?R~CTICAL ASPECTS OP HOME TELETEXT

GARY W. STANTON

SOUTHERN SATELLITE SYSTEMS, INC. TULSA, OKLAHOMA 74136 ABSTRACT transmitted on the vertical blanking interval on a satellite signal. Since There have been many learned papers that date, knowledge has been obtained describing the detailed technical on the various parameters that affect aspects of teletext transmission; most successfull vertical blanking interval in language only understood by others transmission to and through the cable with an intimate knowledge of the environment. subject. This paper defines the During the last four years requirements for the practical knowledge has been gained, various implement~tion of home teletext standards have evolved, equipment has reception in the cable television been designed and services implemented environment. using vertical blanking interval Since data integrity is the measure technology. Numerous degradation factors of technical success, the items which peculiar to satellite television effect data integrity are deliniated. transmission and to the cable Included are the satellite path, receive environment have been identified. To earth station, satellite receiver, cable offset these factors, error correcting modulators, AML link, and the cable techniques have been developed which plant itself. Results of various give several orders of magnitude measurements and observations describing improvement in data integrity. With the effect of each subsystem have been these improvements, home teletext is made and the results will be described. suitable not only for video display, but Practical suggestions for achieving also for electronic mail and downloading desired results in the most critical of computer software and games. areas are presented. HISTORY INTRODUCTION It is generally acknowledged that The first vertical interval the first commercial use of vertical transmission to the cable television blanking interval technology was by the industry was conducted at the NCTA British Broadcasting Company in Great Convention in Las Vegas in 1979. This Britain. The BBC Ceefax system which was demonstration, although only partially designed specifically for the 625 line successfull, showed that data could be PAL television system in use in Great

173 Britain was not usable in the United broadcast standard. ~t the same time,

States without significant change. the video screen format was impr~ved to

Two of the most significant allow the display of 24 r~ws of text differences are data rate and screen instead of 20 rows. format. The 625 line system utilizes a data rate of 6.9375 Mbits/sec. which is CUSTOMER SERVICES faster than can be used in the NTSC system. The second difference is screen The initial services provided were format. In the Ceefax system there is a the delivery of two news services to fixed relation between the data byte's cable television headends. The equipment position in the transmit line and its utilized was a video-in/video-out position on the video display. In Great commercial quality teletext receiver. It Britain, forty usable data bytes are was packaged in a rack mount chassis and transmitted on each vertical blanking specifically designed for direct interval line which correspond to a interface to a satellite receiver. ~t video screen format of forty character this point in time, only problems width. Depending on data rate, only 36 inherent in satellite transmission and or 37 usable bytes can be transmitted in satellite receivers had been addressed. the NTSC system. In order to use a Shortly thereafter a teletext version of the Ceefax system in the transmission format was developed and a United States these two parameters receiver designed featuring a data needed to be modified to fit the NTSC output rather than the video output. The system. The basic incentive for using initial purpose was to provide an the Ceefax system is the availibility of interface between the vertical blanking LSI chips from several sources which interval and a character generator at a make teletext decoders reliable and cable television headend. This provided economically practical. the operator the opportunity to use his

~t the NCT~ Convention in 1979, sophisticated character generator for Southern Satellite Systems displayed a both a national alphanumeric news primitive implementation with a data channel and for local generated text or rate of approximately 3.2 Mbits/sec weather information. ~s is the case with using two vertical blanking interval the video output unit, the data output lines for one display line. By late 1980 unit was packaged in a rack mount a data rate of 5.554 Mbits/sec was in chassis with an interface specifically use and the screen was formated with a designed to interface with a satellite 40 character by twenty row display. The receiver. As may be noted, both of these mapping technique used was formulated by units were designed for installation at Mullard Labs.l In early 1983, the data a cable television headend or other rate was increased to 5.7272 Mbits/sec commercial location where a baseband to be compatible with the proposed video signal was available.

174 l:iOME TELP.TEXT ~s a companion to the above teletext decoder which has a typical Recently Southern Satellite Systems Channel 3 or 4 output, a similar unit is has introduced Keyfax, a home teletext being designed which has a standard service. This service utilizes the RS232C data interface. This will be United States proposed version of the ideal for downloading information to BBC Ceefax standard which was proposed printers and home computers. Its to the Federal Communications addressable funtion will allow specific Commission.2 The service features over users to get only the data to which they one hundred video pages of text from subscribe. which the viewer can select. This The data interface unit, combined service is presently being transmitted with vertical blanking interval on the vertical blanking interval of technology offers the ability to deliver HTBS \vhich is on Sl\TCOM IIIR Transponder one way data from a central point to 6. thousands of locations simultaneously. To implement this service, a This feature is particularly useful to set-top teletext decoder was necessary. the news services, commodity services, The unit designed features a full cable and financial information industries. It television converter with infrared is presently being used by customers in remote control and a built in teletext these industries to deliver information decoder. This unit can be used to to cable television headends and to home replace existing converters in subscribers. non-scrambled systems or as an add on unit where scrambling is used. l\ block diagram is shown in Figure l.

CABLE TV VERTICAL TELE . EXT DROP INTERVAL TIMING PROCESSOR CHAIN f VARACTOR TUI'

() TELE EXT 0 z RECOVERY RAM -t CHIP - :D 0 __..!: CD )> I FRARED 8748 MICROPROCESSOR t (/) DETECTOR WITH RAM & ROM m I CD )> z I 0 0 ' R < TELETEXT ___G__ RGBTO a I m CHARACTER B BASEBAND 0 I GENERATOR y RECONSTRUCT

INFRARED REMOTE DJK CONTROL 1!1-112 TO ANTENNA INPUT OF T.V. R F MODULATOR - CHANNEL 3/4

175 of the receiver. Tests indicate that a THE C~BLE TELEVISION SYSTEM bandwidth of 30 mHz or greater is required for successful teletext There are numerous factors in the 0peration. Some of the home quality cable environment that effect teletext receivers have an IF bandwidth of 25 mHz performance. ~t the headend, the earth or lower: usually to improve apparant station, satellite recei~er, and the video threshold. The lower bandwidth cable modulator all affect performance. will distort the data by limiting the In the cable plant, the ~ML microwave, frequency response of the system. This the amplifiers, and the drops can also limiting will severely round the data affect performance. Each item will be pulses and cause both positive and discussed separately. negative overshoots in the data area. In one extreme case, the negative data 1. Receive Earth Station overshoot exceeded 20 IRE units with the The quality of data received is directly result being the television receiver's proportional to the signal to noise sync circuitry became erratic. performance of the receive earth The test instrument required to station. ~typical 3.6 meter with a 100 ueasure and observe this effect is a degree LN~ in the central United States full function waveform monitor, such as produced an error performance of 1 error a Te"ktronics 1480. Alternately, a 50 mHz in 106 bits transmitted. For some data oscilloscope can be use if it features a applications this error rate may not be television sync horizontal trigger. adequate. ~ larger receive dish would be For most purposes, most commercial necessary to lessen the number of bit quality satellite receivers that have errors. ~ 4.5 meter dish appears been used in the cable industry over the suitable for most applications. last five years meet the 30 mHz requirement. HQwever, due to cost 2. Satellite Receivers pressure, several major manufacturers The quality of the satellite receiver have been considering lower bandwidth can have a major effect on the products to be competitive with the home performance of the teletext system. Of satellite offerings. These lower extreme importance is the IF bandwidth bandwidth units will not function properly on a teletext system.

~ second effect inherent in the satellite receiver is video filtering. It is common practice today to incorporate bandpass filter on the video output which limits the band to 4.2 mHz. The portion of the band above 4.2 mHz contains the aural carrier at 6.8 mHz

176 and most often numerous other with too deep a notch will cause as much subcarriers and/or noise. All receivers degredation as no filter at all. are equipped with either a 6.8 mHz notch filter or the low pass filter. However, 4. Modulators those older receivers with only a 6.8 The cable television modulator can mHz filter will pass the energy of the damage the vertical blanking interval additional subcarriers which will data. Fortunately, it is the adjustment manifest itself as video noise. If an of the modulator that is most often the older receiver is to be used, an problem, rather than the modulator external low pass filter is recommended itself. Of particular importance is if the teletext decoding is to be preventing video overmodulation as well accomplished at the cable television as maintaining the aural level at -15 to headend. For decoding on a cable system, -19 db with respect to the main carrier. the filter is not required as the It is equally important to correctly modulator will provide the filtering. maintain the aural level on the next adjacent lower carrier. 3. Terrestrial Interference Modulators that do not have One of the common problems that plauge bandpass filters can allow harmonics to many receive locations is interference pass into the system. This is typical of from terrestrial microwave sources. the strip amp variety used in some older Unless the interference is obnoxiously cable systems and most frequently used bad, it is frequently ignored. in apartment buildings, hotels, and Unfortunately, with teletext it cannot other institutional systems. This type be ingnored as it produces the same of installation frequently can be effect as if the system was operating at improved by adding filters or replacing threshold. The best solution is the modulator being used for the channel shielding the receive dish from the carrying teletext. interfering signal. If shielding is not possible, 5. Microwave Radio Systems filters are commercially available that There have been no reported problems have produced acceptable results in many with FM microwave systems. The applications. The filter is placed in investigation of one reported problem the IF of the receiver and is either a with an AML system indicated the system 60 mHz or a 80 mHz center frequency was badly out of manufacturers filter, depending on whether the tolerences. Numerous other operators offending carrier is 10 mHz below or 10 have successfully passed the vertical mHz above the desired signal. The filter blanking interval data over AML chosen needs to be optimized to remove microwave systems. It can be concluded just enough of the offending signal to that a properly operating AML system provide optimum data reception. A filter will not affect teletext data.

177 6. The Cable Plant 7. Test Equipment A properly adjusted cable plant will The equipment used to test a cable plant correctly pass the vertical blanking can have an adverse effect on teletext

interval. Typical proble~ areas are performance. Of particular importance is malfunctioning AGC amplifiers, DC power the high level sweep equipment used on supplies with high AC ripple, and leaks many systems. This type of sweep

in the cable system. The leakage is equip~ent will completely destroy extremely significant for on-channel teletext integrity. The newer low level operation of off air signals with sweep techniques can coexist with teletext. The leakage generates the same teletext data, however some degradation effect as multipath reflections, except has been observed. the weaker signal is before the desired signal rather than after it. In some 8. System Maintenance systems, channels 18 and 19 (E and F) It has been observed that proper are susceptible to two-way radio maintenance and adjustment of the system interference. This interference will is critical if low error rates are destroy the integrity of teletext data. expected. Of critical importance is A third cable plant problem is that maintaining the proper levels between of poor drops. This may be due to satellite receiver and modulator, and inadequate levels: teletext reliability the correct modulation levels for both is enhanced with a strong drop signal, the video and aural level of all typically between 5 and 10 dbmv. channels. It is common to find improper Installations subjected to high levels levels due to either misadjustment or of RFI may also be unreliable. subsequent unterminated or double It has been observed in terminated equipment. Care in headend conventional cable plants that certain setup and maintenance is mandatory for channels are cleaner than others. This success in teletext transmission. is generally a function of the many combinations that result from the ERROR PERFORMANCE AND IMPROVEMENTS addition and subtraction of the carriers and their harmonics. A spectrum analyzer Several techniques have been is generally required to track this developed to improve the data integrity. phenomena, nowever changing to a The basic code structure provides for different channel is often easier and parity checks on all data bytes. The may solve the problem. address codes are further protected by Hamming codes. This is a code where a single bit error in a byte will be corrected and a double error will be corrected. These methods are in current

178 use. For certain data a method of CONCLUSIONS multiple transmission has been developed. For the video type of The use of the vertical blanking teletext unit, the second transmission interval for delivery of information to overwrites the original t~ansmission; the cable television headend and to the however if a parity error is detected in homeowner is now feasible and practical. any given byte, the byte from the first Reliable equipment is available at transmission is left on the display. reasonable costs. A well maintained This technique gave a significant cable system will be able to pass the improvement in data integrity and is vertical blanking interval information used for critical transmissions. successfully. Small changes in other The same technique has been used systems may be required, most often in for the data output teletext unit. A the maintenance practices. control code is transmitted which tells the decoder whether it is receiving the REFERENCES first or second transmission of the identical data. Test have shown that G. 0. Crowther, TELETEXT AND this technique generates 100 times VIEWD~TA SYSTEMS AND THEIR POSSIBLE improvement in data integrity. EXTENSION TO EUROPE AND USA, IEEE

~nother technique usable on the Transactions on Consumer Electronics, data output type decoders is that of July 1979 Volume CE-25 Number 3. longitudinal parity check or LPC. In this technique, a byte is t~ansmitted at 2. United Kingdom Teletext Industry the end of each vertical blanking Group Petition for Rulemaking before the interval line which is the result of Federal Communications Commission, March calculating parity on a bit by bit 26, 19Bl. basis. From the LPC byte a single byte error can be corrected in any given vertical blanking interval line. The improvement is almost as good as the multiple transmission method when basic error rates of less than 1 x 106 exist. This improvement diminishes for worse error rates and improves at better error rates. Of significant importance is the added overhead is only 3% rather than the 2:1 overhead in the multiple transmission technique.

179