Present Vertical Blanking Interval Usage Line Field 1
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CONSIDERATIONS FOR IMPLEMENTING TELETEXT IN THE CABLE SYSTEM John J. Lopinto Home Box Office ABSTRACT PRESENT VERTICAL BLANKING INTERVAL USAGE In a few years, commercial teletext service will begin in the United States. LINE FIELD 1 FIELD 2 Technical standards, regulatory consi derations and semiconductor designs will 10 have been established to permit the fea 11 sibility of a teletext service for every 12 market segment. The cable industry has 13 Teletext Teletext an opportunity to exploit the versati 14 Teletext Teletext lity inherent in any teletext service. 15 Teletext Teletext This paper is intended to identify the 16 Teletext Teletext key parameters within the cable environ 17 VITS* VITS* ment as they impact on the implementa 18 VITS* VITS* tion of teletext. 19 VIR* VIR* 20 S.I.D.* Unassigned 21 Captioning* Captioning* A BASIC TECHNICAL DESCRIPTION The term teletext refers to an *F.C.C. Broadcast Standard Allocation electronic system where a data base of information is constantly transmitted via existing television signal paths to TABLE 1 home TV receivers which are suitably modified or supplemented. The TV viewer, through a control box, can access and display this information in lieu of the The data present on the scan lines normal TV program which is carrying the are in the form of bits. Groups of bits teletext signal. can represent, when decoded, letters, The teletext signal, unlike the geometric forms, colors or merely system normal TV program, is a data signal. It housekeeping. Figure 1 gives a basic is transmitted along with program on illustration of the hardware used for a unused scan lines in the vertical inter teletext system. val. As viewed on a TV receiver, these lines are not normally displayed as they are located above the top of the picture. Baseband This is the same vertical interval where Video Data Channel test signals to monitor transmission Source Inserter Modulator performance (VITS) and to provide auto matic color adjustment in the home TV (VIR) occupy non-picture scan lines. Cable Planti Table 1 illustrates this vertical inter val region. The line numbers which teletext occupies, at present, varies from user to user since there are no TV Decoder/ standards, as yet, governing this. Converter Figure 1: Basic Teletext System 45 The viewer gains access to this informa THE TELETEXT DECODER tion through a decoder which is either internal to his TV or a set top adapter. The standards which will eventually The viewer controls the mode of teletext be adopted will be primarily concerned display with a calculator type keypad. He with over-the-air broadcast teletext can select either normal TV programs or transmission. Only a few lines in the one of several teletext display modes. vertical interval will be used for tele These modes may be full screen display of text as this is all the braodcaster is teletext, overlay of teletext on program, allowed for this type of service. While captioned teletext, or conditional display this will be fully compatible for the CATV of teletext. The teletext information can operator, it will fall short of the poten be organized as pages and magazines. A tial he has. page is a full screen of text and/or The system operator has the ability grafic information. A magazine may be to send teletext on all TV lines instead composed of several hundred pages. of programming. This will greatly in The number of pages present in the crease both throughput and the amount of data base to be transmitted along with information to be delivered to the sub the data rate at which they are trans scriber. In addition, the cable system mitted will determine the maximum waiting has the ability to provide an interactive time (access time) for display of a page system which is considered the logical which the viewer randomly accessed. Pages extension of teletext. are transmitted sequentially by number The important thing to realize is with the sequence repeated when all pages that if the subscriber has available to are transmitted. The number of TV lines him a TV set with a conventional teletext used to transmit teletext will determine decoder in it, then the system operator the data rate. A general rule of thumb can do no more than provide teletext ser is a transmission rate of 1.7 pages/ vice in the same manner and format as do second for each TV line used to transmit the broadcasters. What must be developed teletext data. In a four scan line syst~ is an upward compatible system which will 100 pages would have a maximum access time allow cable to expand and exploit the of 15 seconds. This, however, varies con conventional teletext service and TV set siderably with the type of data encoding decoder to fully utilize the available used and the density of the pages trans bandwidth on cable. mitted. The teletext decoder can take on many Graphic information can be trans forms. Basically the decoder accepts an mitted on a teletext system. Pictures NTSC baseband video input with the tele are composed of colored squares placed in text data encoded on it. The output is such a manner on the TV screen to give a usually Red, Green, and Blue signals to low resolution picture much like a mosiac. drive the picture tube in the TV set Systems have been proposed which can pro directly. This is illustrated in Figure duce a high resolution graphic display 2. The set top decoder would have cir with a more advanced decoder. These cuitry to convert the R, G, B signals to graphic techniques are called Alpha-Mosaic an NTSC video signal modulated on a TV and Alpha-Geometric. channel frequency. A final term should be introduced here. Viewdata refers to the transmission of data via ordinary telephone lines for vliJI::U display on a modified home receiver. In ,!, stead of the information entering the home DATA I ~-PAGE ~ J PAG£ via a television signal, it enters through SLICER ~ S~~~i~r--'1 MEMORY the home owners telephone line where it -·-' - ____ I connects to a decoder for display on the TV receiver. It is an interactive system in that the user accesses information directly from the data base computer through his telephone. The data trans mission rate is about one-tenth that of teletext but, because it is interactive, it need only transmit the information requested by the user. Most teletext sys KEYPAD R G B tems were originally viewdata systems. Hybrid schemes are possible, where the user requests information via a telephone FIGURE 2: Basic Teletext Decoder line but receives the requested informa tion over a television channel, either cable or off-air. 46 For cable, however, this set-top not have existing provisions for decoding decoder function can be made an integral teletext, a teletext decoder can be incor part of channel converters and descram porated into the 80 channel data selector blers presently used for cable TV. The at little additional cost and connected to converter already performs channel selec the antenna terminals of simple TV sets. tion. The IF can be demodulated, decoded The system would generate its revenue and remodulated. Teletext functions and by "renting out" these 80 data channels to channel selection can share a common key various information suppliers or use sev pad. eral of them for his own purposes. Each Teletext decoders will be offered by one of these 80 data channels would have TV receiver manufacturer's as an integral the capacity to transmit the equivalent part of the TV set. Initially, however, information of 10 billboard channels as there will be a market for external deco they now exist on a system. ders to be used with existing TV receivers. One final extension of this type of The combination converter/decoder can be system would be where the operator would offered at a premium to cable subscribers take the broadcasters teletext signal and giving them the ability to make use of place it on one of these 80 data channels existing teletext services. The imple and offer one package containing all the mentation of the decoder function in the teletext information available on his sys converter, need only be the addition of tem channels. field installable circuits so that a sep Some thought has been given to using arate box need not be inventoried. the CATV channel as a wideband data chan nel without "packaging" it on TV lines. This would make distribution of the data THE TRANSMISSION OF THE TELETEXT SIGNAL impractical since it could no longer be monitored by conventional TV test equip The system operator can implement ment and no longer be handled and "dealt teletext by simply allowing teletext on with" like a TV signal. A new frame of his off-air signals to be passed on to his reference and way of thinking would need subscribers for decoding by subscriber pro to be established by the system operator vided decoding equipment (i.e., teletext in maintaining such a service. TV set or set-top adapter). Unfortunately, this does not allow the operator to gener ate any additional revenue. COST PROJECTIONS The next level of implementation is where the operator buys teletext generating It is assumed that the cost of the equipment, the same the broadcaster buys, decoder during the first years of teletext and inserts teletext the same way as the service in the U.S. will determine how broadcaster, into the vertical interval many consumer users there will be. It is of his locally originated channels. He for this reason that the cost of the tele would then generate revenue from his lo text decoder is so sensitive an issue.