Probably one of the more difficult processors to understand when you are a newbie to electronic music creation is the compressor. I have had many friends new to electronic music asking me what this processor does, as well as I have had to explain this to many an executive when trying to get my equipment budgets approved, so I thought I would post an overview here for anyone interested.
Compression was originally created to control the range of dynamics of a sound. Think of it as an automatic volume control. Although, compression has many applications aside from recording, a perfect example of how compression is used and what it does, is easily illustrated in a typical vocal recording scenario. When you record vocals you want to get the signal to noise ratio as high as you can. Meaning that you want to have a loud voice signal and a low noise signal on your recording. However, vocalists can be very dynamic and if you set your recording levels to be perfect when they are singing loud your signal to noise ratio will drop when they sing a quiet part. If you set your levels to be perfect when the vocalist sings quietly, then you will distort your input when they sing louder. To solve this problem you need to reduce the overall dynamics of the incoming signal. Back in pre-historic times the engineer would ride the faders to keep a signal's volume under control. Nowadays, fortunately for all of us, you can apply a compressor to this problem that will automatically adjust the volume to fit with the levels that you specify. So in a nutshell, compression controls the volume of a sound, keeping it within levels that the user sets.
Controls on a Typical Compressor
On most of the software compressors there are pre-sets that you can use for the most standard compressor applications. However, like anything its better to know what the controls on the compressor actually do. This will give you the power to make intelligent tweaks to get the compressor to do exactly what you want it to. Don't be beholden to presets!
Threshold - This is the signal level at which the compressor will start reducing the incoming signal. This is usually measured in dB (decibel). Typically you want to set this so the signal level is just below the threshold. Notice in the picture below that the peak incoming signal is -8.1 dB. You can see that I've set the threshold to -10.0 db. This will reduce the signal of anything that is louder than -10 dB.
Ratio - This setting refers to the amount of gain reduction that will take place on the signal if it surpasses the threshold. For example if the ratio is 5:1, then a signal that exceeds the threshold by 5dB would be reduced so you would only see a 1dB increase instead of a 5dB increase at the output.
Attack & Release - On the most simple level, the attack refers to how long it takes the compressor to pull the volume down on the signal once it reaches the threshold and the release refers to how long it takes the compressor to returns the signal to its original level.
Soft & Hard Knee Compression - Hard knee refers to compression that happens abruptly where soft knee compression is more gradual. As you can see in the graph below, hard knee compression squashes the signal as soon as it reaches the threshold.
Output or Make Up Gain - This setting specifies how much the over all signal should be boosted after compression. Some compressors have a setting to do this for you automatically.
Additional Features Some Compressors May Have
Side Chaining - Some hardware and software compressors allow for side chaining. This means that there is a separate input aside from the original audio input. This input can be used to introduce an audio signal that can act as an envelope for the compressor acting on the signal coming in through the normal inputs. A good easy example of this is when you hear an announcer on the radio talking over music. When the announcer starts talking you can hear the music get softer so you can hear the announcer more clearly. This is done by having the signal of the announcer informing the compressor when to attenuate the music. When the announcer stops talking, the absence of the signal returns the compressor back to its original level bringing the music back up in volume.
This setting is typically found on compressors that have a side chain feature. This allows you to have more control on the compressor's release value when side chaining. For example, using our announcer scenario up above, if the announcer took a long enough pause while speaking the compressor would release, bringing the volume of the music back up momentarily until the announcer started speaking again. This would sound unexpected and sloppy. To fix this you would set the Hold control to 40 to 60 ms after it detected silence from the announcer before it would return to the original volume level allowing for pauses.
So there ya have the very basics of the compressor. There are obviously more nuances and things to know to be well schooled in the sorcery that is compression. Stay tuned for some more advanced compression topics in the weeks to come :)
Free VST Compressors to get your feet wet with: (Click on the bold face names to go to the VST website for download)
Until next time... Over N Out