Audio level compression

From Academic Kids

Note: This article is about audio level compression, which reduces the dynamic range of audio signals. This should not be confused with audio data compression, which reduces the data size of digital audio signals.


The terms "compression" and "limiting", meaning audio level compression are used in the sound recording and live sound reinforcement fields. Compression is the broad term for a process whereby the dynamic range of an audio signal is manipulated. A compressor is the device use to create compression.

Contents

Controls

A compressor reduces the dynamic range of an audio signal if it passes a set threshold. The amount of gain reduction is usually determined by a ratio control. That is, with a ratio of 4:1, if the input level is 4 dB over the threshold, the gain will be reduced so that the output level will only be 1 dB over the threshold.

Compressors usually have controls to set how fast the compressor responds to changes in input level, known as attack, and how quickly the compressor returns to no gain reduction once the input level is below the threshold, known as release. These parameters can be adjusted for different effects.

An engineer wishing to soften the attack of a snare drum might choose a fast attack time and a moderately fast release time. To accentuate the attack of the snare, he might choose a slower attack time, to avoid affecting the initial transient. It is easier to successfully apply these controls if the user has a basic knowledge of musical instrument acoustics.

Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of make-up gain at the output is provided so that an optimum level can be used.

Limiting

A limiter is a compressor with a stronger ratio. Two commonly accepted definitions are that more than 4:1 or more than 8:1 ratios are limiting, anything less is compressing. They are no different in process. Soft and hard limiting are also differences of degree. The "harder" a limiter, the lower its threshold and the higher its ratio.

Brick wall limiting means that nothing passes the threshold. In practice, this is a ratio of 50:1 or possibly 100:1 or greater. The sonic results can be undesirable, unless applied to only the loudest transients; it is most appropriate as a safety device in live and broadcast applications.

Side-chaining

Some compressors implement side-chaining. This feature uses the dynamic level of another input to control the compression level of the signal. This is used by disk jockeys to lower the music volume automatically when speaking, for example (known as ducking). Another use is in music production, to maintain a loud bass track, while still keeping the bass out of the way of the bass drum when the drum hits.

Multiband compression

Multiband compressors are compressors that can act differently on different frequency bands. It is as if each band has its own compressor with its own threshold, ratio, attack, and release. They are primarily an audio mastering tool, but their inclusion in digital audio workstation plug-in sets is increasing their use among mix engineers.

Common uses

Compression is often used to make music sound louder without increasing its peak amplitude. Instead, it boosts the perceived loudness by increasing the root mean square volume. In other words, the peak of a sound wave of, say, a drum hit may be at roughly the same amplitude, but the sound is louder relative to the peak as the sound decays than it would be without compression. This reduction of the dynamics makes the drum sound louder even though it peaks at the same place. It is common to compress rock music or pop music heavily to make it sound louder without introducing undesirable distortion from higher amplitudes. This idea is also used in broadcasting to boost the perceived volume of the sound track, since broadcasters have limits on the instantaneous peak volume.

Compression can also be used on instrument sounds to give certain effects unrelated to boosting loudness. For instance, drum and cymbal sounds tend to decay quickly, but a compressor can make the sound appear to have more sustain.

Underlying electronics

A compressor accomplishes its task of reducing dynamic range by using a variable gain amplifier to reduce the gain of the signal. This is typically carried out in analog systems by using a voltage controlled amplifier which has its gain reduced as the power of the input signal increases. Optical compressors use a light sensitive diode to detect changes in signal gain, as expressed via a physical light (an optocoupler). This vintage technique is believed by some to add smoother characteristics to the signal, because the response times of the light and the diode soften the attack and release.

Other uses

A compressor is sometimes used to reduce the dynamic range of a signal for transmission, to be expanded afterwards. This reduces the effects of a channel with limited dynamic range. See Companding.

See also

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