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Transcode
From Wikipedia, the free encyclopedia
Transcoding is the direct digital-to-digital conversion from one (usually lossy) codec to another. It involves decoding/decompressing the original data to a raw intermediate format (i.e. PCM for audio or YUV for video), in a way that mimics standard playback of the lossy content, and then re-encoding this into the target format.
Transcoding can also refer to the encoding of files to a lower bitrate without changing video formats, a process that is also known as transrating.
Compression artifacts are cumulative, therefore transcoding between lossy codecs causes a progressive loss of quality with each successive generation. For this reason, it is generally discouraged unless unavoidable. For instance, if an individual owns a digital audio player that does not support a particular format (e.g., Apple iPod and Ogg Vorbis), then the only way for the owner to use content encoded in that format is to transcode it to a supported format. It is better to retain a copy in a lossless format (such as TTA, FLAC or WavPack), and then encode directly from the lossless source file to the lossy formats required.
Transcoding can be found in many areas of content adaptation however it is commonly used in the area of mobile phones content adaptation. In the world of mobile content, transcoding is a must due to the diversity of mobile devices. This diversity requires an intermediate state of content adaptation in order to make sure that the source content will adequately present on the target device it is sent to.
One of the most popular technologies in which transcoding is used is MMS (Multimedia Messaging System) which is the technology used to send or receive messages with media (Image, Sound, Text and Video) between mobile phones. For example, when using a camera phone to take a digital picture you are actually creating a high-resolution JPEG image, usually at least 640x480 with 24 bits of color. However when sending the image to another phone this high resolution image might be transcoded to a lower resolution image with less amount of color in order to better fit the target device's screen size and color limitation (e.g. 120x160 and 16 bits of color). This size and color reduction not only improves the user experience on the target device but is sometimes the only way for content to be sent between different mobile devices.
Transcoding may also refer to the process of directly changing assembled 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 this is either overkill or impossible, usually when the source code is unavailable. One example would be taking Direct3D function calls and turning them into OpenGL calls.