Transcoding: Difference between revisions

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{{Refimprove|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 [[video]] 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 [[Byte|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 conversion is irreversible, it is then more correctly known as ''destructive''.

==Process==

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: 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.

; 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|date=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]]

: 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 [[downsampled]] image requiring a lower bitrate).