thus allowing cable companies to strip tele- In 1982 RCA became an active partic- at the expense of transmission efficiency. text signals, but this definition is being ipant in, and a major contributor to, the Level 1is primarily a text -transmission appealed. development of the NAPLPS and NABTS system with a primary character set plus In May of 1983, the NBC and CBS tele- standards, working through ANSI, EIA, two mosaic sets. Each mosaic set contains vision networks began transmission of and CCIR committees. the 64 graphic shapes formed by dividing NABTS teletext services. In August the a character cell into an array of blocks, Special Working Group of EIA working two horizontal by three vertical. In one set with the Canadian Videotex Consultative Present status the blocks adjoin whereas in the other Committee (CVCC) completed the draft There are two major contending systems they are separated. A fixed text resolution NABTS. for the U.S. market: the U.K. system, re- of 23 rows of 40 characters plus a page Although this discussion has concentrated cently renamed the World System, and header row is provided. Using mosiacs, on the history of the standards activities the NABTS. graphic images of resolution 80 pixels hori- within ANSI and EIA, there was, and is, The NABTS is favored by the NBC zontal by 69 pixels vertical may be ob- considerable activity in other standards fo- and CBS broadcasting networks, some local tained. A fixed palette of eight colors is rums which affects teletext. The Interna- stations, and several manufacturers includ- provided. tional Standards Organization (ISO) issued ing RCA. The NBC and CBS television Level 2 includes the addition of accents ISO 20229 on which the code -extension networks are currently broadcasting for showing languages other than English, techniques of the NAPLPS are based, and NABTS teletext services. Norpak, Pana- smoothed mosaics for enhancing the appear- is actively revising the standard to include sonic, RCA, Sony, and VSA (Videographic ance of mosaic graphics, and the inclusion the added functional requirements of the Systems of America) have shown proto- of 16 pastel colors. NAPLPS. For example, ISO 2022 permits type NABTS receivers or decoders. Time Level 3 includes the addition of a Dynam- only sets of 94 characters whereas the PDI Inc. is currently conducting trials of an ically Redefinable Character Set (DRCS) character set in the NAPLPS requires 96 NABTS-compatible full -field teletext ser- that allows other character shapes to be characters. ISO has also defined the basic vice on two cable systems. The U.K. sys- downloaded by the broadcaster and inter- layered model upon which the NABTS is tem is favored by some local stations, mixed with the predefined characters. Level based.0 The International Radio Consul- Zenith Radio Corporation, and several 4 includes alphageometric coding that al- tative Committee (CCIR) is developing cable systems. Taft in Cincinnati is cur- lows geometric images consisting of lines, teletext standards.11 ,12 Further details on rently broadcasting the U.K. system level rectangles, polygons, circles and arcs to be the historyof the PLPS can be found in 1 (see the next section), and compatible drawn. Level 5, which is not yet com- Reference 13. decoders manufactured by Zenith can be pletely specified, will include alphaphoto- purchased for about $300. graphic coding to obtain high -resolution History of teletext atRCA The first three levels of the U.K. system full -color images. RCA explored the use of the VBI for are specified in Reference 11, together with analog transmission of information during an experimental version of level 4. Level 5 NorthAmerican Broadcast the 1960s with a system called Homefax." is not yet completely specified. Teletext Specification (NABTS) After that project ended, little work was The NABTS service is defined by the done at RCA until a corporate Teletext "North American Broadcast Television Spec- The NABTS is specified by the EIA. It is Committee was established in the Fall of ification" from the Electronic Industries primarily concerned with the interface be- 1980. This committee reported in March Association. The NABTS is in draft form tween the broadcaster and the TV receiver. 1981 recommending that teletext work be and has been approved by the joint But NABTS also specifies some receiver started and that an asynchronous system EIA/CVCC Special Working Group. Ap- processes, together with some features of be supported for the U.S. It stated that, of proval by the EIA as a recommended the interface between the receiver and the the existing system proposals, the Antiope practice is expected to be completed in the user. system came closest to meeting its criteria fall of 1983. The presentation layer of the Although the teletext service is a one- for an optimum system, but suggested that NABTS is defined by the NAPLPS.8 The way service, the NABTS contains func- some high -resolution graphics be included NAPLPS has completed its public com- tions to increase the interaction between in the initial decoders. ment period. It is expected that the ANSI the user and the broadcast teletext data RCA Laboratories researchers started tech- technical approval process will also be com- base, and to give the broadcaster control nical work in 1981, putting together simu- pleted in the fall of 1983. Some details of of the structuring of this data base. The lation systems for evaluation and demon- the U.K. and NABTS teletext systems are functions provided to the broadcaster are stration. They also evaluated hardware and given in the following sections. quite extensive and there are sometimes software designs for teletext receivers in several ways of achieving the same end order to contribute to the cost -performance result. These complexities are hidden from tradeoffs to be made in the development United Kingdom (U.K.) the user, whose perceptual model of the of standards. teletext system teletext service may be quite simple. At the end of 1981 RCA filed Com- The U.K. system, as modified for North The NABTS is divided into chapters ments to the FCC NPRM strongly recom- America, is specified in CCIR Report 957.11 that correspond approximately to the lay- mending the adoption of a single standard, It is divided into five levels. These corres- ers of the Open Systems Interconnection and favoring the NABTS as the basis for pond to performance levels, not to the (OSI) model.° In this model the layers such a standard. RCA has also evaluated Open System Interconnection (OSI) lev- are independent of one another. The dif- the ruggedness of the competing systems els. These levels are upwards compatible ferent layers of the NABTS are described for the EIA. with each other, a compatibility achieved in Table I.

Astle: Teletext standards in North America 19