Editing Dynamic range compression (section)

== Controls and features ==
[[File:Compression ratio.svg|thumb|right|220px|Different compression ratios for a signal level above the threshold]]
A number of user-adjustable control parameters and features are used to adjust dynamic range compression signal processing algorithms and components.

=== Threshold ===
A compressor reduces the level of an audio signal if its amplitude exceeds a certain ''threshold''. Threshold is commonly set in [[decibels]] ([[dBFS]] for digital compressors and [[dBu]] for hardware compressors),<ref>{{Cite web |url=https://www.audiomasterclass.com/newsletter/audio-compressor-controls-the-threshold-control |title=Audio compressor controls: The threshold control |last=Mellor |first=David |date=2017-11-16 |website=Audio Masterclass |access-date=2019-07-31}}</ref> where a lower threshold (e.g.{{nbsp}}−60&nbsp;dB) means a larger portion of the signal is treated. When the signal level is below the threshold, no processing is performed and the input signal is passed, unmodified, to the output. Thus a higher threshold of, e.g.,{{nbsp}}−5&nbsp;dB, results in less processing, less compression.

Threshold timing behavior is subject to attack and release settings (see [[#Attack and release|below]]). When the signal level goes above threshold, compressor operation is delayed by the ''attack'' setting. For an amount of time determined by the ''release'' after the input signal has fallen below the threshold, the compressor continues to apply dynamic range compression.

=== Ratio ===
The amount of gain reduction is determined by '''ratio''': a ratio of 4:1 means that if input level is 4&nbsp;[[decibel|dB]] over the threshold, the output signal level is reduced to 1&nbsp;dB over the threshold. The gain and output level has been reduced by 3&nbsp;dB. Another way of stating this is that any input signal level over the threshold will, in this case, be output at a level which is only 25% {{nobreak|(i.e. 1 over 4)}} as much over the threshold as its input level was.

The highest ratio of <math>\infty</math>:1 is often known as ''limiting'', and effectively denotes that any signal above the threshold is brought down to the threshold level once the ''attack'' time has expired.

=== Attack and release ===
[[File:Audio Compression Attack and Release-2.svg|thumb|220px|The attack and release phases in a compressor]]
A compressor may provide a degree of control over how quickly it acts. The ''attack'' is the period when the compressor is decreasing gain in response to the increased level at the input to reach the gain determined by the ratio. The ''release'' is the period when the compressor is increasing gain in response to reduced level at the input to reach the output gain determined by the ratio, or, to unity, once the input level has fallen below the threshold. Because the loudness pattern of the source material is modified by the time-varying operation of compressor, it may change the character of the signal in subtle to quite noticeable ways depending on the attack and release settings used.

The length of each period is determined by the rate of change and the required change in gain. For more intuitive operation, a compressor's attack and release controls are labeled as a unit of time (often milliseconds). This is the amount of time it takes for the gain to change a set amount of dB or a set percentage towards the target gain. There is no industry standard for the exact meaning of these time parameters.<ref>{{Cite journal |last1=Jeffs |first1=Rick |last2=Holden |first2=Scott |last3=Bohn |first3=Dennis |date=September 2005 |title=Dynamics Processors – Technology & Applications |url=https://www.ranecommercial.com/kb_article.php?article=2129 |journal=RaneNote |publisher=[[Rane Corporation]] |issue=155 |pages=6–7 |access-date=2012-12-21 |quote=There is no industry standard and different manufacturers define [release time] differently.}}</ref>

In many compressors, the attack and release times are adjustable by the user. Some compressors, however, have the attack and release times determined by the circuit design and cannot be adjusted. Sometimes the attack and release times are ''automatic'' or ''program dependent'', meaning that the behavior may change depending on the input signal.

=== Soft and hard knees ===
[[File:Compression knee.svg|thumb|220px|right|Hard Knee and Soft Knee compression]]
Another control a compressor might offer is hard knee or soft knee selection.  This controls whether the bend in the response curve between below threshold and above threshold is abrupt (hard) or gradual (soft). A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user. A soft knee reduces the potentially audible transition from uncompressed to compressed, and is especially applicable for higher ratio settings where the changeover at the threshold would be more noticeable.<ref>{{Cite magazine |last=White |first=Paul |date=December 2000 |title=Advanced Compression Techniques |url=http://www.soundonsound.com/sos/dec00/articles/adcompression.htm |magazine=Sound On Sound |archive-url=https://web.archive.org/web/20150924122828/http://www.soundonsound.com/sos/dec00/articles/adcompression.htm |archive-date=2015-09-24 |url-status=dead}}</ref>

=== Peak vs RMS sensing ===
A peak-sensing compressor responds to the peak level of the input signal. While providing tighter peak level control, peak level sensing does not necessarily relate to human perception of loudness. Some compressors apply a power measurement function (commonly [[root mean square]] or RMS) on the input signal before comparing its level to the threshold. This produces a more relaxed compression that more closely relates to human perception of loudness.

=== Stereo linking ===
A compressor in ''stereo linking'' mode applies the same amount of gain reduction to both the left and right channels. This is done to prevent image shifting that can occur if each channel is compressed individually.<ref>{{Cite book |chapter-url=http://media.uaudio.com/support/manuals/v9100-vp4lQz6/UAD%20Plug-Ins%20Manual.pdf |title=UAD Plug-Ins Manual |publisher=Universal Audio |edition=190724 |pages=219–220 |chapter=Fairchild Tube Limiter Collection}}</ref>

=== Make-up gain ===
Because a downward compressor only reduces the level of the signal, the ability to add a fixed amount of ''make-up gain'' at the output is usually provided so that an optimum output level is produced.

=== 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 implemented by splitting the input signal and delaying one side (the audio signal) by the look-ahead time. The non-delayed side (the gain control 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 added [[audio latency]] through the processor.