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 How does audio compressor work? What exactly it is?


                                                            Compression of audio tracks


Audio compressor is maybe the least understood of all processors commonly used in multimedia production, whether it is studio, radio or television industry. But they are indispensable in obtaining truly professional sound. Without them the sound would be much more dull and lifeless.

The audio compressor is a device used to intervene in the spread of dynamic range of a recording. It's name suggests compression and so it is. It is like the equivalent of an automatic sound engineer, who quickly turns down the  level of the recording, when it begins to fall in the red area on the meters and turns it up, when very quiet sounds are poorly audible. The audio compressor works on a similar basis as it used to be with manual riding the level of the recording on analog tape machines. The only difference is that the audio compressor reacts much more quickly than the person who governs the knob or slider level and it acts fully automatically. The task of the audio compressor is not only protection against overdriving the sound, which causes distortion in the signal. Compressors generally improve the sound of a recorded track. They make it sound  more compact, powerful, warm and round. In today's highly dynamic digital recordings audio compressors have even greater significance for the shaping of sound, than ever

A typical compressor has 5-knob regulators:

1 Threshold
2 Ratio
3 Attack
4 Release
5 Gain Make-Up


The audio compression is widely applied, because it affects the positive aspects of sound recordings and the improvement takes place mainly because the quiet and hidden sounds are raised up to the general level, which makes the recording sounding more full and wide. This is nicer for the ear. Some of the compressors have additional knobs for adjusting the input and output level. But not all the audio compressors have these regulators-early audio compressors were only equipped with the gain knob, and the whole process took place almost automatically by raising the level of the input.
At a time when the recording gear used only reel-to reel tape some compression of audio happened automatically just by the nature of this technology and this sounded really nice for the ear. Also the studio equipment like the mixing consoles, effect processors or various preamps had far less dynamic range of sound than the modern gear. Today, in an age of digital audio, compression is more desirable than ever. Sterile digital signal with a very large differences in dynamic range and containing a lot of transients, (violent peaks and jumps of the sound waves), after proper compression is gaining much smoother character, which is nicer to listen to and accommodate by the human ear.


In the short characteristics of the audio compressor below we will not only learn about the rules for its operation, but also we will gain potential for certain tricks of advanced audio compressor, that can give us a very interesting sound.


Principles of adjusting and operation of the audio compressor

Let's use  here an example of good software compressor which has nice and clean "interface", a part of Ultrafunk plug-in bundle. The two most important adjusting elements in the compressor are the Threshold and Ratio. Threshold determines the level of the signal from which the compressor starts to operate, for example -16 db.
 

The following image shows an arrow pointing the threshold slider set to -16 db.


The box "RATIO" shows the value of "2.0:1." Let's look at the following picture:


This means in practice that at this setting the  compressor will reduce the dynamics over -16 db about 2 times. The lower level signals, staying below-16 db remain intact, but over the - 16 db threshold every decibel  will be reduced by half. So, for example, if the threshold would be set at  -8 db, at this 2:1 Ratio the dynamics there theoretically will be compressed by 4 db. However, this is only theory, because depending on the time of the  "attack" parameter, the compressor can pass even big quantities of sound impulses, which raise the theoretical reductions at least by half. So if the original signal was coming up to zero db, when you turn on the compressor set to -16 db threshold and 2:1 ratio the signal will be passing  -8 db, and hitting around -4 to -5 dB below zero.

The ratio is the amount of reduction which the audio compressor applies to the dynamics of the signal. For example, if you set the Threshold at -16db, and Ratio at 2:1 it means that if only the signal exceeds minus 16 db compressor will take action and will cut every decibel of dynamics about that level by half, for example if the signal raises by 8 db above the  -16 db Threshold, it will be reduced to only about 4 db above that threshold. Because the overall signal limited this way  becomes quieter, the overall level of sound after the compressor action can be raised by the regulator "Gain Make Up". It brings the level of the signal back to the state prior to compression. But then the sound becomes more saturated, dense and squashed, sounding punchy and making stronger impression. Other two sliders  - Attack and Release are used to set the time of reaction for the  compressor and the time to return the entire compressor system back to the neutral state


Generally quick setting of "Attack"  within 1-5 milliseconds makes very little transients passing through, which peak when there is a lot of fast high frequency waves present in the audio signal. When the high-speed signals enter the compressor which reacts quickly and stops them the sound becomes more compact, focused, with less content of high frequencies, these so-called transients. Attack set for longer times of for example 50 ms will cause the compressor to pass those transients (particularly present in high tones), such as drum cymbals or the trumpet, so that the sound will become less dense, brighter, more spacious. If the compressor is  more transparent in operation, it does not react too quickly. The "release" determines how long the compressor is coming back to neutral state after the sound was compressed. Carefully use the longer times of the release parameter, because it may cause the effect of pumping compressor, which is rather negatively perceived feature of the sound. It is safer to use a shorter time of release, but too short time of release may in turn cause a distortion of sound sometimes. Generally, the release time should be set between 1 and 15 milliseconds. In some cases, producers deliberately and knowingly use the effect of pumping, which can be an intentional artistic effect.

But always the proper setting of an audio compressor  depends on the sound of the recording and there can be no universal setting that sound good for all.

Personally, I recommend the use of a fairly long time of attack, short time of release (which can be extended if the music has slow rhythm) and low-value "ratio", typically between 1.6:1 to 4:1. Then we will be able to find a very full, natural sound without transformation of it's dynamic structure. In mastering we generally use very little "ratio" value between 1.4:1 and 2.0:1.

What should be the order of setting the knobs or sliders on a compressor? There are no rigid rules, but perhaps the following example would help here:
Let's say that we have a drum set in the multi-track project and we want it to glue better  with the rest of the tracks, and at the same time was well distinguishable in the mix. We have two options here: one if we want to get the dramatic compression and hear the  effect, another one we want if more delicate and natural sound. In the second case, we set the "ratio" at about 3.0:1, in the first at at least 6.0:1. Of course, it should be remembered that as long as "Threshold" will be set to 0 dB we will not hear any effect. Compressor always operates from a set threshold below 0 decibels, which is below the maximum possible volume. So when you play back the track slowly turn down the "Threshold height". As soon as you turn it down below the highest peaks in the recording you will have compressor starting to take action. Therefore, there are absolutely no rules, what is the best universal setting of the "Threshold". Everything depends on the level of the track at which it was recorded, some recordings have peaks at -0.5 db, some at - 8 db an some at -26 dB! And there are those that have peaks at -32 db. Therefore it is important to note that the "Threshold" must always be set slightly below the peaks on the recording, then the compressor starts working. If you set it deeply below the peaks, you be able to get quite a dramatic effect. If you set it  only a few decibels below the peaks, you get very slight difference in sound but maybe cutting just tops of the signal make the sound more natural. When the "Threshold" is deep below the peaks we have to deal with a big reduction in the track volume, which should be compensated by "GAIN MAKE-UP". When the "Threshold is only a decibel or two below the peaks in the signal, the reduction in gain is very slight, and thus compensation with "Gain Make-up" will not be needed.

So the drums in this example will sound natural and smooth if we just cut the peaking portion of the signal or will become big and punchy if we get much deeper with the threshold. Remember, too deep sounds bad. 

Values of the "ratio" and "threshold" should be set the first of all audio compressor settings. These are the elements that have the most influence on the sound.

Most compressors have the "GAIN REDUCTION" meter, which shows, how many decibels of the overall level the compressor has reduced. In typical applications of the audio compressor, this value should not exceed 5 db, unless our objective is to get a dramatic effect on sound. In the case of a reduction of 5 db  it is necessary to turn up the signal using the "GAIN MAKE-UP" by the equal or slightly smaller value of about 3-5 dB. Then you will able to get a perfect, powerful and clear sound, which will be perceived as natural and pure.

We still need to clarify on one element: the "KNEE". On the picture below we can see that the point of threshold is gently rounded, and "KNEE" value is 10.  It is clear that the compressor starts to "catch" from -16 db but in a fairly benign way, and about -13 db its operation is fully established:


However, let's try to change the value of the "KNEE" and see what happens then:


 

When the "KNEE" is at 0 we have a very sharp transition to a compressor actuation just above -16 db, and steady, constant level of compression to the end. In practice, this means that compression will give us more focused sound. You can also obtain the extreme opposite effect, "KNEE" set an extremely soft, so soft that tripping compressor is well below a certain "Threshold"!. Here's an example:


Such a "KNEE" of a compressor is called the "soft KNEE" - The "Soft Knee", giving mild compression is suitable particularly for vocals or string instruments. In contrast is the sharp shift actuation of "HARD KNEE", useful when mixing such instruments as drums or electric guitars.

Hardware equivalent of compressors used in the past have generally one type of the "KNEE", different brands showed a different sound, so famous studio engineers held the whole arsenal of different compressors at hand, using them for different purposes in accordance with their pleasure and experience.

Here is a famous Teletronix LA-2A "Leveling Amplifier":


 

As it possess the specific sound, it was (and still is) regarded as the first league studio equipment. It's sound can be heard on numerous hit records.

Another legend of great sound is the Fairchild 670. Compressors of this type were used in EMI Recording Studios at Abbey Road in London during the recording sessions of The Beatles:
Here is the famous Fairchild 670 in stereo version:

A successful program to the Fairchild is its software equivalent, the Blue Tubes FA-770:
 


 

There is a very great variety of both hardware and software compressors. It is impossible to tell which one is best, much less to recommend the optimal settings for any kind of sound that is passed through the device. The successful use of the audio compressors in the mix and mastering depends mainly on qualifications and experience of the technician that operates these devices. Without the human element the equipment remains only the dead matter.

For all your questions and inquires please contact us at:

info@studiomastering.net

 

 
 



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Copyright ® by Mariusz Wojto˝, 2007-2008. Copying, the use of excerpts for any purpose without the agreement of the author prohibited.
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