Audio level compression

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Audio level compression is distinct from audio data compression, which is used to reduce the size of digital audio files.

Audio level compression, also called dynamic range compression, compression, or limiting, is a process that manipulates the dynamic range of an audio signal. Compression is used during sound recording, live sound reinforcement, and broadcasting to improve the perceived quality of audio. A compressor is the device used to create compression.

1 Controls
2 Limiting
3 Side-chaining
4 Multiband compression
5 Common uses
6 Underlying electronics
7 Other uses
8 See also
9 External links

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Controls

A compressor reduces the dynamic range of an audio signal if it becomes louder than 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 falls below the threshold, known as release. Because the loudness pattern of the source material is modified by the compressor it may change the character of the signal in subtle to quite noticeable ways depending on the settings used.

An audio engineer might use a compressor in a subtle way in order to reduce the dynamic range of source material in order to allow it to be recorded optimally on a medium with a more limited dynamic range than the source signal, or he might use a compressor in order to deliberately change the character of an instrument being processed.

An engineer wishing to achieve dynamic range reduction with few obvious effects might choose a relatively low threshold and low compression ratio so that the source material is being compressed very slightly most of the time. To deliberately soften the attack of a snare drum, he might choose a fast attack time and a moderately fast release time combined with a higher threshold. 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.

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Limiting

A limiter is a compressor with a stronger ratio, and generally a quicker attack time. There is no absolute consensus on what ratio constitutes limiting as compared with compression, but most engineers would consider anything with a ratio greater than 10:1 as limiting. Compression and limiting are no different in process, just in degree and in the perceived effect. Engineers sometimes refer to soft and hard limiting which are differences of degree. The "harder" a limiter, the lower its threshold and the higher its ratio.

Brick wall limiting effectively ensures that an audio signal never exceeds the amplitude threshold that is set. In practice, this is a ratio of 50:1 or greater. The sonic results of more than momentary and infrequent hard limiting are usually characterized as harsh and unpleasant; thus it is more appropriate as a safety device in live and broadcast applications than as a sound-sculpting tool. Template:See also

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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 disc jockeys to lower the music volume automatically when speaking, for example (known as ducking). A frequency-specific compressor (often known as a de-esser) may be constructed by feeding a high-pass filtered copy of the original signal into the side-chain input of the compressor so that volume reduction on the original signal only takes place when a specific frequency is present in large amounts (such as sibilant sounds that tend to overdrive preemphasized media such as phonograph records and FM radio). Another use of the side-chain in music production serves to maintain a loud bass track, while still keeping the bass out of the way of the bass drum when the drum hits.

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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. Hardware multiband compressors are also commonly used in the on-air signal chain of a radio station, either AM or FM, in order to increase the station´s apparent loudness without fear of overmodulation. Having a louder sound is often considered an advantage in commercial competition. However, adjusting a multiband output compressor of a radio station also requires some sense of style and a good pair of ears. This is because the constantly changing spectral balance between audio bands may have an equalizing effect on the output, by dynamically modifying the on-air frequency response.

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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 by way of the make up gain that can be applied after compression takes place (compression only reduces the volume of a signal). The peak signals and the non-peak signals become more similar in amplitude after compression and the non-peak signals become relatively louder through the make up gain that is applied to the entire signal after compression. In other words, the peak of a sound wave of, say, a drum hit may be at roughly the same amplitude (because of make up gain), but the body of 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 its peak may be at the same dynamic level it was before compression and make up gain were applied. It is common to compress rock music or pop music heavily to make it sound louder without introducing undesirable distortion that would be introduced by exceeding the dynamic limits of a reproduction device or medium. Compression is used extensively in broadcasting to boost the perceived volume of the sound track while keeping it within strict limits (broadcasters in most countries have legal limits on instantaneous peak volume they may broadcast). Normally these limits are met by permanently inserted hardware in the on-air chain (see section Multiband compression above).

Most television commercials are compressed heavily in order to achieve near-maximum perceived loudness while staying within permissible limits.

Compression can also be used on instrument sounds to create effects not-primarily focused on boosting loudness. For instance, drum and cymbal sounds tend to decay quickly, but a compressor can make the sound appear to have a more sustained tail. Guitar sounds are often compressed in order to obtain a fuller, more sustained sound.

Compression is also often used in music production to make performances more consistent in dynamic range so that they "sit" in the mix of other instruments better and maintain consistent attention from the listener. Vocal performances in rock music or pop music are usually compressed in order to make them stand out from the surrounding instruments.

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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.

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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.

Gain Pumping, where a regular amplitude peak (such as a kick drum) causes the rest of the mix to change in volume due to the compressor, is generally avoided in music production. However, many dance musicians use this phenomenon in a creative way, causing the mix to 'pump' rhythmically in time with the beat.

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