A bit bout compression
Since last Friday was Dynamic Range Day, I thought this week I would start talking about compression. This is a huge topic, so I think I will break it up into a few parts. Today I am just going to cover the basics.
A compressor is a tool that controls the dynamic range of an audio signal. Basically, this means that it makes the quiet elements of a sound louder, and/or the loud elements quieter. Imagine the sound of a bass guitar playing a single pluck of a string. The sound begins loud as the string is plucked, and gradually decays to silence. A compressor could be used to reduce the change in volume and keep most of the sound at a steady level.
The top wave in the image below is a heavily compressed bass guitar. The bottom wave is the original raw recording. You can see how the original recording has peaks and spikes, where the compressed bass is all one solid level.
Controls:
Input gain – Increases the signal going into the compressor
Threshold – This designates the level at which the compressor will start compressing. Sounds above this level will be reduced.
Ratio – This controls how much the signal is reduced once it gets above the threshold. With a ratio of 4:1, peaks that are 4db above the threshold will be reduced to 1db above the threshold.
Attack – This controls how quickly the compressor acts. With a slower attack, some of the transients are allowed through before the compressor acts. With a fast attack, the compressor immediately reacts to anything above the threshold.
Release – This controls how long it takes the compressor to ‘let go’ after the signal has dropped back below the threshold. You can kind of set this depending on the tempo of the song or the speed or length of an instruments sound. Faster songs generally work better with a faster release and slower songs with a slower release. Likewise, quick sounds like a drum hit work better with a fast release, long sounds like bass might be better with a slower release.
Output gain – This controls the level of the signal coming out of the compressor. Anytime you compress something, the signal is attenuated by however much is compressed, the output gain is used to make up for any loss in volume due to the compression. It’s good to try and match the average volume with the signal before compression so you can listen to the track with and without and make an accurate judgment about whether the compressor is helping or not.
If you are having trouble hearing the effects of the compressor, set the controls to extremes and work from there. For example, It is helpful to set the threshold really low and increase the input gain while setting the attack or release. It will cause a pumping effect and you can really hear what the compressor is doing making it easier to adjust release or attack.
Limiters:
A limiter is basically a compressor with a ratio of 20:1 or more and usually a fast attack. Technically, I used a limiter on the bass example above, to visually exaggerate the effect of a compressor.
Uses:
Compressors can be used for a variety of applications. Frequently compressed instruments include, but are certainly not limited to, drums, bass, vocals, horns, etc.
One use of a compressor is to smooth out a dynamic performance. Maybe you’ve recorded an acoustic guitar with some parts with overly heavy plucking, a compressor might be able to tame that.
Sometimes, particularly with outboard compressors(and other hardware), a signal is sent though without actually being compressed, just to get the character or ‘color’ of the compressor.
All compressors have a different sound when they compress (or as with hardware, even if they aren’t compressing). Some are clean and transparent; others can cause coloration, distortion, and other effects. It’s important to chose the right compressor for the job.
Compressor cautions:
Be careful when adding compression, too much can cause that ‘pumping’ effect, where you can actually hear the volume changing as the compressor reacts. In some cases this might be exactly what you want, but otherwise be careful not to over-compress. It usually sounds worse than no compression at all. Another thing to be aware of is that when a compressor compresses it is essentially increasing the noise floor as well, so if you have a noisy recording, compression can make it even noisier.
Sometime in the future I will get into some more detailed uses of compression and advanced functions such as sidechaining.