Big List: Here. From samplecraze.com, whose article has expired.
Compression is one of the most important elements in modern audio work, but it is perhaps one of the least understood and abused. Compression is used at every stage of the audio process and can be used for many different reasons from the mundane to the extravagant. In the first part of the MPC-Tutor compression series, we’ll be looking at the fundamental controls and theory behind compressors.
What is a compressor?
In very simple terms, a compressor is a device that can automatically control the level of an audio signal. Imagine an engineer using the volume fader to keep a singer’s performance at a constant level, making fine adjustments at the right time – well a compressor does this automatically and a lot more accurately!
Type of Compressor
There are many types of compressor, each one uses different internal components to achieve the desired compression. This leads to each compressor imparting its own unique sound on the effected material. Tube and valve compressors are certainly top of the drool factor, with their ability to add 'warmth' to a signal - and also add a couple more zeros to the price! Most budget compressors use solid state components, Joe Meek tend to use Photo-optical components that again, impart their own sound. Many modern compressors use a combination of technologies (such as the Behringer Tube Composer), and of course there are the software compressors designed to emulate all the aforementioned. What type you use depends on the type of music you make, the way you like to work, and your budget. Always remember that basically, a £100 Joe Meek is doing the same job as a £3000 Uriel - controlling the signal level - but each one will approach the task slightly differently and 'colour' the sound in its own unique way.
Compressors come with a variety of controls, but these are the most important.
Threshold – This sets the threshold level, measured in dB (decibels). When a signal exceeds this level, compression will be applied.
Ratio – This is the amount that the signal is reduced by the compressor. A compression ratio of 5:1 means that if the signal exceeds the threshold level by 5dB, the output signal only exceeds the threshold by 1dB. As you can see, the higher the ratio, the nearer the output level will be to the threshold level. When this happens (at ratios of around 20:1), we refer to it as limiting.
Attack – This is how long it takes for the compressor to act after a signal has exceeded the threshold level.
Release – This is how long it takes for the compressor to stop acting after the input signal has fallen back below the threshold level.
Output – After reducing the overall level of a signal, the output gain can be increased to bring the whole signal back to it’s peak level.
Hard knee/soft knee - While attack decides how fast a compressor acts, the compressors hard/soft knee characteristics decide how gradually the full amount of compression is applied. Hard knee compression applies full compression as soon as the attack allows, as shown below.
Soft knee compression actually occurs as the signal reaches the threshold, and is much more gradual than hard knee, as shown below. Soft knee is a much smoother, less noticeable compression over hard knee.
What can compression do for my music?
Compression can be used for many purposes, including:
- help instruments 'sit' more comfortably in a mix
- bring out certain instruments from the rest of the mix
- even out differences in levels on an individual instrument or whole mix
- make all the instruments in a mix 'gel' together
- subtly change the sound of an instrument
- dramatically change the sound of an instrument
- impart its own unique character to a sound
Hooking it up
A compressor isn't strictly an effect, it is a signal processor. With an effect, we tend to mix the effected signal back with the direct signal via the Aux send and returns on a mixer. With a compressor we want to affect the whole sound and only output the effected version - the original, unprocessed signal must not be heard (but we'll see later that this 'law' is regularly broken to good effect).
The easiest way to hook up a compressor to a signal is to use the insert points on your mixer channel. You'll need a TRS jack-to-Y lead. That is a lead with a stereo jack at one end (Tip, Ring, Sleeve), and two mono jacks at the other end. The stereo jack goes into the insert point on your mixer channel, and the mono ends go into the compressor - one in the input and one in the output. The signal is then re-directed from your mixer, through the compressor and back to the mixer again. If you have a stereo signal, you'll need two leads and a stereo compressor - never use 2 mono compressors on a stereo signal as this can lead to stereo discrepancies.
Now that your compressor is hooked up, let's look at some applications.
Limiting a signal
There are many situations where you need to make sure your signal never exceeds a particular level. The most common would be when you are recording vocals or 'real' instruments like bass and guitar - it's not uncommon for the performer to get a bit carried away and start playing too loud, causing your recorder to start distorting (particularly with digital recorders). Remember in the first part that we said that a compressor set to a high ratio like 1 in 20 basically stops your signal exceeding your threshold? Well, that's the exact effect we'd like to see here.
Firstly, set your compressor threshold as high as possible, then set your ratio to 20:1 or infinity:1 if you have it. The compressor wont do anything at the moment, as it's threshold is set so high.
When our signal reaches the threshold, we want the compression to kick in quickly, and then return back to normal as fast as possible once the peak has passed. This means setting a fast attack, a fast release, and preferably using a hard knee setting if you have one. An attack time of 1ms should do that trick, with a release of around 0.1 ms.
All that's left is to set the threshold. This needs to be comfortably above the 'average' playing level, but below the 'danger' level. While someone plays at a comfortable level, start dropping the threshold until you begin to hear a dip in the volume. Then get them to start playing a bit harder, and set your threshold again, but this time back off a little bit more. This should be about right. Get the performer to give it some welly, and note that the level never exceeds the threshold you have set. The idea is to keep most of the player's dynamics, but control the nasty peaks when they hit things a little too hard for your equipment's liking.
Compressing a vocal
Vocalists tend to cover quite a large dynamic range, from very quiet part to very loud powerful elements. A compressor can help smooth out the level differences in the performance, making the vocal sit more comfortably in the mix and help it stand out just when it needs to.
We want the compressor to act reasonably fast, but we don't want it to return to normal too quickly as this may sound a bit unnatural. So set the attack quite fast, while the release should be a medium setting around 0.5 seconds. A soft knee compressor will sound the most natural.
Ratio will vary for different singers. Start with a setting of 2:1. Get your vocalist to sing at a quiet level and start playing with the threshold, until your gain reduction meter shows a gain reduction of 1 or 2dB. finally, adjust your output gain to bring your levels up to normal.
The idea is to bring the up the quiet parts and drop the louder parts so that your vocal becomes tighter and more compact.
Smoothing out a bass part
Bass notes tend to have a large initial attack peak that drops rapidly to a more constant, lower level. To decrease this initial peak, we set a fast attack time so that the compressor kicks in immediately, taming the peak. Also set a fast release. A ratio to start with would be around 4:1. Set the threshold so that the compressor only acts on the peaks of the bass part. In most cases a hard knee setting works best, although Jazz bass can sometimes benefit from a soft knee compressor.
After this type of compression, you should find that your bass part feels tighter and less 'all over the place', so it should sit nicely in the mix and have good presence.
Adding punch to a guitar/bass part
If you have a recorded guitar or bass that lacks some 'oomf' and isn't cutting through the mix well, you usually need to increase the initial attack transient. Yes, a compressor can do this as well! The idea here is to allow the initial part of the bass note through uncompressed, then compress the rest of the note. The resulting note should then have an increased attack transient compared to the rest of waveform.
It's important to set a slow attack time - experiment to see what works best. A reasonably slow release works well. Reducing the threshold should increase the punchiness.
Increasing guitar sustain
Sustain refers to the length of time a note lasts after the initial pluck. The higher the ratio setting, the longer the sustain will be. Try a fast attack and a slow release, then play a note on your guitar. While it is playing, adjust the ratio until you are happy with the level of sustain.
In the same way that we reduced the initial transient on a guitar or bass note, we tend to want the same effect on drum hits. By reducing this initial peak, we bring out the true sound of a snare - the crack - which actually occurs slightly after this peak. A fast attack and release works well, with a starting ratio of around 3:1.
Experiment with the attack time - by slowing it down very slightly can completely change the sound of a snare, bringing out more of the actual stick than the drum. As with all examples, increasing the threshold increases the squashed sound.
A fast attack, but a slow release is necessary here, in order to keep the natural sustain of a cymbal. Try a ratio of 2:1 to start with. On the other hand, a shorter release time with a high ratio and low threshold can lead to the classic 'pumping' effect, where you can hear the compressor actually working - in most situations this sounds bad, but with cymbals it can actually sound good in some songs.
Using a compressor as an Aux effect
Compression can even out the levels of a drum performance, but sometimes this can be a bad thing, leaving the drums sounding weak with no feel. By placing the compressor into the send-return loop, we can mix the compressed signal with the uncompressed signal, leaving some of the dynamics, while benefiting from the definition that compression gives us. Try short attack and release time, with a ratio of around 2:1 and a low threshold.