1. The development of teletext

Teletext is a system of transmitting digitally coded alphanumeric data in the field blanking interval (FBI) of a television signal without disturbing the normal vision or sound signals. The data signal is decoded in the receiver and displayed as a page of information on the screen as an alternative to the video picture. Various methods for utilizing the spare time in the field blanking interval have been proposed as a means of transmitting additional information to the home, but it was not until about 1970, when Random Access Memory (RAM) and Read Only Memory (ROM) integrated circuits (ICs) became available, that practical systems become potentially viable.

The BBC Research Department had at that time been studying methods of subtitling programmes for the hearing impaired, which, in isolation, did not seem to be an economic proposition. However, the availability of larger memories made it feasible to expand the system so that many pages of information could be transmitted which would then extend the scope of the system and make it useful to all viewers. This proposed system was called CEEFAX. The IBA had for some time been using digital data carried in the FBI for the transmission of information in the television network, but this data was not normally broadcast. As a result of development work undertaken by the IBA the system was extended to include pages of information that could be broadcast in an arrangement similar in principle to that proposed by the BBC; and was called ORACLE.

In 1972 both of the UK Broadcasting Authorities had proposed these systems for transmitting information in digitally coded form in the FBI, but the systems had rather different technical parameters. Development work was undertaken which involved hah of the Broadcasting Authorities and a number of receiver manufacturers, notably Philips, GEC and Rank Radio International. This work was coordinated by a BREMA (British Radio equipment Manufacturers’ Association) Working Group. The group produced the specification for a ‘unified system’, the parameters of which were first published in 1974 [1]. Extensive field tests had been undertaken during the development of the unified system and these were continued in order to gain experience and also to increase confidence in the proposals. Further work was undertaken by receiver designers on various aspects of reception and decoding of data signals with high data rates. The influence of the frequency and phase response of IF amplifiers and video detector circuits in teletext receivers was studied and the performance of existing receivers evaluated.

A viable teletext service would need to have a service area substantially similar to that of the television service. The broadcasting authorities implemented studies which involved measurements, using receivers at fixed and mobile locations, to determine that this aim was achievable with the proposed unified system. They also established the necessary transmission standards. The amplitude and the position of the teletext signals in the FBI had to be chosen so that no disturbance would be caused to the picture or sound reproduction on existing television receivers.

The amplitude of the teletext signal was chosen so that no ‘sound buzz’ occurred, even on receivers with barely adequate intercarrier sound selectivity. Sound buzz is caused by the high frequency components of the data signal interfering with the intercarrier sound signal. If the teletext signals are inserted onto the FBI near to the field synchronizing pulses, they can show up as ‘busy dots’ on the vertical flyback across the picture on receivers with inadequate blanking. If they are inserted too close to the video signal they can then show up as ‘busy dots’ at the top of the picture when the display is slightly underscanned.

Demonstrations and tests were also carried out with non-technical viewers to gain commercial confidence in the proposed service and also to confirm the acceptability of the chosen page format. A page format consisting of 40 characters per row and 24 rows per page had been chosen, as a result of experiments, to be a satisfactory compromise between character size and the amount of information contained in a displayed page, when viewed on a normal domestic receiver. (Computer displays normally use 80 characters per row, but are viewed by an operator sitting relatively close to the screen.)

Field tests were also undertaken in Germany by the BBC and IBA in conjunction with the IRT to study the performance at VHF, with a 5 MHz bandwidth television system [2]. In 1976 a revised specification, which included an improved graphics facility, was published [3]. This specification confirmed the broadcast standards that had been verified by the extensive range of field trials.

The UK teletext specification, first published in 1974, had been evolved bearing in mind the performance of future television receiver designs, which would have improved IF response and also improved tuning arrangements. An essential feature of the proposed system was that the receiver decoding could eventually be achieved using only two or three large scale integrated (LSI) circuits so that the additional receiver cost would only be some 5% to 10%. This was essential to ensure that teletext would eventually be incorporated in all television receivers, not only the expensive ones, and hence gain widespread public acceptance. To help meet this objective, it was essential for the teletext signal to be both functional and robust so that satisfactory reception could be achieved in difficult reception areas. In addition, the system had to be capable of future expansion to accommodate languages other than English and more advanced displays incorporating high resolution graphics [4k].

The main technical features of the proposed system were:

  1. Data pulses were transmitted on otherwise unused television lines during the field blanking interval (FBI) using a bit rate of 6.9375 Mbit/s (444 x nominal line frequency).
  2. Each television data line carried all the information for a complete 40-character display row.
  3. Each page consisted of 24 rows of 40 characters, using both upper and lower-case characters, including a special top row called the ‘page header’ carrying information for control and display purposes.
    The format of the transmitted data was mapped directly to the displayed page (‘fixed-format data’).
  4. The system allowed a maximum of 800 decimally numbered pages to be used which could be extended using continuation pages.
    The pages were divided into eight magazines, each of 100 numbered pages.
  5. Using one data line per field, the system allowed two full pages per second to be transmitted.
    Additional data lines could be used to increase the transmission rate.
  6. All the data words were 8 bits in length; parity protection was used for the character data words.
    Hamming codes [5] were used to protect addressing and control words, permitting the correction, at the receiver, of single errors in those data words.
  7. Provision was made for news flashes and subtitles.
  8. Every page header carried clock-time information, for a time display.
  9. Control characters were used to provide colouring and flashing of selected words.
  10. A colour graphics facility was provided.

In formulating the system, a number of factors, some of them conflicting, were considered as follows:

  1. access time and the number of pages
  2. page format, content and legibility
  3. data rate and the television system bandwidth
  4. flexibility of the system, and future developments
  5. receiver options and additional cost

It was considered that a reasonably short access time should be a prime objective, but, at the same time, adequate capacity should be available to justify the cost of the system. In 1976, two lines were used per field blanking interval; hence approximately four pages were transmitted per second. The consequent transmission time for a 100-page magazine, 24 seconds, meant that the mean time for a page to be displayed, after selection, was 12 seconds. This ‘access time’ could be maintained when more than one magazine was used, only by employing additional data lines in the fbi.

For a page of 960 characters (24 rows of 40 characters per row), the required page store was approximately 7000 bits: well within the capacity of one LSI circuit. To simplify the receiver decoder, all the data preamble and the information for one row of characters, 360 bits in all, was transmitted in approximately 52 microseconds within one line period. Hence, the required bit rate was some 6.9 Mbit/s. That bit rate was within the capacity of an existing 5 MHz video channel, but extra care in the design of the receiver IF amplifier and demodulator was required to prevent the performance being degraded by poor pulse shape. Aerial matching was also important since short-duration reflections also degraded the data signal, even in a good reception area.

As a result of experience gained during the two-year experimental period (1974 – 1976), a number of minor modifications were made to improve the information presentation. These included the display of double-height characters, the addition of coloured background to discrete areas of text and a ‘graphics-hold’ facility to prevent discontinuities in the graphics when changing colour. Provision was also made for the system to operate with the magazines transmitted in series (one after the other), using common FBI lines, or in parallel, using separate lines. A revised specification was issued in 1976 [3].

Alternative teletext system proposals were made at about this time both in France and Canada. The French proposal, ANTIOPE [6], had a transmission format to make it compatible with data transmission over telephone lines, but the system did not take any advantage of many of the characteristics of the television signal [7]. The Canadian proposal, TELIDON [8], was designed to produce very high quality graphics; but a much more complex decoder was required and such a decoder would not be appropriate for a domestic receiver for many years. Both these systems are ‘variable-format data’ which required considerably more data to format the page than a ‘fixed format’ system. The former also requires a considerably more elaborate error correction arrangement to prevent disruption of the page format.

A fixed-data format system has a considerable economic advantage and it also provides a much higher measure of protection against potential reception errors [4f]. As a result of continued development in several countries, the UK teletext system had, by 1984, evolved into ‘World System Teletext’ (WST) [9,10,11]. The system is now in use in more than 30 countries throughout the world and the decoder LSI circuits are incorporated into television receivers with very little increase in cost. Whilst the teletext standard is independent of the colour television standard being used – PAL, SECAM or NTSC – it is dependent on the line frequency and video bandwidth of the television channel. A different standard was therefore developed for 525-line receivers [4j, 12]. Teletext first became established in Europe and then extended quite rapidly to many other 625-line countries. Its penetration into 525-line countries is at the moment relatively small, but this situation is likely to change now that 525-line decoder LSI circuits are becoming available.

Fixed data format, as used by WST, permits a relatively simple decoder to be used but this has not inhibited further development of the system. New enhancements include high resolution graphics, the addition of special characters required for a wide range of different languages and the transmission of still pictures.

The various enhancements have been specified as a number of different ‘levels’ [13]. The main features are briefly summarized as follows:

Level 1 Ninety-six character font, double height, flashing, mosaic graphics and eight colours.
Level 2 Additional data packets (non-displayed rows) and ‘pseudo pages’ for transmission of additional characters and non-spacing attributes; a wider choice of background and foreground colours; a ‘smoothed graphics’ capability and other display features.
Level 3 DRCS (dynamically redefinable (downloaded) character sets). The character sets may include picture elements as well as special writing characters.
Level 4 Alphageometric displays. The drawing instructions assume data processing in the receiver (not needed at lower levels).
Level 5 Alphaphotographic displays - still pictures.

The higher levels, in particular levels 3, 4 and 5, require increasing amounts of page storage, and for levels 4 and 5 associated data processing, so that decoders are progressively more expensive. An example of a level-3 page is shown in Figure 1.1(a) and a corresponding level-1 page in Figure 1.1(b). Page creation for these levels requires more complex digitizers and editing software. The additional data required for each page also increases the access time, which effectively reduces the response time of the system.

The maximum number of FBI lines that can be used at present is 16 but when no video signal is present teletext data can occupy all transmitted lines, providing an increased transmission capacity of some 18 times. Such systems are called Full-field Teletext Systems.

Figure 1.1 (a) Level-3 page

Figure 1.1 (b) Corresponding level-1 page

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