Transcode
Encyclopedia
Transcoding is the direct digital-to-digital data conversion of one encoding
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...

 to another, such as for movie data files or audio files. This is usually done in cases where a target device (or workflow
Workflow
A workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...

) 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. Transcoding can be performed just while files are being searched, as well as for presentation. For example, Cineon
Cineon
Cineon System was a computer based system, integrating a film scanner and recorder hardware, digital software for compositing and effects, image restoration, color management and the development of a proprietary file format designed by Kodak. These components were created to support the work flow...

 and DPX
DPX
Digital Picture Exchange is a common file format for digital intermediate and visual effects work and is an ANSI/SMPTE standard...

 files have been widely used as a common format for digital cinema
Digital cinema
Digital cinema refers to the use of digital technology to distribute and project motion pictures. A movie can be distributed via hard drives, optical disks or satellite and projected using a digital projector instead of a conventional film projector...

, 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, trancoding 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
Generation loss
Generation loss refers to the loss of quality between subsequent copies or transcodes of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be considered a form of generation loss...

; however, transcoding can be lossless if the input is losslessly compressed and the output is either losslessly compressed or uncompressed. The process of lossy-to-lossy transcoding introduces varying degrees of generation loss
Generation loss
Generation loss refers to the loss of quality between subsequent copies or transcodes of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be considered a form of generation loss...

. In other cases, the transcoding of lossy to lossless or uncompressed is technically a lossless conversion because no information is lost, however the process is irreversible and is more suitably known as destructive.

Process

The most popular definition of transcoding refers to a two-step process in which the original data/file is decoded to an intermediate uncompressed format (i.e. PCM
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...

 for audio or YUV
YUV
YUV is a color space typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, thereby typically enabling transmission errors or compression artifacts to be more efficiently...

 for video), which is then encoded 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
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...

 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
Wine (software)
Wine is a free software application that aims to allow computer programs written for Microsoft Windows to run on Unix-like operating systems. Wine also provides a software library, known as Winelib, against which developers can compile Windows applications to help port them to Unix-like...

, is taking Direct3D
Direct3D
Direct3D is part of Microsoft's DirectX application programming interface . Direct3D is available for Microsoft Windows operating systems , and for other platforms through the open source software Wine. It is the base for the graphics API on the Xbox and Xbox 360 console systems...

 function calls and turning them into OpenGL
OpenGL
OpenGL is a standard specification defining a cross-language, cross-platform API for writing applications that produce 2D and 3D computer graphics. The interface consists of over 250 different function calls which can be used to draw complex three-dimensional scenes from simple primitives. OpenGL...

 calls.

Re-encoding/recoding

One may also re-encode data in the same format, for a number of reasons:
Editing: If one wishes to edit data in a compressed format (for instance, perform image editing
Image editing
Image editing encompasses the processes of altering images, whether they be digital photographs, traditional analog photographs, or illustrations. Traditional analog image editing is known as photo retouching, using tools such as an airbrush to modify photographs, or editing illustrations with any...

 on a JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....

 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.
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
Sample rate conversion
Sample rate conversion is the process of converting a signal from one sampling rate to another, while changing the information carried by the signal as little as possible...

, 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
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

 onto a Video CD
Video CD
Before the advent of DVD and Blu-ray, the Video CD became the first format for distributing films on standard 120 mm optical discs. The format is a standard digital format for storing video on a Compact Disc...

), or over a lower bandwidth channel.
Image scaling
Image scaling
In computer graphics, image scaling is the process of resizing a digital image. Scaling is a non-trivial process that involves a trade-off between efficiency, smoothness and sharpness. As the size of an image is increased, so the pixels which comprise the image become increasingly visible, making...

: 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
Bitrate peeling
Bitrate Peeling is a technique used in Ogg Vorbis audio encoded streams, wherein a stream can be encoded at one bitrate but can be served at that or any lower bitrate....

, that allow one to easily lower the bitrate without re-encoding, but quality is often lower than a re-encode. For example, in Vorbis
Vorbis
Vorbis is a free software / open source project headed by the Xiph.Org Foundation . The project produces an audio format specification and software implementation for lossy audio compression...

 bitrate peeling as of 2008, the quality is inferior to re-encoding.

Drawbacks

The key drawback of transcoding in lossy formats is decreased quality. Compression artifact
Compression artifact
A compression artifact is a noticeable distortion of media caused by the application of lossy data compression....

s are cumulative, so transcoding causes a progressive loss of quality with each successive generation, known as digital generation loss. For this reason, transcoding is generally discouraged unless unavoidable.

It is better to retain a copy in a lossless format (such as TTA
TTA (codec)
True Audio is a free software, real-time lossless audio codec, based on adaptive prognostic filters.Also, .tta is the generic extension to filenames of audio files created by True Audio codec.- Codec overview :...

, FLAC
FLAC
FLAC is a codec which allows digital audio to be losslessly compressed such that file size is reduced without any information being lost...

 or WavPack
WavPack
WavPack is a free, open source lossless audio compression format developed by David Bryant.-Features:WavPack compression can compress 8-, 16-, 24-, and 32-bit fixed-point, and 32-bit floating point audio files in the .WAV file format. It also supports surround sound streams and high frequency...

 for sound), and then encode directly from the lossless source file to the lossy formats required. For image
Image editing
Image editing encompasses the processes of altering images, whether they be digital photographs, traditional analog photographs, or illustrations. Traditional analog image editing is known as photo retouching, using tools such as an airbrush to modify photographs, or editing illustrations with any...

 and digital audio editing, one is advised to capture or save images in a raw
RAW image format
A camera raw image file contains minimally processed data from the image sensor of either a digital camera, image scanner, or motion picture film scanner. Raw files are so named because they are not yet processed and therefore are not ready to be printed or edited with a bitmap graphics editor...

 or uncompressed format and edit (a copy of) that version, only converting to lossy formats for distribution.

Usage

Although transcoding can be found in many areas of content adaptation, it is commonly used in the area of mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 content adaptation. In this case, transcoding is a must, due to the diversity of mobile devices and their capabilities. This diversity requires an intermediate state of content adaptation in order to make sure that the source content will adequately function on the target device to which it is sent.

One of the most popular technologies in which transcoding is used is the Multimedia Messaging Service
Multimedia Messaging Service
Multimedia Messaging Service, or MMS, is a standard way to send messages that include multimedia content to and from mobile phones. It extends the core SMS capability that allowed exchange of text messages only up to 160 characters in length.The most popular use is to send photographs from...

 (MMS), which is the technology used to send or receive messages with media (image, sound, text and video) between mobile phones. For example, when a camera phone is used to take a digital picture, a high-quality image of usually at least 640x480 pixels is created. When sending the image to another phone, this high resolution image might be transcoded to a lower resolution image with fewer colors in order to better fit the target device's screen size and color limitations. This size and color reduction improves the user experience on the target device, and is sometimes the only way for content to be sent between different mobile devices.

Transcoding is extensively used by Home theatre PC software to reduce the usage of disk space
Disk space
The term "disk space" is an amount of computer storage space on random-access memory devices, such as on a hard drive, floppy or USB flash drive. Disk space units are commonly measured in large numbers of bytes, such as kilobytes, megabytes, and gigabytes, with each unit 1024x times larger than the...

 by video files. The most common operation in this application is the transcoding of MPEG-2
MPEG-2
MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio data compression methods which permit storage and transmission of movies using currently available storage media and transmission...

 files to the MPEG-4
MPEG-4
MPEG-4 is a method of defining compression of audio and visual digital data. It was introduced in late 1998 and designated a standard for a group of audio and video coding formats and related technology agreed upon by the ISO/IEC Moving Picture Experts Group under the formal standard ISO/IEC...

 or H.264 formats.

Real-time transcoding in a many-to-many way (any input format to 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.

See also

  • Data conversion
    Data conversion
    Data conversion is the conversion of computer data from one format to another. Throughout a computer environment, data is encoded in a variety of ways. For example, computer hardware is built on the basis of certain standards, which requires that data contains, for example, parity bit checks....

  • Data migration
    Data migration
    Data migration is the process of transferring data between storage types, formats, or computer systems. Data migration is usually performed programmatically to achieve an automated migration, freeing up human resources from tedious tasks...

  • Data transformation
    Data transformation
    In metadata and data warehouse, a data transformation converts data from a source data format into destination data.Data transformation can be divided into two steps:...

  • Lossy data conversion
    Lossy data conversion
    A lossy data conversion method is one where converting data between one storage format and another displays data in a form that is "close enough" to be useful, but may differ in some ways from the original. This type of conversion is used frequently between software packages that rely on different...

  • Comparison of programming languages (basic instructions)#Data conversions
  • Base Station Subsystem: Transcoder
  • Video coding
  • Comparison of free video converters
    Comparison of free video converters
    Video converters are programs for converting files between different formats. Below is a comparison for four freeware video converters in the field of supported input files, supported output files, features, user interface, ready-made presets, technical help/support, tutorials and supported...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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