Process
Transcoding is a two-step process in which the original data is decoded to an intermediate uncompressed format (e.g., 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. ; Lower bitrate : Transrating is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats; 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). 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.Drawbacks
The key drawback of transcoding in lossy formats is decreased quality. Compression artifacts 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 lossless format (such as FLAC, ALAC, TTA, WavPack, and others) that take around half the storage space needed when compared to original uncompressed PCM formats (such as WAV, and 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 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. For video editing, (for video converting), images are normally compressed directly during the recording process due to the huge file sizes that would be created if they were not, and because the huge storage demands being too cumbersome for the user otherwise. However, the amount of compression used at the recording stage can be highly variable, and is dependent on a number of factors, including the quality of images being recorded (e.g. analog or digital, standard def. or high def., etc.), and type of equipment available to the user, which is often related to budget constraints – as highest quality digital video equipment, and storage space, may be expensive. Effectively this means that any transcoding will involve some cumulative image loss, and hence the most practical solution insofar as minimizing loss of quality is for the original recording to be deemed the master copy, and for desired subsequent transcoded versions, which will often be in a different format and smaller file size, to be transcoded only from that master copy.Usage
Although transcoding can be found in many areas of content adaptation, it is commonly used in the area ofHistory
Before the advent of semiconductors and integrated circuits, realtime resolution and frame rate transcoding between different analog video standards was achieved by a CRT/ 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. The setup could also be used as a genlock.See also
; Concepts * Data conversion * Data transformation * Lossy data conversion * Video coding * Adaptive bitrate streaming ; Comparison * Comparison of DVD ripper software * Comparison of video convertersCitations
General and cited references
* Federal Standard 1037C * MIL-STD-188 * List of Portable Multimedia Software * P. A. A. Assuncao and M. Ghanbari,External links