Look-ahead

The look-ahead function is designed to overcome the problem of being forced to compromise between slow attack rates that produce smooth-sounding gain changes, and fast attack rates capable of catching transients. Look-ahead is a misnomer in that the future is not actually observed. Instead, the input signal is split, and one side is delayed. The non-delayed signal is used to drive the compression of the delayed signal, which then appears at the output. This way a smooth-sounding slower attack rate can be used to catch transients. The cost of this solution is that the signal is delayed.

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Use

An audio engineer might use a compressor 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 else in order to change the character of an instrument being processed.

Engineers 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, they might choose a fast attack time and a moderately fast release time combined with a higher threshold. To accentuate the attack of the snare, they might choose a slower attack time to avoid affecting the initial transient. It is easier to apply these controls with a basic knowledge of acoustics.

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Limiting

Limiting and clipping compared. Note that clipping introduces a large amount of distortion whereas limiting only introduces a small amount while keeping the signal within the threshold.

Main article: Limiting

Compression and limiting are not different in process but in degree and perceived effect. A limiter is a compressor with a high ratio and, generally, a fast attack time. Most engineers consider compression with ratio of 10:1 or more as limiting.[10] Engineers sometimes refer to soft and hard limiting which are differences of degree. The "harder" a limiter, the higher its ratio and the faster its attack and release times.

Brick wall limiting has a very high ratio and a very fast attack time. Ideally, this ensures that an audio signal never exceeds the amplitude of the threshold. Ratios of 20:1 all the way up to ∞:1 are considered to be 'brick wall'.[10] The sonic results of more than momentary and infrequent hard/brick-wall limiting are harsh and unpleasant, thus it is more common as a safety device in live sound and broadcast applications.

Some modern consumer electronics devices incorporate limiters. Sony uses the Automatic Volume Limiter System (AVLS), on some audio products and the PlayStation Portable.

See also: Clipping (audio)

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Side-chaining

The sidechain of a feed-forward compressor

	Uplifting Trance Sidechain     MENU   0:00   The first 8 bars are without side-chaining applied, the second 8 are with side-chaining.
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When side-chaining, the compressor uses the volume level of an input signal to determine how strongly the compressor will reduce the gain on its output signal. This is used by disc jockeys for ducking – lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the compressor's sidechain so that whenever the DJ speaks the compressor reduces the volume of the music. A sidechain with equalization controls can be used to reduce the volume of signals that have a strong spectral content within a certain frequency range: it can act as a de-esser, reducing the level of vocal sibilance in the range of 6–9 kHz. A de-esser helps reduce high frequencies 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 without the bass drum causing undue peaks that result in loss of overall headroom.

A linked stereo compressor without a sidechain input can be used as a mono compressor with a sidechain input. The "key", "trigger" or sidechain signal is sent to the left input of the stereo compressor while the signal that is to be compressed is routed through the right channel.

This technique is not to be confused with Parallel compression, which was referred to as "side chain" compression in a 1977 Studio Sound article by Mike Bevelle.

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Parallel compression

One technique is to insert the compressor in a parallel signal path. This is known as parallel compression and can give a measure of dynamic control without significant audible side effects, if the ratio is relatively low and the compressor's sound is relatively neutral. On the other hand, a high compression ratio with significant audible artifacts can be chosen in one of the two parallel signal paths—this is used by some concert mixers and recording engineers as an artistic effect called New York compression or Motown compression. Combining a linear signal with a compressor and then reducing the output gain of the compression chain results in low-level detail enhancement without any peak reduction (since the compressor will significantly add to the combined gain at low levels only). This will often be beneficial when compressing transient content, since high-level dynamic liveliness is still maintained, despite the overall dynamic range reduction.

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Multiband compression

Multiband (also spelled multi-band) compressors can act differently on different frequency bands. The advantage of multiband compression over full-bandwidth (full-band, or single-band) compression is that unneeded audible gain changes or "pumping" in other frequency bands is not caused by changing signal levels in a single frequency band.

Multiband compressors work by first splitting the signal through some number of bandpass filters or crossover filters. The frequency ranges or crossover frequencies may be adjustable. Each split signal then passes through its own compressor and is independently adjustable for threshold, ratio, attack, and release. The signals are then recombined and an additional limiting circuit may be employed to ensure that the combined effects do not create unwanted peak levels.

Software plug-ins or DSP emulations of multiband compressors can be complex, with many bands, and require corresponding computing power.

Multiband compressors 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 artistic sense of style, plenty of time 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. A further development of this approach is programmable radio output processing, where the parameters of the multiband compressor automatically change between different settings according to the current programme block style or the time of day.

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Serial compression

Serial compression is a technique used in sound recording and mixing. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor will generally stabilize the dynamic range while the other will more aggressively compress stronger peaks. This is the normal internal signal routing in common combination devices marketed as "compressor-limiters", where an RMS compressor (for general gain control) would be directly followed by a fast peak sensing limiter (for overload protection). Done properly, even heavy serial compression can sound very natural in a way not possible with a single compressor. It is most often used to even out erratic vocals and guitars.

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Common uses

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