Transcoding

For the command-line audio/video transcoding tool, see Transcode (software).

Transcoding is the direct digital-to-digital conversion from one (usually lossy) codec to another. It involves traditionally 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. The simplest way to do transcoding is to decode a bitstream into YUV format using a compatible decoder and then encode the data using an encoder of a different standard. A better way to transcode is to change the bitstream format from one standard to another without its undergoing the complete decoding and encoding process. Many algorithms exist to achieve this.

Transrating is a process similar to transcoding in which files are coded to a lower bitrate without changing video formats. Need for transrating arises from the fact that the bitrate requirement varies from channel to channel because of vastness in the compression standards in use. Changing the picture size of video is known as transsizing.

Limitations

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 decode directly from the lossless source file to the lossy formats required.

Usage

Transcoding can be found in many areas of content adaptation, however it is commonly used in the area of mobile phone 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 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.

Transcoding is extensively used by Home theatre PC software, such as MythTV and Freevo to reduce disk space of video files. The most common operation in this application is the transcoding of MPEG-2 files to the MPEG-4 format.

On-the-fly or real-time transcoding in a many-to-many way (any input format into 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, this is considered the next killer application for the mobile devices.

References

• Federal Standard 1037C
• MIL-STD-188
• List of Portable Multimedia Software
• Huifang Sun, Xuemin Chen, and Tihao Chiang, Digital Video Transcoding for Transmission and Storage, New York, CRC Press, 2005.

See also

• Data conversion
• Data migration
• Data transformation
• Comparison of programming languages (basic instructions)#Data_conversions

External links

• Multi-Format Real-Time Transcoding Platform
• IDC Report on Video Transcoding
• 2007 IDC Report on On-Demand Video Transcoding