Compression is one of the most useful tools that a recording engineer can use in recording and mixing. After you are done compressing, the level fluctuations of the audio will become less than they were before applying the compression. This will make the instrument more audible and present in the mix. After compression, the lower-level passages or notes won't get lost. In this article I'm giving you an explanation of the common parameters found on compressors and how to set them to the correct value.
Threshold Parameter
What It Means: The level at which a compressor will begin to change gain. Compressors work by reducing signal strength anytime the incoming signal rises above the threshold level. The threshold is usually adjustable by a control marked "threshold." Reducing the threshold level means that more peaks of the signal will trigger the compressor to turn down the gain. Increasing the threshold level means that less peaks will cause the compressor to reduce gain.
How It Is Set: You start with the threshold set high (so that it doesn't cause any gain reduction). As you are watching the gain reduction indicator, reduce the threshold until the desired amount of gain reduction is present on the louder passages or sounds. For general compression, 6 dB of gain reduction is a usual goal. For louder types of music or where the audio has larger fluctuations, sometimes 10 dB of gain reduction is used. Special techniques may require differing amounts of gain reduction.
Gain Reduction Parameter
What It Means: The working of a compressor, reducing gain during high-level passages exceeding the threshold level. There is usually a gain-reduction meter that shows the compressor lowering signal strength on the loud passages. This meter (usually a strip of lights) could read "-6dB" on on loud passages indicating that the level was turned down 6 dB on the loud passages. The meter would indicate "0dB" (showing no gain reduction) on lower-level passages.
How It Is Set: How to set this parameter is set is covered above, under "Threshold." Too much gain reduction causes the louder passages to sound restricted and "squashed."
Compression Ratio
What It Means: The compression ratio is how many dB the input signal has to rise above the threshold for the compressor to allow one more dB out of the compressor. A ratio of 2:1 means that when the input signal rises to 2 dB above the threshold level, the compressor only allows the output to rise 1 dB [if the input rises 4 dB above threshold, the output only rises 2 dB]. When the compression ratio is 10:1 or higher, the compressor becomes a "limiter" and effectively prevents the output signal from increasing above the threshold level.
How It Is Set: The compression ratio is set according to your application
(see chart below):
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General Compression for more presence: 2:1 to 3:1
Level Control for audio with sudden peaks: 4:1 to 5:1
Protection from severe overload or damage: 10:1 or more. |
Compression Ratio:
The illustration on the right shows a swell of level in a signal and how this swell would be changed by a compressor. |
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Attack Time
What It Means: In a compressor, the attack time is how fast the compressor reduces gain when the input signal rises above the threshold.
How It Is Set: There are two approaches to setting attack time, as follows.
1. Attack times very close to 0 ms and less than 5 ms tends to bring down the percussive transients of the audio. This type of setting is used to reduce transients or to provide maximum protection for overload and damage to equipment (such as tweeters) that could be caused by those transients. Digital audio compressors sometimes have "look ahead" circuits to get a true 0 ms attack time, something that is impossible with analog compressors.
2. Attack times over 5 ms (usually 10 - 30 ms) are often used to let the transients and the attack of instruments through. This type of setting is often used for general compression.
3. Attack times over 15 ms coupled with gain reduction of 6 dB or
more can cause a "pumping" sound. "Pumping" is an unnatural
sounding change in level after the instrument sound begins.
Setting of the "knee" (see below) also effects pumping.
Release Time
What It Means: In a compressor, the release time is how long the compressor takes to restore gain after a high-level passage (that exceeded the threshold) is over. Generally the fastest release time that sounds good is best.
How It Is Set: One sets the release time by listening. For general compression of mixed music a release time between 50 and 100 ms usually works well.
Too long of a release time creates a breathing effect where you hear gain being restored during softer passages. If there is noise on the track being requested, you can hear the noise change also. The "squashed" sound of higher-level passages (or "pumping"
as described above) is more noticeable with longer release times. Optimum release time would be fast enough to restore gain as the passage being controlled dies down.
Two quick of a release time causes a reoccurring, unnatural surge in the instruments at the end of their sound. It occurs because the release time of the compressor is quicker than the dieing down of the passage being controlled . Release times under 25 ms may distort bass instruments because the compressor changes gain in the middle of a cycle.
Makeup Gain (Output Level)
What It Means: -The makeup gain is a level control at the output of the compressor. The compressor reduces signal strength of the louder passages. After this compression, the peak level of the signal is less. This makeup gain control allows the compressed signal to be turned up so that its peaks are as high as they were before compression. This action of "making up" gain increases the lower level passages of the signal.
How It Is Set: Looking at a meter during higher-level passages, adjust the make-up gain so that louder passages peak at the same place with the compressor in or out of the circuit. It is best to use a "VU" meter for this or the "VU" or "RMS" meter mode on digital consoles and mixers (when available).
Knee
What It Means: The knee of a compressor is how abruptly or gradually the compression ratio is applied to the signal when the signal is near the set threshold level. A Hard Knee means that the full compression ratio is applied to any signal above the threshold level. A Soft Knee means that a mild compression ratio is applied to signals approaching the threshold level and that the ratio is increased as the signal rises above the threshold level; the full compression ratio is applied to signals well above the threshold level. The soft knee makes the gain reduction less obvious - sort of like turning down the volume of a stereo slowly rather than abruptly. Soft knee is used in applications where you are using a compressor to even out volume changes in an instrument. DBX, a well-known brand of compressors, trademarked their soft knee function with the name "Overeasy."
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The graphs to the right are the traditional way of showing the input/output characteristics of hard knee and soft knee compressors, and the graphs resemble "knees" (hence the name). The above graph shows how the output signal would be different in these two types of compressors. |
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How It Is Set: Use a soft knee to make the compression less noticeable. Use a hard knee for maximum protection. Some compressors have a variable knee from very soft to no knee. Use a softer knee with lower compression ratios and less knee with higher compression ratios.
RMS (Average) Detecting
What It Means: There are two ways you could define the "level" of a signal. One way would be to use the absolute peak level of the waveform; the second way would be to use the "average" level of the waveform. In percussive instruments, such as drums, there is a huge difference between the peak level and the average level. In other instruments, such as a flute or violin, the two levels are much closer. The term RMS is an acronym for the mathematical process of determining a signal's effective average level [the "M" stands for "mean" - another word for average]. Many compressors allow the user to chose between average and peak detecting.
How It Is Set: Electronic circuits overload according to the "peak" level but your ear responds to the "average" level. If you are using a compressor, with a high ratio, to help prevent overload (say of a digital recording) you would want the compressor to respond to the peak level. If you are using a compressor to make an instrument more even in volume, you would want the compressor to respond to the "average" level.
Auto Release
Auto Release is variable release time feature in compressors and helps make the signal's volume even to the ear. With auto release activated, the compressor's release time is longer for peaks going well above the threshold and shorter for peaks going slightly above the threshold. Auto release is often used for general compression.
revised July 9, 2006
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