Compression Basics 
by Tony Koretz http://www.koretzmusic.com/tony.html

In this article I want to present a broad but brief overview on the use of compressors in   the recording of sound, be it music, the human voice or general sound effects. The   compressor is a great tool to have in your arsenal as a sound engineer, but it is one of   those that can either make or break your audio recordings. Used right, it can greatly   improve your tracks, used wrong it can ruin the audio and make it sound squashed and   muffled. The human ear can detect sounds in a wide dynamic range - from a quiet   whisper for example to a huge explosion. 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. So here I will just give   you some tips to help you get started in the use of compressors and limiters in   controlling your sound's dynamics. 

Description

Okay, so here we go. First up I will give you a quick outline on what a compressor or   limiter does. Right oh, a Limiter abruptly cuts off the dynamics of the signal above a   predefined threshold, while by contrast, a compressor gradually attenuates the signal   above the predefined threshold.  Often times the same device can be used as a compressor or a limiter, or it may have   two sections, a compressor section and a limiter section. A compressor is generally   used to control the dynamics within the normal program level of the material, while a   limiter usually has its threshold set above the average signal level, so that it just 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. While there are a number of   different types of compressors and limiters available , either as hardware or software,   most have some or all of the following adjustable features available for user control,   though sometimes they are given different names. 

Threshold

The threshold control enables you to set a point above which the program material   begins to have it's level reduced in amplitude. Therefore, any signal which exceeds the   threshold point in it's volume will begin to be attenuated by an amount determined by the   ratio control. 

Ratio

Once a signal exceeds the threshold ,the way the ratio control is set will determine how   "Hard" the signal above the threshold is attenuated. For instance, a ratio of 1:1 would   mean that no attenuation is occurring at all, at 2:1 the signal above the threshold will   have it's gain reduced.  At this ratio, for every decibel of signal gain that happens, only half a decibel of   increased output will actually occur. Two decibels signal gain would result in one   decibel of increased output, ten db gain would result in a 5 db increase etc.  The higher the ratio, the harder the gain reduction. In practice, any ratio above 8:1 is in   effect pretty close to being labeled as a limiter in my book, as not much extra signal gain   will be output beyond the threshold at that setting. 

Attack Control

The attack control determines how quickly the compressor will react to signals that   exceed the threshold. You might think that instant response would be ideal, but in   practice a slight delay in time between the signal increase and the onset of compression   often results in a more musical and less muffled sound. An example here would be say   a kick drum, where you want to here the quick attack transient of the beater to give it the   punch, before the resonance of the drum occurs, and the signal is squashed. Generally   attack times are set somewhere between 0. 1 to 100milliseconds or so. With a limiter   though, you generally want a very quick attack to catch the peaks before they slam into,   and over your precious 0db digital recording levels and cause distortion, and maybe   equipment damage. 
 

Release Control

The release control determines the amount of time it takes for the signal to return to its   normal gain once the material is safely below the threshold level again. Release can   usually be set from times that are near instantaneous, to times of 5 secs 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, so long as we don't take it above the level   where clipping occurs. 

Setting the Controls

So, having given a brief outline of what the controls do, I will now attempt to give you a   few guidelines as to 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 article.   Remember it is titled "Compression Basics" so I will try to keep it that way here! 

Compression

Okay, begin with a ratio of between 3:1 and 6:1 for most natural sounding results, and   gradually lower the threshold till you achieve the desired amount of gain reduction.   Remember, the lower the threshold, the more the signal will be compressed. For voices and non percussive instruments try setting the attack time moderately fast, let's say at around 10 milliseconds, and the release at between 0. 5- 1 sec. If the attack time is too fast your, S's and T's will disappear, and dynamic distortion is also more likely to occur at fast attack times, particularly if coupled with a low threshold. If the attack is too slow,   the S's and T's may actually stand out too much. If the release is too short the level will   appear to fluctuate and pump, as the compression goes in and out, but if too long,   pumping side affects will be noticeable when loud passages are followed by quieter   ones.  Percussive instruments can be treated by using a faster attack time, to catch the initial   transients, or alternatively use of a slower attack time will let the transients through   while catching the body of the sound. 

Leveling

When set up for leveling, a compressor will keep the overall sound at a more uniform   level, while not affecting the short term peaks in the material. In order to achieve this,   you would set the threshold relatively low (affecting more of the sound), the attack time   relatively slow, and the release time slow as well. 

Peak Limiting

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.

Program Limiting

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, just be careful not to push it beyond the clip level though.

Using a compressor and limiter together

A very common thing to do is to place a compressor first in a signal chain to raise the   overall level of the program 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 actually reverse this, and place a limiter first in the   signal chain, and then compress the remaining material. This can often avoid holes being   punched in the sound by the interaction of these two things, but hey, discussing that is   again getting beyond the scope of this article. 

Avoiding Side Effects

Like taking medicine, there can be side affects to compression. To avoid them…don't overdo it. Too much of a good thing can ruin your health ! Sometimes less is more, so be judicious and don't slam everything to bits with "over compression". Things can sound really lifeless if you do, or can get fatiguing to listen to if there are no dynamics present. If you get distortion in the audio. . . 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

down the attack time and lower the ratio to let some of the transients pass through

before the "squashing" occurs.

Well that's about enough I think . . . this "basics" article is looking pretty darn long to me. AND I'm getting tired hands from typing! Have fun squashing your sound!!!!!!!!!

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