Today we’re going to write a few basic things about compressors and limiters.
First of all, a compressor is a device that reduces the signal volume by a ration, once the signal exceeds a certain threshold. A typical compressor will have the following settings: Ratio, threshold, input gain, output gain, soft knee or hard knee, attack and release.
The threshold determines the Db above which the compressor takes action. The ratio is depicted in fractions such as 2:1 or 4:1. This means that for every 1 Db the signal surpasses the threshold, the compressor reduces it by 2 or 4 db respectively.
This diagram shows what happens. There is a certain threshold level. The signal enters and once it reaches the threshold, the compressor starts working. Α 1:1 ratio means that the signal enters and exits unaltered. An infinity:1 ratio means that the compressor works as a limiter. It doesn’t allow the signal to pass the threshold, no matter how loud it is. We’ll get to this in a while.
A compressor also has a knee setting. This determines whether the compressor starts working in an abrupt or soft way. When the knee is hard, the compression is more apparent in the sound.
The attack of a compressor determines how fast a compressor starts working. When this timing is very fast, for example 10 msec, then the compressor starts working immediately and the whole signal is compressed. However, when we choose a greater attack setting, such as 50 msec, we let the transient pass. That is the first part of the signal that hold no harmonic elements, but makes the sound more "punchy". Release, determines how quickly the compressor goes off.
A few lines above we mentioned the term "limiter". As we said, a limiter is a compressor with a ratio equal to infinity:1, which means that it doesn’t allow anything to pass the threshold. A compressor is useful at the mastering stage. It is actually the very last thing in the mastering chain. After all the process has taken place, we pass the signal through a good brickwall limiter, in order to make the signal louder, while ensuring that nothing exceed the threshold and causes distortion.
PSP Xenon is a very good software limiter
Of course, this doesn’t mean that we can go as loud as we want without distortion. A good brickwall limiter can add character to the sound and go louder than another one, but, after a point, everything distorts. Limiters are an entire different topic altogether, but know that they shouldn’t be used in mixing. If you are mixing and then you are passing your signal through limiters, then expect the quality of your sound to quickly deteriorate.