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ITU-T Group 4 FAX Compression (T.6)

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Table of Contents
Format Description Properties Explanation of format description terms

Identification and description Explanation of format description terms

Full name ITU-T (CCITT) T.6. Facsimile Coding Schemes and Coding Control Functions for Group 4 Facsimile Apparatus.Group 4 Compression (common name)
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

Local use Explanation of format description terms

LC experience or existing holdings As TIFF_G4, used extensively for American Memory printed matter, typescripts, and other documents.
LC preference Both TIFF_G4 and TIFF_UNC are preferred as master images for documents; TIFF_G4 offers some efficiencies in transmission and storage and may be preferred when source documents are largely typographic. Typical applications call for spatial resolution ranging from 200 to 600 dpi, depending on the type and value of original being scanned.

Sustainability factors Explanation of format description terms

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).
    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

Quality and functionality factors Explanation of format description terms

Still Image
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

File type signifiers Explanation of format description terms

Tag Value Note
Filename extension Not applicable.   
Internet Media Type Not applicable.   
Magic numbers Not applicable.   

Notes Explanation of format description terms

General  
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:

  • Group 1: 1968
  • Group 2: 1976
  • Group 3: 1980; uses Modified Huffman (MH) codes for one-dimensional compression and Modified READ (MR; READ is Relative Element Address Designate codes) for two-dimensional compression
  • Group 4: 1984; uses Modified Modified READ (MMR)

Format specifications Explanation of format description terms


Useful references

URLs

Books, articles, etc.

Last Updated: Tuesday, 10-Dec-2013 16:09:23 EST