Transcoding: Difference between revisions

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{{Short description|Direct digital-to-digital conversion of one encoding to another}}

{{Confuse|Transputer|Transpiler}}

{{Other uses|Transcode (disambiguation)|H.264/MPEG-4 AVC products and implementations#Transcoding}}

{{refimproveRefimprove|date=October 2015}}

'''Transcoding''' is the direct digital-to-digital conversion of one [[data compression|encoding]] to another,<ref>{{cite web|author1=Margaret Rouse|title=transcoding|url=http://searchmicroservices.techtarget.com/definition/transcoding|access-date=2018-01-14|archive-date=2018-01-14|archive-url=https://web.archive.org/web/20180114183759/http://searchmicroservices.techtarget.com/definition/transcoding|url-status=dead}}</ref> such as for [[movievideo]] data files, audio files (e.g., [[MP3]], [[WAV]]), or [[character encoding]] (e.g., [[UTF-8]], [[ISO/IEC 8859]]). This is usually done in cases where a target device (or [[workflow]]) does not support the format or has limited storage capacity that mandates a reduced file size,<ref name="SMP">

"Advancements in Compression and Transcoding: 2008 and Beyond",

[[Society of Motion Picture and Television Engineers]] (SMPTE),

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</ref> or to convert incompatible or obsolete data to a better-supported or modern format.

In the analog video world, transcoding can be performed just while files are being searched, as well as for presentation. For example, [[Cineon]] and [[DPX]] files have been widely used as a common format for [[digital cinema]], but the data size of a two-hour movie is about 8 [[terabytesterabyte]]s (TB).<ref name=SMP/> That large size can increase the cost and difficulty of handling movie files. However, transcoding into a [[JPEG2000]] lossless format has better [[data compression]] performance than other lossless coding technologies, and; in many cases, JPEG2000 can compress images to half- their original size.<ref name=SMP/>

Transcoding is commonly a [[lossy compression|lossy process]], introducing [[generation loss]]; however, transcoding can be lossless if the output is either losslessly compressed or uncompressed.<ref name=SMP/> The process of transcoding into a lossy format introduces varying degrees of [[generation loss]], while the transcoding from lossy to lossless or uncompressed is technically a lossless conversion because no information is lost; however, when the processconversion is irreversible, andit is then more correctly known as ''destructive''.

==Process==

Transcoding is a two-step process in which the original data is decoded to an intermediate uncompressed format (e.g., [[Pulse-code modulation|PCM]] for audio; [[YUV]] for video), which is then encoded into the target format.

==Re-encoding/recoding ==

One may also '''re-encode''' data in the same format, for a number of reasons:

;Editing: If one wishes to edit data in a compressed format (for instance, perform image editing on a [[JPEG]] image), one will generally decode it, edit it, then re-encode it. This re-encoding causes [[digital generation loss]]; thus if one wishes to edit a file repeatedly, one should only decode it ''once,'' and make all edits on that copy, rather than repeatedly re-encoding it. Similarly, if encoding to a lossy format is required, it should be deferred until the data is finalised, e.g. after mastering.

; Editing

;Lower bitrate: '''[[Transrating]]''' is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats;<ref name="Ryan">{{cite web|last1=Branson|first1=Ryan (6 July 2015)|title=Why is Bit Rate Important When Converting Videos to MP3?|url=http://converta2z.blogspot.in/2015/07/why-is-bit-rate-important-when.html|website=Online Video Converter|accessdate=10 August 2015}}</ref> this can include [[sample rate conversion]], but may use an identical sampling rate with higher compression. This allows one to fit given media into smaller storage space (for instance, fitting a [[DVD]] onto a [[Video CD]]), or over a lower bandwidth channel.

;Editing: If one wishes to edit data in a compressed format (for instance, perform image editing on a [[JPEG]] image), one will generally decode it, edit it, then re-encode it. This re-encoding causes [[digital generation loss]]; thus if one wishes to edit a file repeatedly, one should only decode it ''once,'', and make all edits on that copy, rather than repeatedly re-encoding it. Similarly, if encoding to a lossy format is required, it should be deferred until the data is finalised, e.g. after mastering.

;[[Image scaling]]: Changing the picture size of video is known as '''transsizing''', and is used if the output resolution differs from the resolution of the media. On a powerful enough device, image scaling can be done on playback, but it can also be done by re-encoding, particularly as part of transrating (such as a [[Downsampling|downsampled]] image requiring a lower bitrate).

; Lower bitrate

;Lower bitrate: '''[[Transrating]]''' is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats;<ref name="Ryan">{{cite web|last1last=Branson|first1first=Ryan (6 July 2015)|title=Why is Bit Rate Important When Converting Videos to MP3?|url=http://converta2z.blogspot.in/2015/07/why-is-bit-rate-important-when.html|website=Online Video Converter|accessdatedate=6 July 2015|access-date=10 August 2015|archive-date=9 August 2015|archive-url=https://web.archive.org/web/20150809101034/http://converta2z.blogspot.in/2015/07/why-is-bit-rate-important-when.html|url-status=live}}</ref> this can include [[sample rate conversion]], but may use an identical sampling rate with higher compression. This allows one to fit given media into smaller storage space (for instance, fitting a [[DVD]] onto a [[Video CD]]), or over a lower bandwidth channel.

; [[Image scaling]]

;[[Image scaling]]: Changing the picture size of video is known as '''transsizing''', and is used if the output resolution differs from the resolution of the media. On a powerful enough device, image scaling can be done on playback, but it can also be done by re-encoding, particularly as part of transrating (such as a [[Downsampling|downsampled]] image requiring a lower bitrate).

One can also use formats with [[bitrate peeling]], that allow one to easily lower the bitrate without re-encoding, but quality is often lower than a re-encode. For example, in [[Vorbis]] bitrate peeling as of 2008, the quality is inferior to re-encoding.

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The key drawback of transcoding in lossy formats is decreased quality. [[Compression artifact]]s are cumulative, so transcoding causes a progressive loss of quality with each successive generation, known as [[digital generation loss]]. For this reason, transcoding (in lossy formats) is generally discouraged unless unavoidable.

For users wanting to be able to re-encode audio into any format, and for [[digital audio editing]], it is best to retain a master copy in a [[data compression#Audio|lossless format]] (such as [[FLAC]], [[Apple Lossless|ALAC]], [[TTA (codec)|TTA]], [[WavPack]], and others) that take around half the storage space needed when compared to original uncompressed [[pulse-code modulation|PCM]] formats (such as [[WAV]], and [[Audio Interchange File Format|AIFF]]), as lossless formats usually have the added benefit of having [[meta data]] options, which are either completely missing or very limited in PCM formats. These lossless formats can be transcoded to PCM formats or transcoded directly from one lossless format to another lossless format, without any loss in quality. They can be transcoded into a lossy format, but these copies will then not be able to be transcoded into another format of any kind (PCM, lossless, or lossy) without a subsequent loss of quality.

For [[image editing]] users are advised to capture or save images in a [[raw image format|raw]] or uncompressed format, and then edit a copy of that master version, only converting to lossy formats if smaller file sized images are needed for final distribution. As with audio, transcoding from lossy format to another format of any type will result in a loss of quality.

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Real-time transcoding in a many-to-many way (any input format to any output format) is becoming a necessity to provide true search capability for any multimedia content on any mobile device, with over 500 million videos on the web and a plethora of mobile devices.

In 2020, Facebook AI Research has announced TransCoder<ref>{{Cite web|title=Pytholabs|url=https://pytholabs.com/blog_page/FACEBOOK%27S%20LATEST%20AI%20MODEL%20CAN%20CONVERT%20CODE%20FROM%20ONE%20LANGUAGE%20TO%20OTHER!|access-date=2020-07-28|website=pytholabs.com}}</ref>, a system that uses unsupervised deep-learning to convert code from one programming language to another. TransCoder was trained on more than 2.8 million open source projects and outperforms existing code translation systems that use rule-based methods.

==History==

Before the advent of semiconductors and integrated circuits, realtime resolution and frame rate transcoding between different [[Composite video|analog video]] standards was achieved by a [[Cathode -ray tube|CRT]]/[[Video camera tube|camera tube]] combination. The CRT part does not write onto a [[phosphor]], but onto a thin, dielectric target; the camera part reads the deposited charge pattern at a different scan rate from the back side of this target.<ref>{{cite web |url=http://www.mif.pg.gda.pl/homepages/frank/sheets/201/7/7828.pdf |publisher=General Electric Corporation |title= ''GEC 7828 Scan conversion tube'' data sheet |date=10 April 1961 |formataccess-date=PDF21 April 2017 |accessdatearchive-date=2126 April 2017 |archive-url=https://web.archive.org/web/20170426062159/http://www.mif.pg.gda.pl/homepages/frank/sheets/201/7/7828.pdf |url-status=live }}</ref> The setup could also be used as a [[genlock]].

==See also==

; Concepts

* [[Data conversion]]

* [[Data transformation]]

* [[Lossy data conversion]]

* Data transformation

* [[Type conversion]]

* [[Video coding]]

* [[Adaptive bitrate streaming]]

; Comparison

* [[Comparison of video converters]]

* [[Comparison of DVD ripper software]]

* [[Comparison of video converters]]

==Notes Citations ==

{{reflistReflist}}

== General and cited references ==

==References==

* [[Federal Standard 1037C]]

* [[MIL-STD-188]]

* [[List of Portable Multimedia Software]]

* P. A. A. Assuncao and M. Ghanbari, "[https://ieeexplore.ieee.org/document/736724 A frequency-___domain video transcoder for dynamic bit-rate reduction of MPEG-2 bit streams],", in ''IEEE Transactions on Circuits and Systems for Video Technology'', vol. 8, no. 8, pp. 953-967, Dec. 1998.

* Huifang Sun, Xuemin Chen, and Tihao Chiang, ''[https://www.routledge.com/Digital-Video-Transcoding-for-Transmission-and-Storage/Sun-Chiang-Chen/p/book/9780849316944 Digital Video Transcoding for Transmission and Storage]'', New York, CRC Press, 2005.

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{{Authority control}}

[[Category:Video conversion software]]

[[Category:Audio format converters| ]]

[[Category:Film and video technology]]

[[Category:Video conversion software]]