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|Full name||ISO/IEC 13818. Information technology -- Generic coding of moving pictures and associated audio information (formal name); MPEG-2 (common name)|
|Description||This description pertains to the video or picture encoding defined by the MPEG-2 family of specifications. The key techniques employed in the MPEG-2 codec include intra-frame Discrete Cosine Transform (DCT) coding and motion-compensated inter-frame prediction; See Notes for more detail. All MPEG-2 streams (picture, sound or data) are based on elementary streams; in the case of picture (video), the elementary stream contains compressed frame images, plus sequence headers, group-of-picture (GOP) headers, and other data needed to decode the stream. An elementary stream is broken up into packets of variable length, forming a packetized elementary stream (PES). Each PES packet includes a header. Picture and sound elementary streams are combined to produce single-program program streams, which may be further combined with other programs in a multi-program transport stream.|
|Production phase||Generally a final-state (end-user delivery) format; MPEG-2_MP, Main Profile, or MPEG-2_422, 4:2:2 Profile, may be used as middle-state (broadcast production) formats.|
|Relationship to other formats|
|Subtype of||MPEG-2_family, MPEG-2 Encoding Family|
|Has subtype||MPEG-2_SP, Simple Profile|
|Has subtype||MPEG-2_MP, Main Profile|
|Has subtype||MPEG-2_422, 4:2:2 Profile|
|Has subtype||Other MPEG-2 profiles not documented at this time: SNR Scalable, Spatially Scalable, and High.|
|Used by||MPEG-2 program stream or file with MPEG-2 video and AES3 Digital Audio Interface, SMPTE Extensions (Dolby and surround audio encoding). Not described at this Web site at this time.|
|Used by||MPEG-2_MP_layer_2, MPEG-2 File with Main Profile Video and Layer II Audio Encoding|
|Used by||MPEG-2_422_layer_2, MPEG-2 File with 422 Video and Layer II Audio Encoding|
|Used by||MPEG_layer_2_audio, MPEG-1 and MPEG-2 Layer II Audio Encoding|
|Used by||Matroska_MPEG-2, Matroska File Format with MPEG-2 Video Encoding (H.262)|
|LC experience or existing holdings||See MPEG-2_family|
|LC preference||See MPEG-2_family|
|Disclosure||Open standard. Developed by ISO technical program called the Motion Pictures Expert Group (MPEG), Coding of audio, picture, multimedia and hypermedia information. Developed in joint partnership with ITU-T, who published the "common text" as H. 262.|
|Documentation||ISO/IEC 13818; first approvals in 1994. Ten parts have been published; parts 1, 2, 3, and 7 are central. Part 2 concerns the coding and specifies profiles and levels. See list of ISO documents in Format specifications below.|
Widely adopted for filemaking, DVD disks, and other applications. Most significant is the format's required use in digital terrestrial broadcasting to homes in the United States and several other nations, as governed by the ATSC (Advanced Television Systems Committee) specifications. Many software tools exist for encoding and decoding.
Adoption is also indicated by the use of the underlying MPEG-2 encoding in other formats like VOB, MOD and tivo. Each of these formats is described in the Notes below.
Wide adoption may not extend to all profiles, levels, or parts of the standard. [Evidence in various documents suggests that the Simple, Main, and 4:2:2 profiles are the most widely adopted. Comments welcome]
|Licensing and patents||Managed by MPEG LA LLC (http://www.mpegla.com/main/default.aspx). Licenses pertain to tools and not to streams or files per se.|
|Transparency||Depends upon algorithms and tools to read; will require sophistication to build tools.|
Technical (coding) information is contained in the MPEG-2 bitstream in macroblock headers, slice headers, picture headers, Group of Picture (GOP) headers, and sequence headers. The sequence header provides data needed before decoding can begin, such as the size of the picture and the frame rate. Because it is so important, the sequence header is usually repeated as often as twice a second.
The lack of metadata of the type called bibliographic by librarians motivated the MPEG group to develop MPEG-7, a separately standardized structure for metadata to support discovery and other purposes.
|Technical protection considerations||MPEG-2 shares with MPEG-4 elements of a standardized Intellectual Property Management and Protection (IPMP) scheme; see Intellectual Property Management and Protection in MPEG Standards.|
|Normal rendering||Good support.|
|Clarity (high image resolution)||Moderate to very good, given that this is a format for lossy compression. Outcome will depend on the type and extent of compression, and the encoder used.|
|Functionality beyond normal rendering||The MPEG-2 transport stream permits the multiplexing of multiple programs.|
|There is no explicit MPEG-2 file format; MPEG-2 content "ready to be delivered" is exchanged in a de facto file format that may carry one of these extensions.|
|Internet Media Type||video/mpeg
||Registered with IANA [see http://www.iana.org/assignments/media-types/video] through RFC 2046 from IETF (Internet Engineering Task Force).|
|Internet Media Type||video/mpv
|Registered with IANA [see http://www.iana.org/assignments/media-types/video] through RFC 3555; the four subtypes are glossed as "MPEG-1 or -2 Elementary Streams," "MPEG-2 Transport Streams," "MPEG-1 Systems Streams," and "MPEG-2 Program Streams" from IETF (Internet Engineering Task Force).|
|Internet Media Type||video/mpg
|Additional examples selected from The File Extension Source.|
|Magic numbers||Hex: 00 00 01 Bx
|From Gary Kessler's File Signatures Table.|
Regarding MPEG-2 encoding: the following has been excerpted and paraphrased from http://www.bbc.co.uk/rd/pubs/papers/paper_14/paper_14.shtml:
One MPEG-2 coding element employs discrete cosine transform (DCT). Each frame is broken into small blocks (8 pixels by 8 lines) of each component of the picture. A two-dimensional DCT is performed on the small blocks to produce blocks of DCT coefficients; the magnitude of each DCT coefficient indicates the contribution of a particular combination of horizontal and vertical spatial frequencies to the original picture block.
A second coding element employs motion-compensated inter-frame prediction, a technique that exploits temporal redundancy by attempting to predict the frame to be coded from a previous 'reference' frame. The prediction cannot be based on a source picture because the prediction has to be repeatable in the decoder, where the source pictures are not available; consequently, the coder contains a local decoder which reconstructs pictures exactly as they would be in the decoder, from which predictions can be formed. The simplest inter-frame prediction of the block being coded would take the co-sited (i.e. the same spatial position) block from the reference picture. Motion-compensated inter-frame prediction, however, is more sophisticated and offsets any translational motion that has occurred between the block being coded and the reference frame.
In an MPEG-2 system, the DCT and motion-compensated interframe prediction are combined: the coder subtracts the motion-compensated prediction from the source picture to form a 'prediction error' picture. The prediction error is transformed with the DCT, the coefficients are quantized and these quantized values coded using a variable length code.
MPEG-2 defines three picture types: Intra pictures (I-frames) are coded without reference to other pictures. They are used periodically to provide access points in the bitstream where decoding can begin. Predictive pictures (P-frames) can use the previous I- or P-picture for motion compensation and may be used as a reference for further prediction. Bidirectionally-predictive' pictures (B-frames) can use the previous and next I- or P-pictures for motion-compensation, and offer the highest degree of compression. To enable backward prediction from a future frame, the coder reorders the pictures from natural 'display' order to 'bitstream' order so that the B-picture is transmitted after the previous and next pictures it references. This introduces a reordering delay dependent on the number of consecutive B-pictures. The different picture types typically occur in a repeating sequence, termed a 'Group of Pictures' or GOP. A typical GOP in display order is: B1 B2 I3 B4 B5 P6 B7 B8 P9 B10 B11 P12. For these, the corresponding bitstream order is: I3 B1 B2 P6 B4 B5 P9 B7 B8 P12 B10 B11.
Regarding the use of MPEG-2 encoding in other video formats:
VOB (DVD Video OBject) files are closely related to MPEG-2 files. VOB files are assembled by DVD producers, and they contain the actual Video, Audio, Subtitle, and Menu contents in stream form. The Wikipedia entry for VOB (as of November 28, 2006), reports that "VOB files are encoded very much like standard MPEG-2 files. When the extension is renamed from .vob to .mpg or .mpeg the file will still be readable and will continue to hold all information, although most players supporting MPEG-2 don't support subtitle tracks. In order to burn the VOB files to a DVD-R disc, other standard DVD-Video files are needed as well, including IFO and BUP files."
MOD is a file format used in JVC, Panasonic and Canon tapeless video cameras. According to the Wikipedia entry MOD files "are file-based formats that are stored on a random-access media. Standard definition video is stored in MPEG program stream container files with MOD extension; in most other systems these files have extension MPG or MPEG. MOD video can be viewed on a computer with a player that is capable of reproducing MPEG-2 video. This video can be easily authored for watching on a DVD player without recompression, because it is fully compliant with DVD-video standard." Most nonlinear editing software cannot natively ingest MOD files. These files must first be converted using transcoding software like FFmpeg.
From zatznotfunny: "TiVoToGo files, having the .tivo extension, are essentially encrypted and fingerprinted mpeg [MPEG-2] files. MPEG is a common video format. Your unique Media Access Key (MAK) is appended to all shows as text, and perhaps embedded as a watermark in other ways. The TiVo Desktop software includes a .dll that is used to decrypt, or unlock, your show using your MAK. TiVo files can be viewed, converted, edited, transferred, and/or burned."
|History||MPEG stands for the Motion Pictures Expert Group, which began developing video compression standards in the 1980s. The group was founded by two men described by one commentator as "the fiery Leonardo Chiariglione (CSELT, Italy)" and "the peaceful Hiroshi Yasuda (JVC, Japan)." MPEG's initial development (of MPEG-1) was partly inspired by the H.261 video coding standard published by the ITU (International Telecom Union).|
1 The italicized blurbs in the specifications section are derived from the Berkeley Multimedia Research Center. This link is to the Internet Archive's saved copy of this page: MPEG-2 FAQ