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Understanding Compressors

UNDERSTANDING AND USING COMPRESSORS

compressors

A BRIEF INTRODUCTION

The compressor is often one of the least understood pieces of equipment, or software in a sound engineer or amateur sound recordist’s arsenal.
In this tutorial I intend to present a brief but broad overview on the use of compressors in the recording and mixing of audio. The principles discussed here will applicable for any sound application, whether it be in music, the spoken voice, or general sound effects and foley. For an audio engineer, the compressor is a great tool to have in your bag of tricks, but it is one of those things that can either make or break your recordings and mixes. Used right, compressors can greatly improve audio tracks, but used wrongly they can ruin them, leaving them sounding squashed, lifeless and muffled. The human ear is able to detect sounds in a wide dynamic range – from quiet whispers to a huge explosions. However, our recording and playback equipment has physical limitations which mean we have to squash or compress these sounds into a smaller dynamic range in order to reproduce them. The purpose of this article is to give you some tips to help you get started in the use of compressors and limiters for controlling the dynamics of your sound.

DESCRIPTION

Here I aim to give you a quick outline on what a compressor or limiter does. Firstly, a Limiter cuts off the dynamics of the signal above a predefined threshold abruptly, while in contrast to this, a compressor gradually attenuates the signal above the predefined threshold.
Often times the same piece of gear can be used as a compressor or a limiter, or it may have two sections, a compressor section and a limiter section and both can be used together. A compressor is generally used to control the dynamics within the normal program level of the material, while a limiter normally has it’s threshold set above the average signal level, in order to catches peaks that rise above those levels. With digital equipment this is often necessary to prevent “overs” or digital clipping if the level exceeds 0 db. There are a number of different types of compressors and limiters available, either as hardware or software, but most have some, or all of the following adjustable features available for user control, though different manufacturers do use different names to label these functions at times.

THRESHOLD

The threshold control enables you to set a point above which the program material begins to have it’s level reduced in amplitude. Any signal which exceeds the threshold point in it’s volume will begin to be attenuated by an amount that is determined by the setting of the ratio control.

RATIO

When a signal exceeds the threshold, the setting of the ratio control will determine how “Hard” the signal above the threshold is attenuated. For example, if a ratio of 1:1 is used, it would mean that no attenuation is occurring at all. However, at a ratio of 2:1 the signal above the threshold will have it’s gain reduced.
At a 2:1 ratio, for every decibel of signal gain that happens, only half a decibel of increased output will actually occur. Two decibels of signal gain would result in one decibel of increased output, while ten dB of gain would result in a 5 dB increase etc.
The higher the ratio, the harder the gain reduction will be. In practice, any ratio above 8:1 is in effect pretty close to being labelled as a limiter, and not much extra signal gain will be output beyond the threshold at settings above 8dB.

ATTACK

The attack control determines how quickly the compressor will react to signals that exceed the threshold. You may think that instant response would be ideal, but in practice a slight delay in the time between the signal’s increase and the onset of compression will often result in a more musical and less muffled sound. For example with a kick drum, where you want to be able to hear the initial attack transient of the beater hitting the drum to give it some punch before the resonance of the drum occurs, and the signal is squashed you don’t want instantaneous compression to stifle this. Generally attack times are set somewhere between about 0.1 and 100 milliseconds. With a limiter however, you will generally want a very quick attack to catch the peaks before they slam into the 0 dBFS of digital recorders causing distortion, and maybe equipment damage.

RELEASE

The release control determines the amount of time it takes for the signal to return to it’s normal gain once the material has dropped below the threshold level again. Release can usually be set from times that are near instantaneous, to times of 5 seconds or more.

OUTPUT OR GAIN MAKEUP CONTROL

When we compress or limit a signal, the result is an overall level output that is lower than the original program material, and we normally use this control to bring the level back up to, or beyond the original level, as long as we don’t take it above the level where clipping occurs. Some compressors automatically apply makeup gain to compressed signals.

SETTING THE CONTROLS

So, having given a brief outline of what the controls do, I will now attempt to give you a few guidelines on how to use them. There are many ways that compressors can be used for interesting and creative effects, but that is beyond the scope of this tutorial.

Compression
Begin with a ratio of between 3:1 and 6:1 for the most natural sounding results, and then gradually lower the threshold until you acheive the desired amount of gain reduction. Remember, that the lower the threshold is set, the more the signal will be compressed. For voices and non percussive instruments try setting the attack time moderately fast, perhaps at around 10 milliseconds, and the release time set between 0.5-1 sec. If the attack time is too fast your, S’s and T’s will begin to disappear, and dynamic distortion will also be more likely to occur at fast attack times, particularly if coupled with a low threshold. Conversely, if the attack is too slow, the S’s and T’s may actually stand out too much. If the release time is too short, the level will appear to fluctuate and pump, as the compression goes in and out, but if set too long, then pumping side affects will become noticeable when loud passages are followed by quieter ones.
Percussive instruments can be treated either by using a faster attack time to catch the initial transients, or alternatively using a slower attack time will let the transients through while catching the main body of the sound. How you use these settings can be quite a big part of the whole creative process, and used for shaping the sound of an instrument to suit the rest of the material. The settings used may depend on the desired effect and the end result you are hoping to attain.

Levelling
When set up for levelling, a compressor is used to keep the overall signal at a more uniform level, while not affecting the short term peaks in the material. In order to acheive this, you should set the threshold relatively low (affecting more of the sound), the attack time relatively slow, and the release time slow as well.

Peak Limiting
Peak limiting can be used for tracking instruments during recording, or when mixing, and is often used in broadcast program feeds. To set up as a peak limiter, use a high ratio (8:1 or more), a high threshold, and a fast attack and fast release time.
Programme Limiting or Average Limiting
Applications for average limiting include the tracking of instruments and vocals during recording. This method is also often used to increase the apparent loudness of broadcast feeds and programs. Such things as “punching up” an on-air presenter’s voice are done this way. Often average limiting is done in conjunction with a peak stop limiter.
If you desire only to limit the average signal level of the audio and not to catch the peaks then set the attack time at 20 ms or slower.

Makeup Gain
Once you have done your compressing, use the output control to make the gain up so that you raise the signal level back up to, or beyond the unprocessed sound. However, you must be careful not to push the signal back up beyond the clip level.

USING A COMPRESSOR AND LIMITER IN TANDEM

A very common thing to do is to place a compressor first in a signal chain to raise the overall level of the programme material, and follow it with a limiter set only to catch the peaks that the compressor misses, and thereby keep the level close to the zero decibel mark. However, another way is to reverse the order, and place the limiter first in the signal chain, followed by the compressor to massage the remaining material. The second method can often avoid holes being punched in the sound by the interaction of these two processors.

AVOIDING SIDE EFFECTS

Just like in the taking of medicine, there can be side affects to compression. In order to avoid unwanted side effects, don’t overdo the compression and limiting. Too much of a good thing can ruin your health ! Sometimes less is more, so be judicious and don’t slam everything to pieces by “over compressing”. Music and other audio can sound really lifeless if you squash them too hard, and can get fatiguing to listen to if there are no dynamics present. If you find you are getting distortion in the audio, then back off on the attack time a little, or set the threshold higher, and the ratio lower. If your audio starts sounding muffled, then slow the attack time down, and lower the ratio to let some of the transients pass through before the onset of “squashing” occurs.

CONCLUSION
So there you have it. Aside from being a little confusing to use if you don’t know what the controls do, once you understand them and have a play round with compressors, they can be fun tools and can really make or break your final mixes. Start conservatively and don’t overdo it. Repairing damage from poorly compressed audio is very difficult. Better to be safe than sorry until you have gained experience with different compressors, in different scenarios. But don’t be put off using them. When used well they can really make a great difference in the world of audio. So go to it and have fun doing some squashing!

By Tony Koretz
© copyright March 2012