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 recompressing files to a lower bitrate without changing formats.
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, Trasncoding is a must due to the diversity and large amount of different mobile devices. This diversity requires an intermediate state of content adaptation in order to make sure that the source content will adequatly present on the target device one sent to it.
For exapmle: When using your phone camera to take a digital picture you are actually creating a high resolution JPEG image, usually at least 640x480 with 24 bits of colors, however when sending it to another phone this high resolution image might be Transcoded to a lower resolution image with less amount of colors in order to better fit the target device screen size and color limitation, for exmplae 120x160 and 16 bit's of color. This size and color reduction does not only imporove the user experience on the target device but is sometimes the only way for content to be sent between different mobile devices.
One of the most popoular technologies in which Transcoding is used is MMS (Multimedia Messaging System) which is the technology used to send/recieve messages with media (Image, Sound, Text and Video) between mobile phones.
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