Content deleted Content added
→Stereo linking: Image only moves back to centre if you are not 100% one side and 0% the other. Tags: Mobile edit Mobile web edit |
Citation bot (talk | contribs) Added article-number. Removed parameters. Some additions/deletions were parameter name changes. | Use this bot. Report bugs. | Suggested by Abductive | Category:Audio engineering | #UCB_Category 13/238 |
||
| (37 intermediate revisions by 23 users not shown) | |||
Line 1:
{{Short description|Audio signal processing operation}}
{{About|a process that intentionally reduces the dynamic range of audio signals|similar reductions caused by circuit imperfections|Gain compression|processes that reduce the size of digital audio files|Data compression{{!}}Audio compression (data)}}
{{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.]]▼
▲[[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 162; Dbx 165; Empirical Labs Distressor; Smart Research C2; Chandler Limited TG1; Daking FET (91579); and 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
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 [[limiting]] are identical in process but different in degree and perceived effect. A
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 ==
{{multiple image
| align = left
| total_width = 500
| header = '''Two methods of dynamic range compression'''
Line 24 ⟶ 26:
| caption2 = Upward compression
}}
There are two types of compression
''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.
Line 32 ⟶ 34:
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
''Upward'' expansion
{{Clear}}
Line 42 ⟶ 44:
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]]
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}}
Line 58 ⟶ 60:
The amount of gain reduction is determined by '''ratio''': a ratio of 4:1 means that if input level is 4 [[decibel|dB]] over the threshold, the output signal level is reduced to 1 dB over the threshold. The gain and output level has been reduced by 3 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
=== 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>
Line 76 ⟶ 78:
=== 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
=== Make-up gain ===
Line 102:
=== Voice ===
A compressor can be used to reduce [[
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>
Line 111:
Broadcasters use compressors in order that their station sounds louder than comparable stations. The effect is to make the more heavily compressed station jump out at the listener at a given volume setting.<ref name=broadcastbridge>{{Cite web|url=https://www.thebroadcastbridge.com/content/entry/7278/using-compression-for-recorded-and-live-audio|title=Using Compression for Recorded and Live Audio - The Broadcast Bridge - Connecting IT to Broadcast|first=The Broadcast|last=Bridge|date=November 23, 2016|website=www.thebroadcastbridge.com}}</ref> This is not limited to inter-channel differences; they also exist between programme material within the same channel. Loudness differences are a frequent source of audience complaints, especially TV commercials and promos that seem too loud.
The [[European Broadcasting Union]] (EBU) has been addressing this issue in the EBU PLOUD group, which consists of over 240 audio professionals, many from broadcasters and equipment manufacturers. In 2010, the EBU published [[EBU R 128]] which introduces a new way of metering and [[Audio normalization|normalizing audio]]. The Recommendation uses [[ITU-R BS.1770]] loudness metering. {{as of|2016}}, several European TV stations have announced their support for the new norm<ref>{{Cite web|url=https://www.ebu.ch/news/2011/loudness-france-chooses-ebu-r128|title=Loudness: France chooses EBU R128 to bolster audio laws|date=25 October 2011|website=[[European Broadcasting Union]]|language=en|access-date=8 April 2020}}</ref><ref>{{Cite web|url=https://www.svgeurope.org/blog/headlines/sky-confirms-formal-adoption-of-r128-loudness-specification/|title=Sky confirms formal adoption of R128 loudness specification|last=Davies|first=David|date=9 December 2013|website=SVG Europe|language=en|access-date=8 April 2020}}</ref> and over 20 manufacturers have announced products supporting the new ''EBU Mode'' loudness meters.<ref name="EBU3341">{{Citation |url=https://tech.ebu.ch/publications/tech3341 |title='EBU Mode' metering to supplement EBU R 128 loudness normalisation |date=2016-01-25 |publisher=European Broadcasting Union |series=Version 3.0 |language=en |id=EBU Tech 3341 |access-date=2019-11-03}}</ref>{{failed verification|reason=Does not support
To help audio engineers understand what loudness range their material consists of (e.g. to check if some compression may be needed to fit it into the channel of a specific delivery platform), the EBU also introduced the ''Loudness Range'' (LRA) descriptor.<ref name="EBU3342">{{Citation |url=https://tech.ebu.ch/publications/tech3342 |title=Loudness range: A measure to supplement EBU R 128 loudness normalisation |date=2016-01-25 |publisher=European Broadcasting Union |series=Version 3.0 |___location=Geneva |language=en |id=EBU Tech 3342}}</ref>
=== Marketing ===
Most television commercials are heavily compressed to achieve near-maximum perceived loudness while staying within permissible limits. This causes a problem that TV viewers often notice: when a station switches from minimally compressed program material to a heavily compressed commercial, the volume sometimes seems to increase dramatically. Peak loudness might be the same—meeting the letter of the law—but high compression puts much more of the audio in the commercial at close to the maximum allowable, making the commercial seem much louder.<ref>
=== Over-usage ===
[[File:Cd loudness trend-something.gif|right|thumb|305px|The trend of increasing loudness as shown by waveform images of the song "[[Something (Beatles song)|Something]]" by [[The Beatles]] mastered on CD four times since 1983
{{See also|Loudness war}}
Line 133:
[[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 [[
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>
Line 150:
{{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 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==
== Multiband compression == <!-- [[Multiband compression]] redirects here -->
Line 163 ⟶ 167:
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.
==Serial compression==
Serial compression is a technique used in [[
== Software audio players ==
Some [[audio player (software)|software audio players]] support [[Plug-in (computing)|plugins]] that implement compression. These can increase loudness of audio tracks, or level out the volume of highly-variable music (such as [[classical music]], or a playlist that spans multiple music types). This improves the listenability of audio played through poor-quality speakers, or when played in noisy environments (such as in a car or during a party).
== Objective influence on the signal ==
In an article
Five signal descriptors were considered: [[RMS power]],
=== Limiters ===
Line 183 ⟶ 189:
The tested limiters had the following influence on the signal:
* increase of RMS power,
* increase of
* decrease of crest factor,
* decrease of
* increase of clipped sample density.
Line 192 ⟶ 198:
=== Compressors ===
As far as the compressors are concerned, the authors performed two processing sessions, using a fast attack (0.5
Set with a fast attack, the tested compressors had the following influence on the signal:
* slight increase of RMS power,
* slight increase of
* decrease of crest factor,
* decrease of
* slight decrease of clipped sample density.
Line 205 ⟶ 211:
Set with a slow attack, the tested compressors had the following influence on the signal:
* decrease of RMS power,
* decrease of
* no influence on crest factor,
* decrease of
* no influence on clipped sample density.
Line 213 ⟶ 219:
== See also ==
* [[
* [[Automatic gain control]]
* [[Gain compression]], similar (but generally undesired) reductions in waveform gain, caused by amplifier circuit imperfections
* [[LA-2A Leveling Amplifier]]
* [[
* [[Tone mapping]], the photographic equivalent
== References ==
Line 229 ⟶ 230:
== External links ==
*
* [https://web.archive.org/web/20070707155321/http://www-ccrma.stanford.edu/~jos/filters/Nonlinear_Filter_Example_Dynamic.html Dynamic range compression]
* [http://www.sweetwater.com/expert-center/techtips/d--03/20/2002 Article on Optical Compressors] from [[Sweetwater Sound]]
* [https://www.youtube.com/watch?v=STtSV-QHxVM Information on Compression in Home Recording]
| |||