Another piece of equipment found in studios, which is responsible for that clean professional sound, is the compressor/limiter.
Compression vs limiting
The dynamic range of a piece of music is the difference in volume level between the softest and loudest notes in the music. This difference is measured in db (decibels). A piece of music may have a 90 db dynamic range, but this would make the softest notes too soft to hear and the loudest notes distort your system. Also, if you are recording onto tape, all recording tape has a tape hiss noise floor. If the loudest portion of the music was recorded at a low enough level to prevent it from distorting the system, the softest portion would be masked by the tape hiss and you wouldn't hear every note in the song.
A compressor/limiter could correct this problem by reducing the dynamic range of the music. If the compressor was set for 50 db of gain reduction, it would change a 90 db dynamic range to 40 db (the difference between the original dynamic range [90 db] and the amount of gain reduction [50 db]), which would keep the softest passages loud enough to be heard above the noise floor and the loudest passages soft enough to keep from distorting. A compressor would reduce the dynamic range by increasing the level of the softest passages by 25 db and decreasing the level of the loud passages by 25 db. A limiter would reduce the loudest passage by 50 db and would not affect the softest passages. You would however, then have the option of boosting the overall signal manually for a better signal to noise ratio.
If a vocalist doesn’t know how to work a microphone or doesn’t have any dynamic control of their voice, they may hit some notes too hard and some not hard enough. This problem can be quickly corrected by compressing the dynamic range of the vocalist. With instrumentalists, someone may hit a note too hard or not hard enough. Rather than go back and re-record the track, the problem can be corrected by adding a compressor or limiter to the signal while recording or during the mix.
The more expensive compressor/limiters have the ability to control various parameters.
The threshold adjustment allows you to set the level at which the compressor or limiter begins to reduce the level of the signal. If you are using it as a limiter, then it will begin reducing the level at the point where the threshold is set. If you are using it as a compressor, the reduction point begins at the threshold as with the limiter, but everything below the threshold is raised in volume level.
The compression ratio is the adjustable difference between the input and the output. An example would be a 2:1 compression ratio. This means that for every 2 db of input, only one db of output occurs. In other words, the dynamic range is cut in half. A 4:1 compression ratio would mean that for every 4 db of input there is only 1 db of output, or the dynamic range has been reduced to ¼ of the original signal. A ratio setting of ∞ (infinity) will reduce the dynamic range to 0. Settings such as these seem extreme, but are usually used as a peak limiter rather than a compressor. Used as a limiter, the recording can play it's full dynamic range until it hits the threshold. Then everything above the threshold is reduced by whatever compression ratio you have selected.
Expansion is the opposite of compression. For instance if you have a 1:2 expansion rate, that means that for every 1 db of input, there are 2 db of output. In other words, the dynamic range of the original signal is doubled. An expander is used for several types of applications.
If a signal has been over compressed, you can restore some of the dynamic range with an expander.
An expander can be used to add a little extra "thump" to your peaks.
It can also be used as a noise reduction device similar to a noise gate.
Tape noise reductions units such as Dolby and DBX use both compression and expansion as their encode and decode processes.
The attack adjustment determines how quickly the compressor jumps into operation when the signal hits the threshold. In most cases, you may want the fastest possible attack, but there are times you may want a slower attack. For instance, if you are recording a bass that is boomy and sustaining and you need to get more punch out of it. Slowing down the attack will allow the pluck of the bass to come through first, then the compressor will suppress the sustain of the bass, thus giving it more pop.
The release adjustment determines how quickly the compressor lets go after the signal drops below the threshold. With a wide dynamic range, drastic reduction in levels quickly released will result in a pumping sound. Slowing down the release will remove the pumping sound, and result in a smoother recovery of dynamic range when the compressor lets go of the signal.
The input adjustment determines the amount of signal that will be allowed to enter the unit. If for instance, you have a mixer with lots of head room and you want to push it to the threshold of distortion for the best signal to noise ratio, the output level of your board may be too hot and overload the input of the compressor. An adjustment of the input control on the unit will correct this problem. On the other hand, if the signal coming from the source is too weak to trigger the threshold of the compressor, raising the input level could correct this problem.
The output control determines the level of processed signal that will be coming from your compressor and going to the next unit in your signal chain.
RMS (Root mean square)
This setting averages out the sum of all of the parameters. When your unit is in RMS mode, it is working as a compressor. Everything below the threshold will be increased in volume level and everything above the threshold will be decreased in level. The amount of increase or decrease will be determined by the compression ratio you set.
When in peak mode, the unit functions as a peak limiter. Only the signal above the threshold is reduced in accordance with your compression ratio setting and everything below the threshold is unaffected.
The knee adjustment determines whether the operation at the threshold is abrupt or subtle. A hard knee will make it operate abruptly and a soft knee, subtly. Some of the more expensive compressors allow you to adjust the amount of subtly that it will operate at.
Some compressors have a noise gate. When compressing a signal, you sometimes pick up unwanted noise. If your unit is set for RMS and the softest passages are being increased in level, other things below the threshold also increase in level, such as buzzes from guitars and amplifiers and other system noise. You may not hear the noise when the instrument is playing, but when the instrument stops playing, it sticks out like a sore thumb. Setting the threshold of the noise gate just above the noise level will ensure that you don’t record any noise from that instrument’s output. Noise gates also have a release adjustment to allow for the natural decay of a sustained note before closing off that channel to signal. If you are not playing long sustains that need to fade gradually, then you can shorten the release time of the gate.
Most units have a “bypass” mode that allows the signal to pass through the unit unprocessed. This allows you to compare the original signal with the processed signal or turn the compressor on and off at will during a mix.
Compressor/limiters are used extensively in recording studios. Usually there is a little compression or limiting used in the process of recording each instrument. Then when it comes time to mix, the entire mix of instruments is processed through another compressor/limiter. The end result is a punchy sound with the best possible signal to noise ratio. Additionally, a compressed sound gives the perception of being louder, making the compressor a great tool for recording rock music.
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