mackie ONYX mixer, I have a really old DBX Compressor / Expander because it makes good audio better, and I would prefer to get the audio right when recording than to have to use a stack of plugins in the editor after the fact, even though there are some that are excellent.
When I talk to other players, Compressors rarely come up as a “must-have” pedal and there are still lots of misconceptions about them. Whether you use or even like compressors, is absolutely and entirely your own decision. Whether other people use them, or any other tool is mostly irrelevant. Like I said, I’m a compressor nerd and try as many as I can in the hope of finding the next great thing.
To understand this affliction, let’s take a look at what a compressor actually does, what it does not do, and why it can be so hard to differentiate amongst the many options.
A compressor compresses a signal. Sounds simple enough, but how it does this compression is where the magic happens. The idea is to a signal with a very wide dynamic range, the difference between the loudest and quietest parts and bring them closer together. Typically we are looking for the maximum dynamic range, but very low volume signals can get lost in the noise background and very loud signals could push things into distortion where you do not wan it to happen.
All compressors use an algorithm of some type, in hardware or software or some combination thereof to make the compression happen. Unfortunately learning to get the most out of a compressor takes time and the usual approach of turning everything to full will result in an overly compressed sound that for the most part, you are going to hate because you have effectively squeezed the life out of it.
Moreover different makers take different approaches too compression. The old Ross compressors have established a cult like following and some folks love them. Choice of any signal modifier is always subjective. I am more inclined to like a compressor that sounds like the compressors of old from recording studios that increased sustain but not at the expense of loss of highs, brittleness or the dreaded “stuffed in a box” sound. All compressors of any real use will have configurable parameters. Those with only one knob may work for a particular guitar on a particular tune into a particular amp. I’ve tried a ton and have yet to find one that is really usable. By examining some of the parameters, you can make informed decisions on what would serve you best.
The Threshold defines at what point the compression effect is engaged. This level is actually measured in decibels and the compressor only kicks in when the threshold is achieved. Most guitar pedal compressors don’t express this in dB, but many have a threshold knob. They probably don’t call it threshold though, which is annoying.
The next parameter, which you will likely never find in an effects pedal is the Knee. This is common in studio compressors and it defines how the compressor transitions between when compression is not applied and when it is applied. Simplistically a soft knee makes for a smoother transition than a hard knee. A listener will hear a hard knee, but may not notice the soft knee.
The Attack parameter defines how long the signal takes to achieve the maximum set compression. Attack is typically a knob but the attack duration is actually measure in microseconds. The faster the attack, the faster the compression effect kicks in. If you have ever heard a compressor sound like it’s clicking when you pick a note, the attack is probably set too short, although in some units this happens when the knob is turned up. The TC Electronic Hypergravity compressor gets clicky quickly, one of the reasons that I do not care for it.
The Release time defines how long it takes before the compression effect releases the signal to an uncompressed state. Some folks find a longer release time increases the perception of sustain, although the normal practice is to use as short a release as possible without the compressor producing a pumping sound heard first in the bass frequencies. Like Threshold and Attack, the Release settings are going to very instrument specific with different pickups needing different settings for similar results.
Studio Compressors offer what is called the Compression Ratio, and some pedal compressors offer these as selections such as 2:1 or 4:1. The higher the ratio the more aggressive the compression effect. A 3:1 ratio would be medium compression, reducing the db above threshold by ⅔. 20:1 is very squishy compression and acts like a hard limiter. The way to find the ratio that works in your scenario is to experiment, but be aware that your compressor may not have the option to set this or use some idiotic naming to refer to this.
Compression lowers output because it is performing signal attenuation. Most pedal compressors have a gain control to allow you to boost the output to match the level of an uncompressed signal without any compressor in the chain at all. This gain control is not meant to be used as a boost function although some players use it this way. Personal choice on your part.
There are basically four types of Compressor. The old style tube compressors are slower to respond and tend to colour the sound. Some folks, like myself, love them. Others find them too warm. Personal choice. It is not likely to find a tube compressor on a pedal board or in a gig rack. Optical compressors use a light element and an optical cell to control the compression. Compressors like the Diamond and Diamond Jr, use optical elements. I like this type of compressor for guitar the most. The FET or Field Effect Transistor compressor is the sound that most players know. It uses transistors to act like tubes. The Universal Audio 1176 is the poster child for this kind of thing, and is on thousands of recordings. Jimmy Page used them when producing the Led Zeppelin records for example. VCA based compressors vary enormously in sound, from smooth and subtle to a very hot-rodded compression as found in the DBX 160 compressor used by tons of folks.
When using Compression, a little goes a long way. Professional recordists will often use different compressors at different places in the recording chain, rather than hitting the signal with a big compressor at the end. For a playing guitarist, this is less practical so a pedal or rack based compressor makes the most sense. Finding a compressor that suits your needs means trying them out in your working space, and this means having a good relationship with your seller so you can either evaluate before buying or buy and return in a simple turnaround process. If you are recording in a DAW, look to multi band compressor plugins that allow different compression settings for different frequency ranges, but that’s a different discussion.
I own and use both Diamond Compressors. I have auditioned a bunch of other units and returned all of them, even ones with great reviews in magazines and on websites. Partly this is because compression is subjective and partly because some so called compressors don’t do a good job at all. I have recently found the Empress lineup and have bought and kept their compressor. It’s about the same money as a Diamond Jr. but offers more control for the real compressor dork. I like the Empress a lot, and am comfortable recommending it to the player willing to spend time to learn to use it. A product specific review will appear on this site at some point.