Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | ISO/IEC 13818. Information technology -- Generic coding of moving pictures and associated audio information (formal name)MPEG-2 (common name) |
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| Description | Compression encoding for video and audio data multiplexed with signaling information in a serial bistream, identical to ITU-T (International Telecommunications Union-Telecommunication Standardization Sector) recommendation H.262. Initially developed to serve the transmission of compressed television programs via broadcast, cablecast, and satellite, and subsequently adopted for DVD production and for some online delivery systems. The term stream is used to name the format's sequence of encoded bytes. Elementary streams contain video, audio, or other data; a video elementary stream contains compressed video frames, 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. In many applications, the audio and video are multiplexed, thus combining the two elements. Packetized and multiplexed elementary streams may take the form of single-program program streams or be incorporated 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 | |
| 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. |
| Has subtype | MP3_ENC, MP3 audio encoding |
| Has subtype | AAC_MP2, Advanced Audio Coding (MPEG-2) |
| LC experience or existing holdings | MPEG-2 (specific profile unknown) is used as the "mezzanine" computer-file format for reformatted video recordings, e.g., from VHS tapes, in the American Folklife Center Veterans History Project. (The VHP preservation file follows the MXF-JPEG 2000 specifications employed by the Library's National Audio-Visual Conservation Center Packard Campus.) The mezzanine file serves as the source for streaming files placed online; in 2011 and 2012, the use of RealVideo is being replaced and by a low-bitrate version of MPEG- 4 AVC |
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| LC preference | For file-based compressed video, conform to or approximate MPEG-2_422 (4:2:2 Profile) at Main Level (aka MPEG-2 422@ML) or MPEG-2_MP (Main Profile) at Main Level (aka MPEG-2 MP@ML). Uncompressed or losslessly compressed copies are preferred to compressed (for future development). |
| Disclosure | Open standard. Developed by ISO technical program JTC 1/SC 29 (WG11), aka 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. |
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| 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. |
| Adoption |
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. MPEG-1 and MPEG-2 are recommended data formats in the list of FCLA recommended formats, although no profiles or levels are specified. 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/). 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. |
| Self-documentation |
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. |
| External dependencies | Playback of surround sound requires multiple loudspeakers. |
| Technical protection considerations | None |
| Moving Image | |
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| Normal rendering | Good support. |
| Clarity (high image resolution) | Moderate to very good, given that this is a format for 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. |
| Sound | |
| Normal rendering | Good support. |
| Fidelity (high audio resolution) | Moderate to very good, given that the options for audio employ compression. Widespread use is made of MPEG-2 layer 3 audio, aka MP3_ENC. Some producers make use of Advanced Audio Coding, aka AAC, considered to be superior to MP3 at a given bit rate. Producers are not limited to the audio structures inherent in MPEG-2. The Society of Motion Picture and Television Engineers (SMPTE) provides specifications for incorporating AES3_SMPTE data into the MPEG-2 transport stream. Among other things, this permits the use of AC-3 compressed audio (aka Dolby Digital). For all types of compressed audio, the quality of the outcome will depend on the type and extent of compression, and the encoder used. |
| Multiple channels | Multi-channel sound is supported in both the backwardly compatible (BC) audio structures shared with MPEG-1, which includes layer 3 audio (MP3_ENC), and in the non-backwardly compatible AAC audio structure. MPEG-2 BC can deliver five main channels and an optional LFE (Low Frequency Encoding or Effects) channel, i.e., 5.1 surround sound. AAC provides a capability of up to 48 main audio channels, 16 LFE channels, 16 overdub/multilingual channels, and 16 data streams. The use of AC-3 compressed audio (aka Dolby Digital), described above in connection with AES3_SMPTE data, provides another means for presenting surround sound. |
| Functionality beyond normal rendering | None |
| Tag | Value | Note |
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| Filename extension | mpg, mpeg |
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. |
| Filename extension | mod |
Reported to be used by the Panasonic SV-AV100 camcorder (from Shopping.com, as documented April 13, 2006) |
| Internet Media Type | video/mpeg |
From IETF (Internet Engineering Task Force) RFC 2046 |
| Internet Media Type | video/mpv video/mp2t video/mp1s video/mp2p |
From IETF (Internet Engineering Task Force) 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." |
| Internet Media Type | video/mpg video/x-mpg video/mpeg2 application/x-pn-mpg video/x-mpeg video/x-mpeg2a |
Additional examples selected from The File Extension Source. |
| Magic numbers | Hex: 00 00 01 Bx ASCII: .... |
From Gary Kessler's File Signatures Table. |
| General | 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." 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." |
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| 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
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