NABTS, the North American Broadcast Teletext Specification, is a protocol used for encoding
NAPLPS
NAPLPS (North American Presentation Level Protocol Syntax) is a graphics language for use originally with videotex and teletext services. NAPLPS was developed from the Telidon system developed in Canada, with a small number of additions from AT&T ...
-encoded
teletext
A British Ceefax football index page from October 2009, showing the three-digit page numbers for a variety of football news stories
Teletext, or broadcast teletext, is a standard for displaying text and rudimentary graphics on suitably equipp ...
pages, as well as other types of digital data, within the
vertical blanking interval
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...
(VBI) of an analog video signal. It is standardized under standard EIA-516, and has a rate of 15.6 kbit/s per line of video (with
error correction
In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communica ...
). It was adopted into the international standard
CCIR 653 (now
ITU-R
The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU) and is responsible for radio communications.
Its role is to manage the international radio-frequency sp ...
BT.653) of 1986 as CCIR Teletext System C.
History
NABTS was originally developed as a protocol by the Canadian
Department of Communications, with their industry partner
Norpak
Norpak Corporation was a company headquartered in Kanata, Ontario, Canada, that specialized in the development of systems for television-based data transmission. In 2010, it was acquired by Ross Video Ltd. of Iroquois and Ottawa, Ontario.
Norpa ...
, for the
Telidon
Telidon (from the Greek words τῆλε, ''tele'' "at a distance" and ἰδών, ''idon'' "seeing") was a videotex/teletext service developed by the Canadian Communications Research Centre (CRC) during the late 1970s and supported by commercial ...
system. Similar systems had been developed by the BBC in Europe for their
Ceefax
Ceefax (, punning on "seeing facts") was the world's first teletext information service and a forerunner to the current BBC Red Button service. Ceefax was started by the BBC in 1974 and ended, after 38 years of broadcasting, at 23:32:19 BST ( ...
system, and were later standardized as the
World System Teletext
World System Teletext (WST) is the name of a standard for encoding and displaying teletext information, which is used as the standard for teletext throughout Europe today. It was adopted into the international standard CCIR 653 (now ITU-R BT.653) ...
(WST, aka CCIR Teletext System B), but differences in European and North American television standards and the greater flexibility of the Telidon standard led to the creation of a new delivery mechanism that was tuned for speed.
NABTS was the standard used for both
CBS
CBS Broadcasting Inc., commonly shortened to CBS, the abbreviation of its former legal name Columbia Broadcasting System, is an American commercial broadcast television and radio network serving as the flagship property of the CBS Entertainm ...
's
ExtraVision
ExtraVision was a short-lived teletext service created and operated by the American television network CBS in the early to mid-1980s. It was carried in the vertical blanking interval of the video from local affiliate stations of the CBS network. ...
and
NBC
The National Broadcasting Company (NBC) is an American English-language commercial broadcast television and radio network. The flagship property of the NBC Entertainment division of NBCUniversal, a division of Comcast, its headquarters are l ...
's very short-lived
NBC Teletext
NBC Teletext was a teletext service provided by the NBC, based on the NABTS standard.
Initial trials started in Los Angeles in 1981. Transmissions started as a regular service on May 16, 1983 after FCC approval, in parallel with CBS similar '' Ex ...
services in the mid-1980s. Due to teletext in general not really catching on in North America, NABTS saw a new use for the
datacasting
Datacasting (data broadcasting) is the broadcasting of data over a wide area via radio waves. It most often refers to supplemental information sent by television stations along with digital terrestrial television (DTT), but may also be applied ...
features of
WebTV for Windows, under
Windows 98
Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. The second operating system in the 9x line, it is the successor to Windows 95, and was released to ...
, as well as for the now-defunct
Intercast system. Canadian company
Norpak
Norpak Corporation was a company headquartered in Kanata, Ontario, Canada, that specialized in the development of systems for television-based data transmission. In 2010, it was acquired by Ross Video Ltd. of Iroquois and Ottawa, Ontario.
Norpa ...
sold and manufactured encoders and decoders for NABTS until the end of analog broadcasting in North America in the early 2010s; it was acquired by the
Ross Video
Ross Video Ltd is a privately held Canadian company that designs and manufactures equipment for live event and video production. Ross Video's headquarters and manufacturing operations are located in Iroquois, Ontario, Canada, while their R&D lab ...
consortium in 2010. NABTS is still used in legacy analog video systems for private
closed-circuit data delivery over a
television
Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertisin ...
broadcast
Broadcasting is the distribution of audio or video content to a dispersed audience via any electronic mass communications medium, but typically one using the electromagnetic spectrum ( radio waves), in a one-to-many model. Broadcasting began ...
or
video
Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...
signal.
Description
In a normal
NTSC
The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
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
Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the be ...
(HBI). When the scanning process reaches the end of the screen it returns to the top during the
vertical blanking interval
In a raster scan display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final visible line of a frame or field and the beginning of the first visible line of the next fra ...
(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.
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
Closed captioning (CC) and subtitling are both processes of displaying text on a television, video screen, or other visual display to provide additional or interpretive information. Both are typically used as a transcription of the audio por ...
, 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.
See also
*
Antiope - French teletext standard (CCIR Teletext System A)
*
World System Teletext
World System Teletext (WST) is the name of a standard for encoding and displaying teletext information, which is used as the standard for teletext throughout Europe today. It was adopted into the international standard CCIR 653 (now ITU-R BT.653) ...
– European Teletext Specification (CCIR Teletext System B)
*
JTES
JTES, the Japanese Teletext Specification, is a protocol used for encoding teletext pages, as well as other types of digital data, within the vertical blanking interval (VBI) of an analog video signal in Japan. It was adopted into the internationa ...
- Japanese Teletext Specification (CCIR Teletext System D)
*
NAPLPS
NAPLPS (North American Presentation Level Protocol Syntax) is a graphics language for use originally with videotex and teletext services. NAPLPS was developed from the Telidon system developed in Canada, with a small number of additions from AT&T ...
– North American Presentation Level Protocol Syntax
*
Teletext character set
This article covers technical details of the character encoding system defined by ETS 300 706, a standard for World System Teletext, and used for the Viewdata and Teletext variants of Videotex in Europe.
Character sets
The following tables s ...
*
Text semigraphics
Text-based semigraphics or pseudographics is a primitive method used in early text mode video hardware to emulate raster graphics without having to implement the logic for such a display mode.
There are two different ways to accomplish the em ...
References
{{Teletext
Television technology
Teletext
Canadian inventions
1980s establishments in Canada