Dynamic range compression: Difference between revisions

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{{Use American English|date=June 2020}}

[[File:Comp. rack (Supernatural).jpg|thumb|150px|A rack of audio compressors in a recording studio. From top to bottom: Retro Instruments/Gates STA level; Spectra Sonic 610; [[Dbx (noise reduction)|Dbx]] 162; Dbx 165; [[Empirical Labs Distressor]]; Smart Research C2; Chandler Limited TG1; Daking FET (91579); and [[Altec Lansing|Altec]] 436c.]]

'''Dynamic range compression''' ('''DRC''') or simply '''compression''' is an [[audio signal processing]] operation that reduces the volume of loud [[sound]]s or amplifies quiet sounds, thus reducing or ''compressing'' an [[audio signal]]'s [[dynamic range]]. Compression is commonly used in [[sound recording and reproduction]], [[broadcasting]],<ref name="Follansbee">{{Cite book |url=https://books.google.com/books?id=gEN1GbEgb5AC |title=Hands-on Guide to Streaming Media: An Introduction to Delivering On-Demand Media |last=Follansbee |first=Joe |publisher=Focal Press |year=2006 |isbn=9780240808635 |edition=1 |page=84 |oclc=1003326401 |via=Google Books}}</ref> [[sound reinforcement system|live sound reinforcement]] and in some [[instrument amplifier]]s.

A dedicated electronic hardware unit or audio software that applies compression is called a '''compressor'''. In the 2000s, compressors became available as software plugins that run in [[digital audio workstation]] software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and [[limiter|limiting]] are identical in process but different in degree and perceived effect. A limiter is a compressor with a high [[#Ratio|ratio]] and, generally, a short [[#Attack and release|attack time]].

Compression is used to improve performance and clarity in [[public address system]]s, as an [[Effects unit|effect]] and to improve consistency in [[Audio mixing (recorded music)|mixing]] and [[Mastering (audio)|mastering]]. It is used on voice to reduce sibilance and in [[broadcasting]] and [[advertising]] to make an audio program stand out. It is an integral technology in some [[noise reduction]] systems.

== Types ==

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| caption2 = Upward compression

}}

There are two types of compression,: downward and upward. Both downwardtypes and upwardof compression ''reduce'' the [[Dynamic range#Audio|dynamic range]] of an audio signal.<ref name="Reese">{{Cite book|last1=Reese|first1=David E|url=https://archive.org/details/audioproductionw0000rees|title=Audio Production Worktext: Concepts, Techniques, and Equipment|last2=Gross|first2=Lynne S|last3=Gross|first3=Brian|publisher=Focal Press|year=2009|isbn=978-0-240-81098-0|pages=[https://archive.org/details/audioproductionw0000rees/page/149 149]|oclc=1011721139|url-access=registration|via=Internet Archive}}</ref>

''Downward'' compression reduces the volume of loud sounds ''above'' a certain threshold. The quiet sounds ''below'' the threshold remain unaffected. This is the most common type of compressor. A [[limiter]] can be thought of as an extreme form of downward compression as it compresses the sounds over the threshold especially hard.

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Some compressors also have the ability to do the ''opposite'' of compression, namely ''expansion''. Expansion ''increases'' the dynamic range of the audio signal.<ref>{{Cite web |url=https://ccrma.stanford.edu/courses/192b/192b_Lecture_1_18.pdf |title=Dynamic Range Processing and Digital Effects |last=Kadis |first=Jay }}{{Cite web |url=https://ccrma.stanford.edu/courses/192b/192b_Lecture_1_18.pdf |title=Dynamic Range Processing and Digital Effects |last=Kadis |first=Jay }}</ref> Like compression, expansion comes in two types, downward and upward.

''Downward'' expansion makemakes the quiet sounds below the threshold even quieter. A [[noise gate]] can be thought of as an extreme form of downward expansion as the noise gate make the quiet sounds (for instance: noise) quieter or even silent, depending on the floor setting.<ref name="Reese" />

''Upward'' expansion makemakes the louder sounds above the threshold even louder.

{{Clear}}

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The signal entering a compressor is split; one copy is sent to a [[variable-gain amplifier]] and the other to a ''side-chain'' where the signal level is measured and a circuit controlled by the measured signal level applies the required gain to the amplifier. This design, known as a ''feed-forward'' type, is used today in most compressors. Earlier designs were based on a ''feedback'' layout where the signal level was measured after the amplifier.<ref>{{Cite journal |last1=Giannoulis |first1=Dimitrios |last2=Massberg |first2=Michael |last3=Reiss |first3=Joshua D. |date=2012-07-09 |title=Digital Dynamic Range Compressor Design—A Tutorial and Analysis |url=https://www.eecs.qmul.ac.uk/~josh/documents/2012/GiannoulisMassbergReiss-dynamicrangecompression-JAES2012.pdf |journal=Journal of the Audio Engineering Society |language=en |volume=60 |issue=6 |pages=399–408 |citeseerx=10.1.1.260.1340 |access-date=2019-06-06}}</ref>

There are a number of technologies used for variable-gain amplification, each having different advantages and disadvantages. [[Vacuum tube]]s are used in a configuration called ''variable-mu'' where the grid-to-cathode voltage changes to alter the gain.<ref>{{Cite web |url=http://www.tangible-technology.com/dynamics/comp_lim_ec_dh_pw2.html |title=An Overview of Compressor/Limiters and Their Guts |last1=Ciletti |first1=Eddie |last2=Hill |first2=David |date=2008-04-19 |website=www.tangible-technology.com |access-date=2019-11-03 |last3=Wolff |first3=Paul}}</ref> Optical compressors use a [[photoresistor]] stimulated by a small lamp ([[Incandescent light bulb|incandescent]], [[Light-emitting diode|LED]], or [[Electroluminescence|electroluminescent panel]])<ref>{{Cite web |url=http://media.uaudio.com/assetlibrary/l/a/la-2a_manual.pdf |title=Model LA-2A Leveling Amplifier |website=Universal Audio |type=Manual }}</ref> to create changes in signal gain. Other technologies used include [[field effect transistor]]s and a [[diode bridge]].<ref>{{Cite magazine |last=Berners |first=Dave |date=April 2006 |title=Compression Technology and Topology |url=https://www.uaudio.com/webzine/2006/april/text/content4.html |department=Analog Obsession |magazine=Universal Audio WebZine |publisher=Universal Audio |volume=4 |issue=3 |access-date=2016-08-29}}</ref>

When working with digital audio, [[digital signal processing]] (DSP) techniques are commonly used to implement compression as [[audio plug-in]]s, in [[mixing console]]s, and in [[digital audio workstation]]s. Often the algorithms are used to emulate the above analog technologies.{{citation needed|date=May 2019}}

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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 ∞{{hsp}}<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 ===

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=== 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 Thisbook becomes|chapter-url=http://media.uaudio.com/support/manuals/v9100-vp4lQz6/UAD%20Plug-Ins%20Manual.pdf particularly|title=UAD noticeablePlug-Ins whenManual a|publisher=Universal loudAudio element|edition=190724 that|pages=219–220 is|chapter=Fairchild pannedTube closeLimiter to either edge of the stereo field raises the level of the program to the compressor's threshold, causing its image to shift toward the center of the stereo field.Collection}}</ref>

Stereo linking can be achieved in two ways: The compressor uses the sum of the left and right inputs to produce a single measurement that drives the compressor; or, the compressor calculates the required amount of gain reduction independently for each channel and then applies the highest amount of gain reduction to both (in such case it could still make sense to dial different settings on left and right channels as one might wish to have less compression for left-side events<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 ===

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=== Voice ===

A compressor can be used to reduce [[Sibilant consonant|sibilance]] ('ess' sounds) in vocals ([[de-essing]]) by feeding the compressor's side-chain an [[Equalization (audio)|equalized]] version of the input signal, so that specific, sibilance-related frequencies (typically 4000 to 8000 hz) activate the compressor more.<ref>{{cite magazine|url=https://www.soundonsound.com/techniques/techniques-vocal-de-essing|title=Techniques For Vocal De-essing|magazine=Sound on Sound|date=May 2009|access-date=12 May 2010}}</ref>

Compression is used in voice communications in [[amateur radio]] that employ [[single-sideband modulation|single-sideband (SSB) modulation]] to make a particular station's signal more readable to a distant station, or to make one's station's transmitted signal stand out against others. This is applicable especially in [[DXing]]. An SSB signal's strength depends on the level of [[modulation]]. A compressor increases the average level of the modulation signal thus increasing the transmitted signal strength. Most modern amateur radio SSB transceivers have speech compressors built-in. Compression is also used in [[Two-way radio|land mobile radio]], especially in transmitted audio of professional [[walkie-talkie]]s and [[Tone remote|remote control dispatch consoles]].<ref>{{Cite book |title=HF radio systems & circuits |publisher=Noble Pub |year=1998 |isbn=9781613530740 |editor-last=Sabin |editor-first=William E. |edition=2nd |___location=Atlanta |pages=13–25, 271–290 |oclc=842936687 |editor-last2=Schoenike |editor-first2=Edgar O.}}</ref>

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[[Hearing aid]]s use a compressor to bring the audio volume into the listener's hearing range. To help the patient perceive the direction sound comes from, some hearing aids use [[binaural recording|binaural]] compression.<ref>{{Cite book |title=The textbook of hearing aid amplification |last=Sandlin |first=Robert E. |date=2000 |publisher=Singular Thomson Learning |isbn=1565939972 |edition=2nd |___location=San Diego, California |oclc=42475568}}</ref>

Compressors are also used for [[Hearing protectors|hearing protection]] in some electronic active hearing protection [[earmuffs]] and [[earplugs]], to let sounds at ordinary volumes be heard normally while attenuating louder sounds, possibly also amplifying softer sounds. This allows, for example, shooters wearing hearing protection at a shooting range to converse normally, while sharply attenuating the much louder sounds of the gunshots,<ref>{{cite web |url=https://www.pewpewtactical.com/best-shooting-ear-protection/ |title=10 Best Shooting Ear Protection |date=27 May 2020 |quote=They are comfy for hours with their gel caps, have easily accessible button controls, great sound cutoff and compression, and allow for earplugs if the decent 22db NRR doesn’t cut it. |access-date=2021-05-25}}</ref> and similarly for musicians to hear quiet music but be protected from loud noises such as drums or cymbal crashes.{{citation needed|date=December 2013}}

In applications of machine learning where an algorithm is training on audio samples, dynamic range compression is a way to augment samples for a larger data set.<ref>{{Cite journal |last1=Salamon |first1=Justin |last2=Bello |first2=Juan Pablo |date=March 2017 |title=Deep Convolutional Neural Networks and Data Augmentation for Environmental Sound Classification |journal=IEEE Signal Processing Letters |volume=24 |issue=3 |pages=279–283 |arxiv=1608.04363 |doi=10.1109/LSP.2017.2657381 |bibcode=2017ISPL...24..279S |s2cid=3537408 |issn=1070-9908 }}</ref>

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{{listen | filename = Uplifting Trance Sidechain.ogg| title = Uplifting Trance Sidechain| description = The first 8 bars are without side-chaining applied, the second 8 are with side-chaining. | format = [[Ogg]]}}

A compressor with a side-chain input controls gain from main input to output based on the level of the signal at the side-chain input.<ref name="Colletti">{{Cite web |url=http://www.sonicscoop.com/2013/06/27/beyond-the-basics-sidechain-compression/ |title=Beyond The Basics: Sidechain Compression |last=Colletti |first=Justin |date=2013-06-27 |website=SonicScoop |access-date=2015-03-16}}</ref> An early innovator of side-chain compression in an effects unit was the [[Eventide, Inc|Eventide]] Omnipressor from 1974.<ref>{{cite web |title=50th Flashback #3: The Omnipressor |url=https://www.eventideaudio.com/blog/aagnello/50th-flashback-3-omnipressor |website=Eventide Audio |date=10 March 2021 |access-date=17 May 2021}}</ref> With side-chaining, the compressor behaves in the conventional manner when both main and side-chain inputs are supplied with the same signal.

The side-chain input is used by [[disc jockey]]s for [[ducking]] – lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the side-chain input so that whenever the DJ speaks the compressor reduces the volume of the music.

A sidechain with [[equalization (audio)|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&nbsp;kHz.<ref>{{Cite magazine |last=Senior |first=Mike |date=May 2009 |title=Techniques For Vocal De-essing |url=http://www.soundonsound.com/sos/may09/articles/deessing.htm |department=Sound Advice |magazine=Sound on Sound |access-date=2015-03-16}}</ref> 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 (audio signal processing)|headroom]].<ref name="Colletti" />

==Parallel compression==

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In music production, 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.

The TC Electronic Finalizer included a three band compressor and was a popular [[audio mastering]] tool around year 2000.<ref>{{citation |url=https://www.soundonsound.com/reviews/tc-electronic-finalizer-96k |title=TC Electronic Finalizer 96K |access-date=2023-12-29}}</ref>

On-air signal chains of [[radio station]]s commonly use multiband compressors to increase [[loudness]] while avoiding [[overmodulation]]. Having a louder sound is often considered an advantage in commercial broadcasting.

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==Serial compression==

Serial compression is a technique used in [[Sound recording and reproduction|sound recording]] and [[Audio mixing (recorded music)|mixing]]. Serial compression is achieved by using two fairly different compressors in a signal chain. One compressor generally stabilizes the [[dynamic range]] while the other aggressively compresses 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) is followed by a fast peak-sensing limiter (for overload protection). Done properly, even heavy serial compression can sound natural in a way not possible with a single compressor. It is most often used to even out erratic [[vocal]]s and [[guitar]]s.

== Software audio players ==

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== Objective influence on the signal ==

In an article published in January 2014 by the ''Journal of the Audio Engineering Society'', Emmanuel Deruty and Damien Tardieu performed a systematic study describing the influence of compressors and brickwall limiters on the musical audio signal. The experiment involved four software limiters: Waves L2, Sonnox Oxford Limiter, Thomas Mundt’sMundt's Loudmax, Blue Cat’sCat's Protector, as well as four software compressors: Waves H-Comp, Sonnox Oxford Dynamics, Sonalksis SV-3157, and URS 1970. The study provides objective data on what limiters and compressors do to the audio signal.<ref name="DerutyTardieuAES">{{Cite journal |last1=Deruty |first1=Emmanuel |last2=Tardieu |first2=Damien |date=2014-02-03 |title=About Dynamic Processing in Mainstream Music |journal=Journal of the Audio Engineering Society |language=en |volume=62 |issue=1/2 |pages=42–55 |doi=10.17743/jaes.2014.0001 }}</ref>

Five signal descriptors were considered: [[RMS power]], [[EBU R 128]] integrated loudness,<ref name="EBU3341" /> [[crest factor]], R 128 LRA,<ref name="EBU3342" /> and density of clipped samples. RMS power accounts for the signal's physical level, R 128 loudness for the perceived level.<ref name="EBU3341" /> The crest factor, which is the difference between the signal's peak and its average power,<ref name="DerutyTardieuAES" /> is on occasions considered as a basis for the measure of micro-dynamics, for instance in the ''TT Dynamic Range Meter'' plug-in.<ref>{{Cite conference |last=Vickers |first=Earl |date=4–7 November 2010 |title=The Loudness War: Background, Speculation and Recommendations |url=http://www.sfxmachine.com/docs/loudnesswar/loudness_war.pdf |conference=129th AES Convention |___location=San Francisco |publisher=[[Audio Engineering Society]] |access-date=July 14, 2011}}</ref> Finally, R 128 LRA has been repeatedly considered as a measure of macro-dynamics or dynamics in the musical sense.<ref name="DerutyTardieuAES" /><ref name="SOS_Dynamic_Range">{{Cite magazine |last=Deruty |first=Emmanuel |date=September 2011 |title='Dynamic Range' & The Loudness War |url=https://www.soundonsound.com/sound-advice/dynamic-range-loudness-war |magazine=Sound on Sound |access-date=2013-10-24}}</ref><ref>{{Cite journal |last1=Serrà |first1=J |last2=Corral |first2=A |last3=Boguñá |first3=M |last4=Haro |first4=M |last5=Arcos |first5=JL |date=26 July 2012 |title=Measuring the Evolution of Contemporary Western Popular Music |journal=Scientific Reports |volume=2 |pagearticle-number=521 |arxiv=1205.5651 |bibcode=2012NatSR...2E2..521S |doi=10.1038/srep00521 |pmc=3405292 |pmid=22837813 }}</ref><ref>{{Cite journal |last1=Hjortkjær |first1=Jens |last2=Walther-Hansen |first2=Mads |date=2014-02-03 |title=Perceptual Effects of Dynamic Range Compression in Popular Music Recordings |journal=Journal of the Audio Engineering Society |volume=62 |issue=1/2 |pages=37–41 |doi=10.17743/jaes.2014.0003 }}</ref><ref>{{Cite journal |last=Skovenborg |first=Esben |date=2012-04-26 |title=Loudness Range (LRA) - Design and Evaluation |url=http://www.aes.org/e-lib/browse.cfm?elib=16254 |language=en |publisher=Audio Engineering Society |url-access=subscription |access-date=2019-11-04 |via=AES E-Library}}</ref>

=== Limiters ===

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In other words, slow-attack compressors decrease both physical and perceptual levels, decrease macro-dynamics, but have no influence on crest factor and clipped sample density.

Bulgarian sound expert Petko Mukelov have created a purely analog Audio Compressor Circuit with versatile controllable parameters, which put back to history any other compression algorithms and cemented Compression as a Main Studio Processing Unit.

== See also ==

* [[Squelch1176 Peak Limiter]]

* [[Automatic gain control]]

* [[Gain compression]], similar (but generally undesired) reductions in waveform gain, caused by amplifier circuit imperfections

* [[LA-2A Leveling Amplifier]]

* [[1176 Peak LimiterSquelch]]

* [[Tone mapping]], the photographic equivalent