4 Teletext data management and transmission

The teletext pages from the editing terminals are fed to the main teletext system, which consists of a processor with resident software, the page store with adequate capacity for all the pages that will be used, the teletext data formatter, data pulse shaper and inserter. An archival store may also be provided together with a printer. The essential elements of a typical system are shown in Figure 4.1. The resident system software contains a wide range of system functions which the editors use to produce the magazines of pages to suit particular requirements. These include the more frequently transmitted pages, enabling a page to be transmitted out of sequence; this facility is sometimes used for index pages. A page may be transmitted, say, every 20 pages to provide rapid access for the viewer. The rotating, or extended, page sets are also produced by the system software, so that four, five or more pages may be given the same page number and be transmitted sequentially with a time interval of typically 20 seconds between each. This facility allows a teletext page to have several associated pages. Such instructions to the system processor are sent as commands from the editing terminals and are associated with the particular page (or pages) to which they refer. The system processor will have a number of entry ports for editing terminals and external data feeds. The system software will therefore contain a command structure designed for the particular requirements for processing the pages from memory into the required sequences for the output magazines together with data management software which is associated with the input terminals and other data inputs.

Figure 4.1 Single channel teletext system

A hard disc can be used for both page storage and page archival purposes. However, the use of a hard disc for the page store limits the output capability of the system in terms of data output rate, due to the time delays in accessing the hard disc. Also system reliability is impaired because the mechanical disc drives are in continuous use. Large capacity RAM memories are now available at sufficiently low prices to allow them to be used for the page stores, even on large systems. Such memories are battery backed so that in the event of a power failure, or when the system is switched off, the content of the memory is maintained. It is also becoming increasingly common for the resident software to be contained in EPROM rather than on a hard disc. This again improves system reliability since the software is completely independent of mechanical disc drives. On systems in which the operating software is contained on a disc, updates can be made or design ‘bugs’ cleared using a floppy disc or a modem link. Updating the software contained in EPROM requires the PROM to be changed, but the improved reliability is often considered worthwhile. When a hard disc is used for the archival page store a tape streamer is often used for backup purposes.

The system processor feeds the pages, in the required sequence, to a cache memory for the data formatting function. The header row (row 0), which contains the static information, the real-time clock and the data run-in sequence, is added, so that the page is formatted ready for transmission.

The formatted data is serialized and passed through a fast digital logic circuit to ensure that the data pulses have identical rise and fall times. This data is then fed through a special band shaping filter whose characteristics are carefully chosen so that the output signal meets the required broadcast specification. This signal is then fed to the data inserter for insertion into the video output of the studio. Synchronizing and timing signals are generated from the input video signal to control both the data formatter and the data inserter. Essential features of these circuits are that they time the data correctly, and insert it, once only, onto the selected FBI lines. Under no circumstances must the data ever be inserted during the video period, even when the sync pulses are disturbed by the transients that may occur in television networks (due, for example, to genlocking). The cross-talk from the fast logic circuits into the video circuit must be at a level of less than -70 dB with respect to the video signal. The data inserter must also be fitted with a physical by-pass relay, to operate in the event of a malfunction or power failure.

Backup facilities are also necessary for the teletext system so that a service can be maintained in the event of a malfunction in one of the units. In such a case it may be permissible to operate with a reduced service, for example, by limiting the number of editing terminals in use and having only a single magazine output. If no breaks can be allowed in the service, then a complete backup for the system to cover all possible modes of failure is necessary. A data monitor checks the output data leaving the system and should it detect any form of failure it will cause the standby units to be switched into circuit automatically, and raise the necessary alarm.

In many cases a teletext system may be required to feed two independent television channels with different information, together with some common information. Such a system is likely to use 10 or 12 editing terminals together with data inputs from other sources so that the management of the data inputs is itself a considerable task. In such systems the tasks of input data management and teletext system management are normally separated, with individual processors and software. Separation of teletext systems into smaller units with individual processors, each with relevant software, enables greater flexibility in design; and reliability can also be improved.

A dual-channel system with full backup is shown in Figure 4.2. The various functions are shown as separate units for clarity but in practice some of these might be combined. For example, one transmission processor might control the memory used for the two page stores for one channel, and also provide the two teletext outputs to the data detector. The various editing terminal and data inputs are fed to a data port switcher which feeds them to one of two identical data management systems. These systems operate in parallel so that all the information in one is duplicated in the other. When a large number of editing terminals are in use it is often necessary to restrict the access which they may have to the system, by the use of passwords.

The different magazines of pages are prepared by the data management software from the instructions provided by the editors. Magazines can also be prepared off-line, in advance, and can be ‘enabled’ at a specific time for transmission, entirely under machine control. A page which is required to be inserted into different magazines need be created only once; the data management software will insert it into magazines specified by the editor. The now familiar ‘World Time’ page wilt also be regularly updated automatically.

To simplify editorial tasks, new commands can be produced that cause a sequence of actions to take place from a single entry. Such commands are called ‘macros’. A typical example might be that a news page is entered into its own location and in addition be entered into other magazines and the news carousel. Archived pages that are not in current use but that may be needed at a future date can also be stored on the hard disc. As a hard disc can ‘crash’ and lose the data, the discs are normally backed up by a tape streamer which takes data from the hard disc and stores it on the tape cassette.

Figure 4.2 Dual channel system with full backup

External data feeds are also used by teletext systems so that special pages or data packets can be produced without any editorial action – for example, pages of financial information and racing results and Packet 8/30 for the control of domestic video recorders. The data from these sources is fed direct to the system input ports using modem links for external information sources.

The data management software is programmed so that these unsolicited inputs are correctly page-formatted, the appropriate command instructions added (page numbers etc.), and the data entered into the correct magazine. Such inputs to the system would be archived, as would all the inputs from the various editing terminals, thus keeping a complete record of all data transactions. As many broadcasting authorities have to keep records of what is transmitted, there is an automatic logging system to ensure that all transactions are placed on record for a given length of time. A printer is normally used for transaction logging purposes. The time-code input from the station clock is also an automatic data feed to the system, which keeps the real time clock on the transmitted pages in step.

When several editing terminals are in use a ‘mail box’ facility can be provided to enable messages to be sent between terminals via the system. The communication channel is normally opened simply by specifying the source and designation names.

Each data management system drives a pair of cache memories and associated data formatters. One pair is cross-linked and the outputs combined in the data detector of each channel. The data detector monitors the teletext data activity and if this becomes static it switches the standby system into operation and raises an alarm. The serial data is shaped and inserted onto the FBI of the appropriate television signal. The teletext signal from a transmit-ter is normally continuously monitored to check the actual station output.

When subtitles are being transmitted monitoring the output is most important as the timing of the subtitle is critical. Preparation of subtitles (see Chapter 7) is normally an activity separate from the provision of the main teletext service. The subtitle output is fed to the teletext system as an unsolicited data input but the port must be given priority as the subtitle must be transmitted immediately it is received. The page currently being transmitted must be interrupted by the subtitle page to minimize transmission delay. The subtitle page is transmitted with the suppressed header bit set so that the header is not displayed in the receiver, the subtitle itself consisting of only one or two data lines.

The teletext system is self-contained as all the data is independent of the television service until the formatted data is inserted onto the fbi lines of the video signal. The necessary synchronizing information can be derived from the inserter, which is normally in the studio feed to the transmitter. The teletext system and the associated editorial activity is technically indepen-dent of the television service and can be located to suit the broadcast authority.

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