The Compressor’s Place in Audio Recording and Processing
A Guide to Knowing Your Compressor Inside and Out
A compressor is a type of audio processing tool that is in a group of audio tools know as dynamic processors. This group also includes tools like de-essers, limiters, and gates. It can be a rack-mounted hardware compressor or a plug-in for a software workstation or a pedal for your guitar. The explanations below apply to both hardware and software compressors. In effect, they are no different except that software compressors replace hard-wired elements with algorithms and replicated digital circuitry. Either one you use, whichever your preference and budget allow, this guide should give you a better understanding of what a compressor does and it’s place in your studio workflow.
What is a Compressor’s function? Compressors are used in audio recording and processing to control level fluctuations that occur in the audio signal. At it’s most basic level, a compressor simply turns up the volume on your track whenever the volume of the track dips below a certain threshold (settings which we will discuss later). For example, a singer’s voice may be louder or softer at different points in a track, especially if they have less than perfect microphone technique.
In the past, before modern studio technology, this problem was dealt with by physically moving the fader on the mixing board while transferring the audio from the master recording to a new tape to compensate for the volume discrepancies. This process is known as ‘riding the fader’ and some engineers still swear by it. A compressor will accomplish this more efficiently and more accurately. It has a much faster reaction time than a human and reacts automatically based on it’s set parameters, leaving the engineer with two free hands to do anything else that needs doing. Because a compressor affects the overall dynamic range of the sound, it effectively is ‘squeezing’ the signals waveform. It can be used to increase the relative energy of a track by giving a boost to levels before and after peaking and can (subjectively) give a tighter, more consistent sound or can just to boost overall volume in quiet parts of a track. These days we are all used to hearing compression of some form as it is commonly used at the mastering stage to maximize the energy in a mix, making the song sound as loud as possible.
So how does a Compressor work? In order to understand how a compressor works, we need to know that it has two main components, a voltage controlled amplifier (VCA) which is what will be actually affecting your sound, and a side chain which is a spot on the compressor for “listening” to a signal. The VCA is just like a normal amplifier, like one you might plug a guitar or mic into, except that it’s gain is controlled by the voltage level of a control signal, which is directly related to the settings in your compressor.
The side chain monitors the output of the track and uses this to generate the control signal for the VCA to work with. With the compressor switched to bypass, the side chain will merely ‘follow’ the level of the signal passing through the VCA and will have no effect. The compressor is effectively off when the switch is set to bypass. Switch the compressor on and the fun begins! When the output of the VCA (your track) exceeds a preset threshold level, the side chain generates a control voltage and sends it to the VCA. The control voltage triggers the VCA to reduce the gain of it’s amplifier according to a preset ratio, thus altering the volume of your intended track. The threshold and ratio settings are selected by the user along with the attack and release characteristics of the compressor, which we will cover later.
The controls. Let’s talk about some typical controls that you will probably find on any compressor. Understanding what these controls do is the key to effectively using a compressor. The actual controls and their values may vary slightly so I’ll include the standard controls with an average range of values that should apply to most compressors you’ll encounter.
Threshold: -30dB to infinity. This sets the point at which gain reduction (compression) starts. It is important to realize that it is the signal entering the side chain circuit that is governed by the threshold, not the input signal. So if you have your side chain listening to a different track than the one you are compressing, only the track connected to the side chain will trigger the compression when the audio volume reaches the threshold.
Ratio:1:1 to 20:1. This controls the amount of compression with a preset ratio. A ratio of 5:1 means that a signal that would be 5dB above the threshold is reduced to 1dB above by the compressor.
Attack:50uS to 500mS. (50 micro seconds to 500 milliseconds) This controls the response speed of the compressor. 50uS will react to the fastest of audio signals, a slower attack time will allow transient peaks at the beginning of a peak to escape compression and may give a more natural percussion to a sound.
Release: 50mS to 5S. (50 milliseconds to 5 seconds). This controls the time taken for the compressor to recover from gain reduction. In other words, to stop reducing the gain. A short release time will stop the effects of the compressor as soon as the signal falls back below the threshold. A long release time will continue to compress the signal at the selected ratio even after the peak is over. Too long a release time, with a moderate/short attack may cause a ‘pumping’ effect in the output signal as the sound is never allowed to decay away as it would naturally. This control is sometimes labelled ‘sustain’, especially on compressors designed for guitars.
Output Gain:-20 to + 20dB. This is used to make up for any signal level that is lost during compression and is sometimes called ‘make up gain’. This is a part of a compressor that can mess with people’s mind. Since compression actually makes parts of your track louder by making the loudest parts softer, sometimes this is required to bring the track back into your mix at an appropriate level.
The Knee – Compressors come with two varieties of knee, hard and soft. The term ‘knee’ describes the type of curve that the compressor responds with when it starts its gain reduction. A hard knee will give instant compression at the selected ratio whenever the threshold is exceeded. A soft knee will give a progressive compression up to the set amount of gain. The soft knee gives a less processed sound which can be useful when subtle gain reduction, or ‘transparency” is required. A compressor with a hard knee produces a more aggressive reduction that can work well for more percussive sources such as drums. Try applying some heavy compression ratios combined with a high knee and you will really start to see how it can add punch to a drum sound.
Other controls commonly found on a compressor include: a switch for bypassing the unit; a stereo link button that allows you to control both sides of a dual-stereo compressor with one set of controls for easy stereo compression; a Side chain listen switch that lets you listen to a signal entering the side chain. This is sometimes called ‘key listen’ and is a useful feature if you use compression techniques such as ‘ducking’ or ‘de-essing.’ You will normally also find some kind of level meter, or at least a peak indicator for reference.
Limiters – One way to better understand where a compressor belongs in your song is to look at the difference between a compressor and a limiter. Even though they operate on a similar level, they are very different and when it comes to recording, small changes can produce radical differences in sound.
A Limiter (or limiting amplifier) is essentially an amplifier with a constant output above a preset input level. In other words, no matter how much more volume you put into the amp it won’t let the peaks exceed the set level. Unlike a compressor, this characteristic does not affect the whole dynamic range of the signal but just ‘limits’ the peak levels to prevent clipping. A limiter will have a ratio of at least 10:1 and typically as much as 50:1 with an input threshold set just under the optimal output volume of your mixer. This means that the limiter will simply put a “brick wall” at a certain volume (known as the threshold), preventing any sound from peaking past that volume.
A compressor set with a high ratio and a very fast attack time will work as a limiter but will affect the whole dynamic range of the signal and so raise the level of the noise floor creating added hiss. The amount of room, in sonic terms, that you have between the peaks in your mix and your optimal output level (usually represented as 0db on a mixer) is often referred to as ‘headroom’. Another way a limiter can be understood is to visualize the limiter lowering the levels of the peaks on a waveform and raising the overall volume of the waveform, thus filling the headroom.
This principle can be easily observed by looking at the waveform display on your DAW. Take a dynamic sound such as drums with a lot of peaks, apply some extreme compression, and you can see the peaks are reduced and the overall volume of the track gets louder. Try it, you might be surprised at how visual and easy to understand the whole process really is.
Noticing this difference between limitation and compression is another key to understanding the type of dynamic processing you should use on a track and helps you to more easily understand the nature of a dedicated compressor.
Other compression techniques. There are many ways to use compression apart from traditional volume adjustments. Engineers throughout history have discovered and perfected these techniques to the point where they are now accepted as standard practice in certain styles of music. A few common practices are known as De-Essing and Ducking.
De-Essing – Sibilance caused by air in the teeth during speech or singing is a common issue in recordings. To reduce this unpleasant sounding phenomenon an EQ unit can be inserted into the side chain with the sibilant frequencies (usually between 5 and 10Khz) boosted and the normal signal sent through the compressor as usual. Because of the EQ boost in the side chain, sibilant frequencies will be compressed before the rest of the signal. This technique is commonly known as ‘de-essing.” Although this is commonly sold separately, or done manually, some modern compressors will even have a dedicated de-essing function where the side chain is set up for you.
Ducking – You may have noticed that many DJ’s manage to fade the music down while they are talking so that their speech can be heard. This is known as ‘ducking’ and it is achieved by sending the voice signal through the side chain and the music through the compressor.When the voice exceeds the threshold, the music is compressed. Ducking is used extensively in radio and television for voiceovers and with a little imagination can produce some interesting and unusual effects. Experimenting with ducking is also a great way to understand how the side chain interacts with the VCA in a compressor and demonstrates that it is only the loudest part of a signal that triggers compression, even though the resulting gain reduction will affect the whole sound.
I’ve also heard ducking explained like this; Your grandma is listening to your vocal track on her iPod, but has her hand on the volume of your mix. Every time she hears sound in headphones, she turns down the volume of the mix. In the case of recording, this may be done live or as post-processing but the result is the same. Ducking is quite simply listening to one source, and lowering the volume of another.
Drawbacks – A little compression at the master output/recording stage can maximize your output levels and prevent excessive signal peaks from overloading the input and causing distortion or clipping, which is good. On the flip side, there are a lot of ways in which inappropriate compression can completely wreck your sound. There a few culprits.
Heavy compression ratios, especially on tracks that register heavily in the low end of the frequency spectrum, may cause high frequency loss due to the compressor’s attenuation of peaks in the lower frequencies. A normal compressor is not frequency selective so any gain reduction will affect the whole frequency spectrum. This is one reason why its always good to get a relatively balanced sound before you start adding the huge plug-in ‘mastering suite’ across your mix!
Very fast attack and release times can cause the compressor to attempt to respond to individual wave cycles at low frequencies (as these are the longest cycles) which will lead to distortion as the wave-form is compressed out of its natural shape. Thankfully, some compressors have a ‘hold’ function to delay the release cycle or you can make sure the release is set to longer than the lowest frequency cycle.
Very low compression thresholds can cause ‘proximity effect.’ That is the term used to describe one hearing the frequencies in an audio track that give the sensation that the audio source is pushed too hard into the listener’s ears. Most times, this is caused by an acoustic instrument or voice being too close to the microphone. This effect is natural, but when accented by compression with a ratio that is too low it can become very pronounced and can even sound much like clipping, or can cause actual clipping.
It is very tempting to now include a list of recommended compressor settings for various com-mon recording tasks such as vocals, drums, mix etc.. but I believe that if you take some time to under- stand how the compressor works and experiment on your own, the experience reward you with the ability to quickly dial in just the right settings to get the sound you want on any mix.
Many engineers will tell you that, used correctly, a compressor will be virtually inaudible. Why use a compressor if you can’t hear it? Because having things stand out in your mix as unnatural is what necessitates the use of a compressor. In these cases the compressor is being used to counteract some of the artifacts of the recording process that result in an unnatural and uneven sounding mix and return the sound to one that is as natural as possible. On another occasion an engineer may use the effect of deliberate over-compression to create a unique and interesting sound that is most audible. There really are no rules and ballpark preset settings are just that, ballpark and approximate. The secret is, as always, to understand your equipment so you can get the best out of it. I hope I’ve helped you understand this wonderfully useful device and it’s uses in your recording process.
- By Sedric Pieretti | AssociatedContent
- Loulith Galenzoga
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