How many times have you been sitting behind the front of house console and had someone ask you, "Do you know what all of those knobs do?" If the answer is "No" when you are talking about compressors, then you can get yourself into trouble.
Compressors are arguably one of the most misused, as well as overused, tools in the business. But when used correctly, they can be used to shape the timbre or dynamic range of any given source to give a more desirable sound for many different applications.
All sorts of problems can arise with improperly-set compressors. These include ruining gain structure on a system or channel strip and generating more feedback when the band stops playing than when they are performing.
In my opinion, most people know what sounds good and what sounds bad, but getting there tends to be confusing if you don't how to use the tool. This article is not referring to limiters or compressors for system protection necessarily, but referring more to the artistic side of things, such as instruments and the human voice, or making certain things stick out "in the mix." This month, we'll look at what those knobs do, and offer some tips on how to recognize when to use all these different controls, and when to avoid using them. Next month, we will go over some different types of compressors, such as multi-band and side-chains, as well as different applications for them.
What is a Compressor?
A compressor is a device that is used to reduce the dynamic range of a signal. While these tools were originally used mainly to limit the dynamic range for tape machines and paging systems (and still are), these days they tend to get used in more artistic applications as well. That is, shaping tone, timbre and making certain things "stick out in the mix." While some of the applications of use have evolved, the fundamentals behind these devices are the same as they were on day one.
Some older-style compressors are very simple and have minimal controls, such as input and output. Most modern-day compressors have controls such as threshold, ratio, attack, release and output gain. Having an understanding of all these controls, and knowing how to use them properly, makes getting what you want out of the device much easier.
Ratio and Threshold
The amount of signal affected between the input and output is known as the ratio. As a general rule of thumb, anything from 1:1 to 8:1 is considered a compressor. Anything higher than 8:1 is considered a limiter. A ratio will have two numbers. The last number will be a one. The first number is what defines the amount of signal affected by the compressor. The ratio and threshold work in conjunction with one another. Before we go any farther with ratios, let's take a look at what the threshold is.
The threshold is a point at which the input signal gets affected by the compressor (in other words, gain reduction). Imagine looking at a signal meter and having a bar perpendicular to it that moves up or down over top of it, depending on where you set it. Any signal that rises above that bar will be affected by the compressor.
Fig. 1 shows an example of how a signal is affected by a compressor. There are two examples here, both of which have the unaffected input signal on the left and the compressed output signal on the right. Fig. 1A shows a 2:1 ratio with a threshold at 0dB. What this ratio means is that for every two dB increase that the signal crosses above the threshold, one dB is allowed. If the input signal is +4dB, the compressor will reduce it by 2dB, resulting in a +2dB output signal. Fig. 1B shows what happens if there is a 4:1 ratio instead of a 2:1 ration with the same 0dB threshold. The +4db input signal will be reduced by the compressor to just +1dB in output. A higher ratio means more gain reduction and less dB output.
Attack and Release
Attack and release times can make a big difference in shaping the timbre of a sound source. For example, they can make the crack of a drum hit stick out more in the mix, or cut down the "pumping sensation" that is often less-than-desirable during gain reduction.
The attack refers to the amount of time it takes the compressor to "kick in" or reduce gain from the point at which the signal passes above the threshold. Generally speaking, compressors tend to have adjustable attack times anywhere from less than 1 millisecond all the way beyond 100 milliseconds.
If you have a snare drum that needs a little extra "cut," this can be especially useful. If the attack time is too long, the compressor will hardly do anything, because the note will be over before the compressor has time to kick in. If it is set too soon, no initial "crack" will be heard. But when set just right, even on a dull sounding snare, it can bring out the "crack" of the snare before even touching the EQ.
The release knob does exactly what you'd guess it would – it releases! Or, to be more technical about it, the release is the amount of time it takes for the gain reduction to go back to 0db after the input signal has gone below the threshold. Properly adjusting this can avoid the notorious "pumping" sound.
Some manufacturers also provide buttons such as "overeasy" (or "soft knee") and "auto." Overeasy is a "softer" form of compressing. Instead of the signal reducing right at the threshold point, it slightly starts reducing gain before the threshold point. Compressors with an "auto" function engaged have a set attack and release time. It's sort of a "happy medium" for attack and release times.
Compressors also tend to have an output gain adjustment as well. This will allow the engineer to "make up" for gain reduction, which can help maintain proper gain structure.
These are the basics of controls on compressors. Next month we will go over some different types such as side-chain and multiband compressors as well as some practical applications for them.
