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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.
== 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 [[decibel|dB]] over the threshold, the gain will be reduced so that the output level will only be
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.
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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 '''"
== 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 it's 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
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 jockey|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 a mastering tool, but their inclusion in Digital Audio Workstatio plug-in sets is increasing their use among mix engineers.▼
▲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
== Common Uses ==▼
Compression is commonly used in [[television commercial|TV advertising]] to boost the perceived volume of the sound track. [[Television|TV]] broadcasters have limits on the instantaneous peak volume of the audio track in a broadcast rather than its [[root mean square]] volume over the entire 30 second commercial segment. By applying compression followed by gain compensation the overall volume of the track is increased without increasing its peak volume and the sound is perceived as louder.
== Underlying
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
A
== See also ==
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== External links ==
* [http://www.prometheusradio.org/compressors2.shtml I would have thought of a compressor set for hard limiting as a clipper]
* [http://home.btconnect.com/ssa/whitepaper/whitepaper.htm Description of expansion, compression, and limiting in an audio processor]
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