DO~ffiLOADING AND ADDRESSING VIA TELETEXT

Gary w. Stanton

Southern Satellite Systems

ABSTRACT

A teletext signal has been continuously typewriters, or not a large enough transmitted on the vertical interval supply of "t's" at their printer? These of Satcom I, transponder 6 since the questions aside, let's deal with what's NCTA show in 1980. The original purpose really happening. was the video dissemination of cable news services. Recently, the system There are some cable systems in the has been modified to incorporate data u.s. delivering "videotext," which for transmission which emulates a standard the purposes of this paper will be telephone line, but with far higher considered two-way and interactive. data reliability. In addition, the There are some broadcast stations test modified teletext system for data marketing their "teletext" services, transmission incorporates an which in this article will mean strictly addressability function with over five one-way. UPI, Reuters, Dow Jones, and million addresses possible. A priority Quotrader are all delivering their scheme for national, regional, local services, which are "teletext", to and individual addressing is described. cable systems, today and in the case A description of the system, including of Quotrader, to private users. UP! data from reliability tests is presented. and Reuters began satellite testing This will also define several key of the service in July 1980, and service technical requirements for successful to cable TV headends in October 1980. teletext data reception. But one-way teletext services aren't really what all the excitement is about. It's the interactive services that are General Information getting all the attention, and precipitating the hue and cry of the The cable and communications trades future of communications. The have been full of news about the up Warner-Qube system in Columbus, and and coming information invasion of the other such ventures like The Source home. With the British, the French in Virginia, are the kinds of things and the Canadians, among others, the that the cable and communications battle lines have been drawn about whose industry have been talking about so system is the best for delivering various much. videotext or teletext services into America's wired cable homes. Unfortunately, few of the things being Unfortunately, there's so much talk talked about are ready to happen on and so little really happening that a national basis today. That's why, it may be time to sit back and take if teletext isn't going to remain a a look at where this revolution really thing of the future, the industry needs is, what's really happening and what's to use what's available today to its just in the testing, a few years down full capacity. That means using the the road. existing means of teletext, one-way delivery, and make it two-way, in effect, One problem in all the talk is the fact by using telephone lines as the means that the generic jargon of the of "upstreaming." alphanumeric information industry is confusing. Are videotext and teletext Videotext is basically two-way the same thing? What about viewdata? communications, using a telephone line And why do some people leave the "t" to "call up" specific information, which off of videotex(t)? Do they have lazy is then delivered to the person who

161 requested it, using the same phone line. hours per day. It's "narrow stream," two-way delivery, which is interactive. The key to this application of existing, in-use today technology, is in what's called an "RS-232 decoder." RS-232 Teletext, on the other hand, is a one-way is an EIA defined standard computer "stream" of information (channels or serial data interface used for pages), delivered as a "wide stream." interconnection of many different Phone lines, the standard means of most devices. A printer or home computer operating text delivery today, do not can be attached to a RS-232 decoder, permit wide streams of information, as well as a cable system's character whereas broadcast or cable delivered generator as it's being used by UP!, teletext does. Two-way videotext is Reuters, Dow Jones, and View Weather selective and slow. One-way teletext today. The delivery of teletext is non-selective and fast. It takes information into businesses and homes a lot of transmission space to provide of "specialty" users is one of the two streams of information for the general markets soon to be tapped by the common public. Because of the various carrier Southern Satellite Systems, limitations inherent in both videotext through its teletext system called and teletext, there is need for both CableText. wide stream teletext, as well as interactive videotext. Let's look at the "downloading" application of teletext. An example The major drawbacks to interactive is being accomplished today by Quotrader, videotext services are the long distance a commodities service which utilizes phone charges, the cost of the receive a specially equipped computer along equipment, and the fee for the with the RS-232 decoder. The decoder information service itself. The interfaces with the microcomputer, downloading scenario outlined previously feeding the information into the computer enables users to accomplish most of for use in analyzing the commodities the same things they could accomplish information. The software provided using a two-way system, for less cost. as part of Quotrader's monthly fee also Downloading with satellite delivered enables users to receive buying and teletext information, as done by selling recommendations. Buy/sell orders CableText, and upstreaming via phone are placed by a phone call to a user's lines means teletext for specialty users broker, or by attaching the computer is not a thing of the future. It's being to a modem which permits the user's done today and can be done by any kind computer to talk directly to the broker's of information supplier who wishes to computer. do so. rhat scenario, starting with 1) In this application, subscribers will downloading information into a home be continuously "on-line," with their computer, or in some cases a printer, printer or computer storage device 2) storage of the information on the receiving the information transmitted computer storage device or on a printed for them. page, 3) viewing of the information at the convenience of the user, and While a satellite delivered teletext finally 4) the use of the information system can be delivering hundreds of for a phone transaction (upstreaming), different specialized types of is one application that is ready for information, individual subscribers the marketplace today. will receive only the information they want and need, because the teletext Existing System Formats decoders will be "addressable." A Teletext decoder will feed the data There are a number of teletext and into a user's printer or computer with videotext systems either proposed or storage. Though the information may in existence in either test or be delivered at 3 a.m., the user can operational modes. Some are very still "call it up" when he or she is similiar, differing only in minor ready to use it, either by instructing detail, however, just different enough the computer to display it, or by simply as to be incompatible. Others are reading the printed copy. If a significantly different in one or more subscriber needs to use the information respects. for a transaction, a simple telephone call will make a technically one-way system two-way in effect. DownloadLng the information via teletext makes the For background purposes, a brief system interactive only when needed, description of the various systems with information being delivered 24 is in order.

162 1. The first teletext systems does not specify transmission format, and by far the most widely used is the only presentation level, i.e. viewable United Kingdom (U.K.) system and its level format. This means the CRT and variations. The teletext version is printer presentation codes have been called Ceefax in Great Britain and the defined by the PLP format, but not the videotext version is called Prestal. transmission scheme. This system is also used in Australia and several other European countries. Since AT & T conceived the PLP format, Several variations of it are in use Teledon and CBS have arnrnended their in the United States, such as Field display system protocol to be compatible. Communications' "Keyfax" system and At this point there are basically two Southern Satellites' "CableText" system. major competitive incompatible systems KSL in Salt Lake City and WGN in Chicago in existence, the U.K. system, which also experimented with variations of includes both teletext and videotext the U.K. system. Reportedly, 1.5 million definitions, and the PLP presentation LSI chip sets have been sold for the system with the CBS videotext U.K. system. transmission definitions.

The U.K. system features a mapped video Advantages/Disadvantages screen system where each screen alphanumeric character or mosaic graphic Much has been written on the comparative character's screen position is defined advantages of each system. However, by its position in the transmit stream. the engineering community must stand After the necessary page address or back and realize that the end user, header information is transmitted, whether a horne owner, business, or cable subsequent consecutive transmissions system really does not care what contain screen row numbers and the technical system is in use. All the information to be displayed on that end user is really interested in is: row. For example, the lOth character l) Obtaining the desired data transmitted with a row 4 address will or information be displayed in the lOth screen position 2) Reliability in row 4. In practice, it's not quite 3) Lowest possible cost that simple, but in essence, that is how it works. It is apparent that each of the two major systems has certain advantages 2. The remaining major systems and disadvantages. The particular use a free format where there is not application should dictate the specific a fixed relationship between transmit system chosen. screen positions. This means screen positions must be defined in the Transparent Data System transmitted data, as well as line feed/carriage return at the end of each SSS has been using an early version line. The major systems using variations of the U.K. teletext system for of this protocol are: transmission of the UPI and Reuters a) Antiope: The French teletext news services. There was not a U.K. system. Technically, Antiope is the defined protocol for data transmission screen language and "Didon" is the only, or for addressability greater transmission protocol. The French than the 800 page numbers defined in pioneered the non-mapped concept for the U.K. protocol. Therefore, it was teletext/videotext usage. necessary to define a system protocol b) Teledon: The Canadian system. and have appropriate hardware This system is quite similar to the fabricated. Antiope system, however, it also included In the basic U.K. system, a header row a set of picture description instructions is transmitted, which contains the page (POI's) which gives the ability to draw addresses, various control codes, and circles, lines, and other geometric 22 free bytes usually used for the first figures. part of the displayed text. Subsequent c) CBS system: CBS originally transmit rows contain the hundreds digit adopted the Antiope system to the NTSC of the page number as the row number television system. They added the and the data for display. All transmit necessary transmission protocol to rows have a three byte preface consisting make the system work properly in the of two bytes of clock run-in and a single 525 line 60 cycle television system byte framing code. These three bytes currently in use in the U.S. synchronize the decoder for data d) North American Presentation recovery. The control and addressing level protocol or "PLP". AT & T combined codes in both the header and rows are the Antiope, Teledon, and CBS systems Hamming code protected, while the data into a single system. The PLP format is transrnited with odd parity.

163 Standard Transmission Mode in the receive buffer is moved to the output buffer for spooling out to the Examination of the Header row reveals RS-232 interface. The input buffer ~ number of options. The first option is cleared for a new block of data. 1s to use the last 22 bytes in the header row for transmission of the required For ultra high reliability, it is data. To implement this system, the possible to transmit three or more computer needs to store incoming data times, with the decoder selecting the until 22 bytes are collected, then insert best of the transmissions in a voting these bytes on the next available process. vertical interval line. The decoder will receive these 22 bytes and store Additional Addressability these in RAM. A microprocessor controlled UART then spools the data Examination of the control code out to the RS-232 interface at whatever structures reveals 13 bits that can data rate the UART has been set for. be redefined without jeapordizing needed As long as the UART is set to transmit control functions. Using these 13 bits at a speed equal to or greater than a three tier subaddress mode was the incoming rate for new data, reliable constructed. The tiers contain 64, data recovery is obtained. 16, and B levels, respectively, with each tier operating independently of Since the effective vertical interval each other. transmission speed is 5.554 megabits/second for two or three For example, the group of 64 can be vertical interval lines, many standard defined as states, the group of 16 could 300 to 1200 baud services can be be regions in a state, and the last multiplexed together. The basic U.K. tier group as individual decoders within Teletext chip set has capacity for 800 the region. Alternatively, the 8 tier descrete addresses. For practical group could be used for regions in the purposes, one vertical interval line U.S., and work downward in the opposite will support forty-four 300 baud services direction. A third option is to use in this mode. the bits for discrete decoder address instead of a tier system. This gives Multiple Transmit Mode a total of 6615 discrete addresses per page, or a total of 5,292,000 discrete It has been recognized that a need exists possibilities. for a higher level of data integrity than may exist in a single transmission. The zero address in each tier is not In two way telephone line service, the included in the above address receive device can send a negative possibilities. The zero address is acknowledge (NAK) upon failure to reserved for mass addressing. If a zero receive intelligible data. For one-way address is sent for a given tier address all decoding units in that tier respond transmission this is not the case~ therefore, depending on the reliability to the data in that pocket. This gives the flexibility of mass addressing of required, it may be necessary to retransmit the identical block of decoders without sending identical data information. to units with different address codes.

To implement the multiple transmission Data Error Test mode, two additional control bytes are defined, thereby leaving 20 bytes A number of tests have been performed for information data. The first control and are continuing to quantitize byte is a mode definition which alerts reliability at a TVRO, and subsequent the decoder of the multiple transmission transmission on a cable system. Although mode. The second control byte is a measurements are continuing, various continuity index which is incremented preliminary results are reportable. each time new data is sent, but is not It is apparent that considerably incremented on a repeat of the old data. different factors are involved in cable system data integrity than those which affect TVRO integrity. The decoder is double buffered. In its receive buffer only data with correct At a TVRO, impulse noise appears to parity is retained. Subsequent be the greatest source of errors. The transmissions will not replace previous amount of impulse noise is related to correct parity information, but will the usual known factors such as dish fill in missing information. Therefore, size, LNA, etc. In other words, the the data will integrate upward on higher the carrier to noise ratio, the subsequent transmission. Upon receipt better the data integrity. Tests were of a new continuity index, the data made using a 3.65 meter dish with a

1M 90 degree LNA, with the measured c/n data. One study by a reputable = 11. Byte error rates of 1 x 10-5 manufacturer has concluded that in the dual transmit mode were measured. adjustments of modulation levels is The dual transmit mode uses the technique of critical importance, as over described earlier, with two transmissions modulation is disastrous for teletext in the same TV field. Tests with a data recovery. 10 meter antenna indicate error rates of 1 x 10-6 are obtainable in the single transmit mode. Conclusion

There are numerous factors that affect An economical, addressable, teletext performance of teletext via the cable system has been developed for downloading TV medium. A complete analysis is beyond data to various types of home/business the scope of this paper: however, several data equipment such as computers and comments are in order. One observation printers. The system features is that transmission errors tend to transparent data communication as well appear in large groups, rather than as a flexible addressable format. The singularly as is typical at a TVRO. system is based on the reliable and Various investigators have found many available U.K. teletext chip sets for causes. implementation in today's marketplace, using high speed one way data downloading It has been shown that the modulation and low speed selective return path process is critical in passing teletext via the existing telephone system.