In the mid 1970s a number of firms were rolling out videotex services, although the most popular by far would be in England. Character codes are sent to users' televisions by encoding them as dot patterns in the invisible "vertical blanking" area just off the top and bottom of the visible portion of the TV signal. A circuit in the TV decoded these signals back into text pages, which the user could select among. The data rate was quite slow, about 600bps, resulting in largely static pages.
At about this time the Canadian government decided to create its own "second generation" service that would support both text and graphics, called Telidon. To start they used up considerably more of the "wasted" portion of the TV signal, and increased the signalling rate to about 2400bps. To this they added a "backchannel" that send data back to the hosting computers, typically over phone lines.
But the real effort centered on creating a simple graphics language that would allow a more complex circuit in the TV to decode not only characters, but simple graphics as well. To do this the graphic was encoded as a series of instructions (graphics primitives) like "polyline" which was represented as the characters PL followed by a string of digits for the X and Y values of the points on the line. This system was referred to as PDI (Picture Description Instructions).
Telidon was so impressive that AT&T decided it wanted in on the project. They added a number of useful extensions, notably the ability to define your own graphics commands (macro) and character sets (DRCS). It was sent to the ANSI board for standardization and became ANSI X3.110, NAPLPS. As with all to many standards written by governments or more than one company, the original simplicity of NAPLPS was drowned in a huge document describing many issues that could safely be ignored.
Various systems using NAPLPS appeared in the early 1980s, but it soon became clear that there was no way to make money on a read-only service (a fact many in the web discovered again 20 years later). Unlike the UK where teletext was supported by one of only two large companies whose whole revenue model was based on a read-only medium (television), in North America Telidon was being offered by companies who worked on a subscriber basis and couldn't figure out how to make money.
Nevertheless NAPLPS's legacy lives on today. In the 1980s it became the basis for the GKS microcomputer based standard, which was later implemented in Digital Research's GSX graphics system and later used in their GEM GUI. GKS was later extended into a 3D version, and additions to this resulted in PHIGS (Programmer's Hierarchical Interactive Graphics System), a former competitor to OpenGL. More recently NAPLPS was used as the basis for the SVG standard for vector graphics on the web, proving that all that is old is new again.
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