Transcoding is the direct digital-to-digital data conversion of one encoding to another,[1] such as for movie data files or audio files. 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,[1] or to convert incompatible or obsolete data to a better-supported or modern format. 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 terabytes (TB).[1] That large size can increase the cost and difficulty of handling movie files. However, trancoding into a JPEG2000 lossless format has better compression performance than other lossless coding technologies, and in many cases, JPEG2000 can compress images to half-size.[1]
Transcoding is commonly a lossy process, introducing generation loss; however, transcoding can be lossless if the input is losslessly compressed and the output is either losslessly compressed or uncompressed.[1] The process of lossy-to-lossy transcoding introduces varying degrees of generation loss. In other cases, the transcoding of lossy to lossless or uncompressed is technically a lossless conversion because no information is lost, however the process is irreversible and is more suitably known as destructive.
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The most popular definition of transcoding refers to a two-step process in which the original data/file is decoded to an intermediate uncompressed format (i.e. PCM for audio or YUV for video), which is then encoded into the target format.
Transcoding may also refer to the process of directly changing assembled software code to work on a different platform or operating system. While it is usually preferable to use source code and recompile the application, there are times when doing so is either impractical or impossible. This usually occurs when the source code is unavailable. One example, such as in the case of Wine, is taking Direct3D function calls and turning them into OpenGL calls.
One may also re-encode data in the same format, for a number of reasons:
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.
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 is generally discouraged unless unavoidable.
It is better to retain a copy in a lossless format (such as TTA, FLAC or WavPack for sound), and then encode directly from the lossless source file to the lossy formats required. For image and digital audio editing, one is advised to capture or save images in a raw or uncompressed format and edit (a copy of) that version, only converting to lossy formats for distribution.
Although transcoding can be found in many areas of content adaptation, it is commonly used in the area of mobile phone content adaptation. In this case, transcoding is a must, due to the diversity of mobile devices and their capabilities. This diversity requires an intermediate state of content adaptation in order to make sure that the source content will adequately function on the target device to which it is sent.
One of the most popular technologies in which transcoding is used is the Multimedia Messaging Service (MMS), which is the technology used to send or receive messages with media (image, sound, text and video) between mobile phones. For example, when a camera phone is used to take a digital picture, a high-quality image of usually at least 640x480 pixels is created. When sending the image to another phone, this high resolution image might be transcoded to a lower resolution image with fewer colors in order to better fit the target device's screen size and color limitations. This size and color reduction improves the user experience on the target device, and is sometimes the only way for content to be sent between different mobile devices.
Transcoding is extensively used by Home theatre PC software to reduce the usage of disk space by video files. The most common operation in this application is the transcoding of MPEG-2 files to the MPEG-4 or H.264 formats.
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.