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).
NABTS was originally developed as a protocol by the Canadian Department of Communications, with their industry partner Norpak, 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 encoders and decoders for NABTS.
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.
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.