For: Transcode (software)

Transcoding is the direct digital-to-digital conversion of one encoding to another. This is usually done to incompatible or obsolete data in order to convert it into a more suitable format. When transcoding one lossy file to another, the process almost always introduces generation loss.

Process

In true transcoding, the bitstream format of one file is changed from one to another without its undergoing another complete decoding and encoding process. This usually is possible if the source and target codecs are sufficiently similar. However, support for this process very much depends on the case.

The most popular method of transcoding is to decode the original data to a intermediate format (i.e. PCM for audio or YUV for video), in a way that still contains the content of the original, and then encoding the resulting file 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.

Re-encoding

One may also re-encode data in the same format (also called recode, as in Nero Recode). One may do this 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 decoding and re-encoding it.

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 the same sampling rate but 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 media's resolution. On a powerful enough computer, image scaling can be done on playback, but it can also be done by re-encoding, particularly as part of transrating (a downsampled image requiring a lower bitrate).

In theory, one can have formats with bitrate peeling, that allow one to easily lower the bitrate without re-encoding, but the benefits remain theoretical: in Vorbis bitrate peeling as of 2008, the quality is inferior to re-encoding.