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Setup Suggestions for Audio Compression and Limiting
Written by Bill Gibson - © 2005, Cengage Learning. Reprinted with Permission.
This article is an excerpt from the following book: The S.M.A.R.T. Guide to Mixers, Signal Processors, Microphones, and More.

Typically, the threshold is set above normal operating levels—most of the time, there should be no gain reduction. If there's always gain reduction, the VCA is always working, and you begin to lose the clarity and signal integrity. An experienced engineer tries to eliminate unnecessary amplifying circuits in the signal path—that's our approach here. The VCA should only act when it's needed. Compressors and limiters are generally used while recording tracks as opposed to during mixdown, since one of the main benefits in compressing the signal is that you can get a more consistently hot signal on tape.

Listen to the different versions of the exact same vocal performance in Audio Examples 6-11 to 6-15. I've adjusted the level so that the peak of each version is at the same level. The only difference is the amount of compression. Pay special attention to the understandability of each word, the apparent tape noise, and the overall feel of each track.

Should I Use the Compressor/Limiter on Input or Output?
The compressor/limiter is typically used at the beginning of the signal path, just after the source enters the mixer input; however, there are valid reasons to incorporate this tool at each stage of the signal path. Compression and limiting provide an efficient means of controlling the level that's recorded to tape, hard disk, or any other analog or digital media. Recording instruments with a wide dynamic range often require constant level adjustments to insure a consistently acceptable signal-to-noise ratio. These level changes can be performed manually, although they're typically much more reliable when performed electronically.

It's common to compress or re-compress audio tracks or groups during mixdown. Compressing the lead vocal or a stereo group of backing vocals during mixdown provides the engineer the opportunity to finely craft their positioning within the dynamic audio spectrum. However, avoid over-compressing any signal. Part of the power of audio rests in its dynamic content. When robbed of dynamic contrast, music and other audio sources lose impact. A mix devoid of dynamic contrast is tedious to for the listener. Music is, at its very essence, a balance of tension and release. Sound that remains constant in amplitude and energy provides no release and eventually wears the listener out. Strive to find the best dynamic contrast for your music.

Setup Suggestions for the Compressor/Limiter
· Adjust Ratio to determine function. Setting below 10:1 produce compression; setting 10:1 to infinity:1 produce limiting.
· Set attack time fast or slow, depending on the audio source and desired effect.
· Set release time to about .5 seconds for general use.
· Select Soft Knee for gentle compression, or Hard Knee for limiting applications.
· Select RMS for most compression applications, or Peak for most limiting applications.
· Adjust the threshold for the desired amount of gain reduction.
· Adjust the threshold for the amount of gain reduction that you want. You should typically have 3 — 6 dB of reduction at the strongest part of the track, and there should be times when there is no gain reduction.
· Consider all rules carefully, then break them at will, and intentionally, anytime the music demands.

This is the text book approach for the most natural and least audibly conspicuous compression.

If you've achieved 6 dB of gain reduction, you're able to boost your overall level to tape by 6 dB over what it would have been without the compressor. With the entire track boosted, we can hear the nuances and softer passages more clearly. As an additional bonus, the complete track (including the soft passages) will be 6 dB further away from the noise floor than they were before compression, or the bit resolution will be increased.

Compressors are essential tools for making professional sounding audio recordings. If you are involved in audio for video and television, compressors are essential because of the limited dynamic range in these mediums.

How Much Is Enough
Though the musical and artistic needs of any recording must dictate the use and application of all available tools, there are some guidelines for most tasks which should be considered.

Compression and limiting are generally most effective when gain reduction occurs several times throughout a recording, yet most of the audio is left untouched—beneath the threshold. If the compressor/ limiter is always turning the signal down and back up again, optimum gain reduction is not being achieved; in addition, adverse side effects called pumping and breathing occur. For more information, see The S.M.A.R.T. Guide to Mixers, Signal Processors, Microphones, and More.


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