NABTS
Encyclopedia
NABTS, the North American Broadcast Teletext Specification, is a protocol used for encoding NAPLPS
-encoded teletext
pages, as well as other types of digital data, within the vertical blanking interval
(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).
, for the Telidon
system. Similar systems had been developed by the BBC in Europe for their Ceefax
system, and were later standardized as the World System Teletext
(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's ExtraVision
and NBC
's very short-lived NBC Teletext 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
features of WebTV for Windows
, under Windows 98
, as well as for the now-defunct Intercast
system. NABTS is still used for private closed-circuit
data delivery over a television
broadcast
or video
signal, and Canadian company Norpak still sells and manufactures encoder
s and decoder
s for NABTS.
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.
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. Three bytes are used for hardware synchronization, another three for the packet address, two for sequencing information, and 28 for data and redundant forward error correction (FEC) information. In theory, the system can encode 262 lines of data per field, but in practice the data is instead "hidden" in lines that will not appear on a conventional television. 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.
NAPLPS
NAPLPS 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
Teletext
Teletext is a television information retrieval service developed in the United Kingdom in the early 1970s. It offers a range of text-based information, typically including national, international and sporting news, weather and TV schedules...
pages, as well as other types of digital data, within the vertical blanking interval
Vertical blanking interval
The vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...
(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).
History
NABTS was originally developed as a protocol by the Canadian Department of Communications, with their industry partner NorpakNorpak
Norpak Corporation was a company headquartered in Kanata, Ontario, Canada, that specialized in the development of systems for television-based data transmission...
, for the Telidon
Telidon
Telidon was a videotex/teletext service developed by the Canadian Communications Research Centre during the late 1970s and early 1980s...
system. Similar systems had been developed by the BBC in Europe for their Ceefax
Ceefax
Ceefax is the BBC's teletext information service transmitted via the analogue signal, started in 1974 and will run until April 2012 for Pages from Ceefax, while the actual interactive service will run until 24 October 2012, in-line with the digital switchover.-History:During the late 60s, engineer...
system, and were later standardized as the World System Teletext
World System Teletext
World System Teletext is the name of a standard for encoding and displaying teletext information, which is used as the standard for teletext throughout Europe today....
(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's ExtraVision
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 VBI of the video from local affiliate stations of the CBS network...
and NBC
NBC
The National Broadcasting Company is an American commercial broadcasting television network and former radio network headquartered in the GE Building in New York City's Rockefeller Center with additional major offices near Los Angeles and in Chicago...
's very short-lived NBC Teletext 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
Datacasting 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 television, but may also be applied to digital signals on analog TV or radio...
features of WebTV for Windows
MSN TV
MSN TV is the name of both a thin client which uses a television for display , and the online service that supports it....
, under Windows 98
Windows 98
Windows 98 is a graphical operating system by Microsoft. It is the second major release in the Windows 9x line of operating systems. It was released to manufacturing on 15 May 1998 and to retail on 25 June 1998. Windows 98 is the successor to Windows 95. Like its predecessor, it is a hybrid...
, as well as for the now-defunct Intercast
Intercast
Intercast was a short-lived technology developed in 1996 by Intel for broadcasting information such as web pages and computer software, along with a single television channel. It required a compatible TV tuner card installed in a personal computer and a decoding program called Intel Intercast Viewer...
system. NABTS is still used for private closed-circuit
Closed-circuit television
Closed-circuit television is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors....
data delivery over a television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
or video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
signal, and Canadian company Norpak still sells and manufactures encoder
Encoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another, for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size.-Media:...
s and decoder
Decoder
A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode...
s for NABTS.
Description
In a normal NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
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.
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. Three bytes are used for hardware synchronization, another three for the packet address, two for sequencing information, and 28 for data and redundant forward error correction (FEC) information. In theory, the system can encode 262 lines of data per field, but in practice the data is instead "hidden" in lines that will not appear on a conventional television. Lines 1-9 are used for vertical synchronization, line 21 is used for Closed Captioning
Closed captioning
Closed captioning is the process of displaying text on a television, video screen or other visual display to provide additional or interpretive information to individuals who wish to access it...
, 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.