Sustainability of Digital Formats: Planning for Library of Congress Collections |
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Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact |
Full name | ITU-T (CCITT) T.6. Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus.Group 4 Compression (common name) |
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Description | Bitstream encoding format for still images that generally result from the scanning of paper documents. Offers lossless compression of bitonal (aka "bi-level") images, typically with compression ratios on the order of 15:1, using what is called Modified Modified READ (MMR; READ is Relative Element Address Designate codes). Developed to support the international use of FAX machines, i.e., the "Group 4 facsimile apparatus" for which T.6 images are encoded. |
Production phase | Used for initial-, middle- and final-state (end-user delivery) purposes. |
Relationship to other formats | |
Has earlier version | ITU-T (CCCITT) T.4 ("Group 3"), not documented here |
Used by | TIFF_G4, TIFF Bitmap with Group 4 Compression |
Used by | Other file or wrapper formats, not documented at this time |
LC experience or existing holdings | As TIFF_G4, used extensively for American Memory printed matter, typescripts, and other documents. |
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LC preference | See the Library of Congress Recommended Formats Statement for format preferences for still image works. |
Disclosure | Open standard. Developed by the ITU (International Telecommunication Union) in 1984-85 in the subsidiary group then called The International Telegraph and Telephone Consultative Committee (CCITT) and now known as ITU-T (International Telecommunication Union-Telecommunication Standardization Sector). |
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Documentation |
"Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus," Recommendation T.6, Volume VII, Fascicle VII.3, Terminal Equipment and Protocols for Telematic Services, The International Telegraph and Telephone Consultative Committee (CCITT), Geneva, Switzerland, 1985, pp. 48-57. [There may also be relevant information on pages 40-48, not available to the compiler of this document.] |
Adoption |
Widely adopted. Used by most FAX machines and as the encoding within the files produced by many document-imaging applications. Many software tools exist for encoding and decoding. Generally not natively supported by Web browsers. |
Licensing and patents | None |
Transparency | Depends upon algorithms and tools to read; requires sophistication to build tools. |
Self-documentation | None; this is provided by file wrappers like TIFF_6. |
External dependencies | None |
Technical protection considerations | None |
Still Image | |
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Normal rendering | Good support. |
Clarity (high image resolution) | Excellent, given that this is an encoding for bitonal (pure black and white) images. |
Color maintenance | Not applicable. |
Support for vector graphics, including graphic effects and typography | Not applicable. |
Support for multispectral bands | Bitonal (i.e., single-band) only. |
Functionality beyond normal rendering | None |
Tag | Value | Note |
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Filename extension | Not applicable. | |
Internet Media Type | Not applicable. | |
Magic numbers | Not applicable. | |
Pronom PUID | See note. | No PUID as of March 2023. |
Wikidata Title ID | Q26887334 |
See https://www.wikidata.org/wiki/Q26887334 |
General | |
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History |
From a page at the Web site for the Indian Institute of Technology, Kanpur, dated 1999: "Fax standards have emerged from analog transmission units (Groups 1 and 2) in the late sixties and early seventies to the digital transmission units (Groups 3 and 4) in the eighties. The Group 3 facsimile standard however, has been in wide use for two decades now." The sources cited in Useful references below provide the following outline:
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