NABTS
NABTS, the North American Broadcast Teletext Specification
History
NABTS was originally developed as a protocol by the Canadian
NABTS was the standard used for both
Description
In a normal NTSC video signal there are 525 "lines" of video signal. These are split into two half-images, known as "fields", sent every 60th of a second. These images merge on-screen, and in-eye, to form a single frame of video updated every 30th of a second. Each line of each field takes 63.5 μs to send; 50.3 μs of video and 13.2 μs amount of "dead time" on each end used to signal the television that the line is complete, known as the horizontal blanking interval (HBI). When the scanning process reaches the end of the screen it returns to the top during the vertical blanking interval (VBI), which, like the HBI requires some "dead time" to properly frame the signal on the screen. In this case the dead time is represented by unused lines of the picture signal, normally the top 22 lines of the frame.
NABTS encodes data into the video signal as a series of dots at a fixed rate of 5.7272 Mbit/s. Each line of a field has 50.3 μs of video area that can be used for transmission, which results in 288 bits per line, or 36 bytes. In NABTS, three bytes are used for hardware synchronization, another three for the packet address, two for sequencing information, leaving 28 for data and redundant forward error correction (FEC) information.[14]
In theory, the NABTS codes can be used on any of the 262 lines of the display, allowing up to 262 x 28 = 7,336 bytes of data per frame. Typically, however, the data is instead placed only in the unused lines of the vertical framing area. Lines 1–9 are used for vertical synchronization, line 21 is used for closed captioning, and everything after 22 is the television picture. That leaves 10 lines, lines 10 to 20, that are useful for sending data. At 60 fields per second, those 10 lines at 288 bits each encode a total of 172,800 bit/s, although 20% of that is needed for signaling purposes, so rates of 115,200 for end-user data are more typical. Applications requiring less throughput can simply use fewer lines.
Services using NABTS
- Telidon – videotex/teletext service developed by the Canadian Communications Research Centre (CRC)[4]
- ExtraVision – teletext service created and operated by CBS[5][6]
- NBC Teletext – teletext service provided by NBC[7][5][8][9][6]
- Time Teletext – operated by the Time Video Information Services division of Time, Inc.[11][12]
See also
- Antiope – French teletext standard (CCIR Teletext System A)
- World System Teletext – European Teletext Specification (CCIR Teletext System B)
- JTES – Japanese Teletext Specification (CCIR Teletext System D)
- NAPLPS – North American Presentation Level Protocol Syntax
- Teletext character set
- Text semigraphics
References
- ^ Astle, B. (September 1983). RCA Engineer – Teletext standards in North America (PDF). RCA Corporation. p. 15.
- ^ "Recommendation ITU-R BT.653-3 (02/1998) Teletext systems" (PDF). itu.int.
- ^ "Norpak – TES3/NABTS". October 10, 2006. Archived from the original on 10 October 2006.
- ^ doi:10.22230/cjc.1990v15n2a549 (inactive 31 January 2024).)
{{cite book}}
: CS1 maint: DOI inactive as of January 2024 (link - ^ a b c d "mb21 – ether.net – The Teletext Museum – World". teletext.mb21.co.uk. Retrieved 2022-12-15.
- ^ a b c d Enterprise, I. D. G. (1983-04-25). Computerworld. IDG Enterprise.
- ^ a b "NBC Teletext | UX | Portfolio | Videotex, Prototyping, Thought Leadership, Website Design, Consumer". www.jcvtcs.com. Retrieved 2022-12-15.
- ^ ISBN 978-0-934223-64-5.
- ^ a b Technology, United States Congress House Committee on Science and Technology Subcommittee on Science, Research, and (1984). Developing Technologies for Television Captioning: Benefits for the Hearing Impaired : Hearing Before the Subcommittee on Science, Research, and Technology of the Committee on Science and Technology, U.S. House of Representatives, Ninety-eighth Congress, First Session, November 9, 1983. U.S. Government Printing Office.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - ISSN 0362-4331. Retrieved 2022-12-15.
- ^ a b "Teletext News Briefs" (PDF). World Radio History. March 1983. Retrieved 4 August 2023.
- ^ a b Technology, United States Congress House Committee on Science and Technology Subcommittee on Science, Research, and (1984). Developing Technologies for Television Captioning: Benefits for the Hearing Impaired : Hearing Before the Subcommittee on Science, Research, and Technology of the Committee on Science and Technology, U.S. House of Representatives, Ninety-eighth Congress, First Session, November 9, 1983. U.S. Government Printing Office.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - ^ "Intercast", The Free Dictionary, retrieved 2022-12-15
- ^ "Norpak – NABTS Data Throughput". October 20, 2006. Archived from the original on 20 October 2006.