Transcoding is the direct digital-to-digital conversion of one encoding to another,such as for movie data files (e.g., PAL, SECAM, NTSC), 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, 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 terabytes (TB). That large size can increase the cost and difficulty of handling movie files. However, transcoding 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.
Transcoding is commonly a lossy process, introducing generation loss; however, transcoding can be lossless if the output is either losslessly compressed or uncompressed.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, the process is irreversible and is more correctly known as destructive.
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.
Transrating is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats;e 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.
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.