The teletext data signal parameters were chosen and specified so that the data could be inserted in the field blanking interval of the normal television signal without disturbing the normal vision and sound signals. The video signal can therefore be considered as a common carrier for the data signal. The data signal, consisting of high speed pulses, is sensitive to amplitude, group delay and non-linear distortion. Such distortion causes overshoots and a deterioration in the separation between the 0 and 1 levels of the signal.
When a television signal is distributed by a wide spread distribution network, it passes through various links and switching centres. Although the quality of the colour television signal is maintained to broadcast standards, the data signai can suffer some degradation. To provide the maximum teletext service area for a given transmitter and also to ensure that the data signal does not disturb the reception of the transmitted television sound or vision signals, it is essential that the data signal be radiated without degrading the pulse shape, and the amplitude of the data pulses must not exceed the correct value. If the amplitude of the data pulses is low, then teletext reception will be impaired and is more likely to be further impaired by poor received signal quality and noise. On the other hand, if the amplitude is high teletext reception may be improved, but receivers are liable to exhibit sound buzz caused by the excessive amplitude of high frequency components of the data signal interfering with the inter-carrier sound signal.
The digital data signal can be processed independently of the video signal. This enables the data to be regenerated or linked to other television networks as required for the teletext service, without disturbing the normal television network.
Television networks often have many special arrangements for distributing video signals which can vary during the day depending on where programmes have been originated and which transmit-ters are required to radiate them. This is particularly the case for television services which have different transmitters for different programmes during the day. The teletext service, however, is likely to be produced centrally, therefore the teletext signal needs to be distributed throughout the television network independently of the studio signal sources.
The teletext data signal can be transferred between different parts of a network using a 'data bridge'. The data bridge has two video input circuits, each of which provides sync, blanking and data clock pulses. The teletext data signal from the A channel is separated from the video signal, error-corrected, and clocked into a buffer memory as shown in Figure 6.1. The memory capacity must be adequate for buffering all the teletext data that is contained in one FBI period of up to 16 lines, that is 16 x 360 bits. If field integrity is required to be maintained then double this capacity is required as the data must be stored for two field periods. The teletext data need not be inserted on consecutive FBI lines as the insertion test signals (VITS) are inserted onto agreed lines and the teletext data can be inserted on lines either side of such signals. It is therefore necessary to maintain the teletext data line integrity, that is the data on line 21, say, is always bridged onto line 21, irrespective of teletext signals on other data lines. Maintaining data line integrity ensures that page headers (row 0) for new pages, subtitle data on dedicated lines or data services using teletext format do not become line shifted.
The memory output circuit is controlled by signals generated from the B channel. The output is serialized and bandshaped to the required specification. This is achieved by passing the fast digital bit stream, which has identical rise and fall times, through a special band shaping filter. Teletext data is then inserted onto the B video channel, the video lines being erased prior to insertion to remove any noise and thus ensure the highest possible quality of the teletext signal on the second network. The two video signals can be asynchronous but the number of data lines on each channel must be the same. The line numbers, however, need not be the same. A teletext signal can therefore be passed through very complex video television networks or can bypass studio centres as required, without any disturbance to the normal television operations. Furthermore, the data is completely regenerated by the bridge at the point of transfer to full specification.
Whilst at first sight it might appear that the input requirements for a data bridge are less stringent than those for a receiver, they are in fact rather more critical. The data bridge provides the input teletext signal to the television network which will then be transmitted to a very large number of receivers. It is therefore imperative that the data signal meets full broadcast specification at the bridge output in spite of signal variations, sync genlocking and other phenomena that occur in television networks. It follows that the data inserter must also meet full broadcast specifications and under no circumstances must the bridge insert data during the actual video signal period.
The data bridge incorporates a test page generator which is initiated whenever no data signal is received from the input network. The page can be programmed to contain an apology caption, which can include the location of the bridge in the network, and it can also maintain test signals at all times for the use of technicians installing or maintaining teletext equipped television receivers.
The data bridge can also shift the teletext signal to different television lines. Thus various teletext sources may be combined onto one video signal, which then acts as a carrier for distribution purposes.
The normal teletext signal consists of 7 bit characters plus an odd-parity bit. Although each character is then represented by an 8-bit code, the code will always be of some combination of 0 and 1. These data transitions are often used in decoding circuits to help maintain the phase of the regenerated clock. When software code or computer data is being transmitted characters which consist entirely of 0s or 1s may occur and transients are not then available to help maintain the phase of the regenerated clock. The transmission of such 8-bit code therefore requires special tests to check that equipment in the television network will not produce errors. This aspect is of particular importance for data transmission to 'closed user groups' as the data is not repeated and an error in transmission could invalidate the message (Chapter 9, page 106).
![]() Figure 6.1 Functional diagram of a data bridge |
Some distortion inevitably occurs in the transmission of television signals in widespread networks. The specifications for video links, and associated measurement techniques, are designed to ensure that the television signal meets broadcast standards. A somewhat different weighting of distortion criteria is appropriate for fast data signals which are particularly sensitive to group delay distortion and to the non-linear distortion produced by some re-broadcast links (RBL).
Provided the teletext data can be decoded correctly, its characteristics can be fully restored by regeneration. At the present time, regeneration of the data signal must be carried out at video. Regenerators can therefore be used only where a base band signal is available, such as at the input to a transmitter.
The regenerator strips the data from the video signal, completely reprocesses and retimes it, carries out band shaping and then reinserts the data onto the video signal one field or one frame period later. The video line is 'erased' prior to insertion. The video inserter section of the regenerator must meet the full colour television broadcast specification as it is in the main programme path, and it must also be equipped with a physical bypass relay which operates in the event of a malfunction. The output circuits of a regenerator are therefore similar to the output circuits of a data bridge.
In addition to their use at transmitting sites, regenerators are also necessary at the outputs of video tape machines to restore and line shift the data when teletext subtitles are recorded together with the programme material.
Data regenerators cannot be used at transposer stations because the vision signal is converted only to an intermediate frequency, not to base band. Current transposer designs are broad band to accommodate combined sound and vision signals and, since they have no sound notches, they do not introduce significant group delay errors at high video frequencies. The need for regeneration equipment in transmission chains incorporating transposers is therefore much less as the data signal degradation is smaller.
A teletext signal is independent of the video signal which acts as a common carrier. In a 625-line/50 Hz teletext system, 40 characters are carried in one data line period. There are, in addition, the clock run-in, framing code and row numbers, so that the total number of 8-bit characters per line is 45. The data rate for one data line per field in a 625-line system is therefore 45 x 8 x 50, i.e. 18 000 bits per second. The bandwidth of 525-line/60 Hz television systems is narrower and only 32 characters can be transmitted in one television line period, which has approximately the same time duration as that of 625-line television systems. The corresponding data rate is therefore 37 x 8 x 60, i.e, 17760 bits per second. The data rates for 525- and 625-line television systems are therefore substantially the same, the 525-line system having a slightly lower data rate.
Teletext transcoding from one television standard to another, in either direction, therefore requires a unit rather like a data bridge but with the input and output circuits each operating on a different standard. The data rates for the 525- and 625-line systems are similar and the displayed page format is the same (40 characters per row and 24 rows per page). However, transmission format is different, as in the 525-line system only 32 characters are transmitted in one data line, the remaining eight characters of each of the preceding four rows are transmitted together as a separate data line. So four rows require five data lines to transmit and each page of 24 rows therefore requires 30 data lines. The buffer memory needs to have a storage capacity of at least one page. In conversion from 525-line to 625-line teletext, the input data rate is 240 bits/second, or 30 characters/second, lower than the output data rate. It might therefore be expected that the 625-line teletext output signal would occasionaly have blank lines, when there was no data available for immediate output. However, the transmis-sion of blank lines can give rise to misleading reception and measurements in a broadcast system and the normal practice to avoid this is to duplicate a line of data.
Conversion from 625 lines to 525 lines requires the buffer memory to be capable of storing the complete transmitted magazine of teletext pages, as the output data rate is lower than the input. The 625-line input signal is decoded and the pages are written into the buffer store on a continuous basis. The output of the store is under the control of the 525-line signal which continuously reads from the memory and outputs the data at the 525-line rate.
There are a number of advantages in having a memory capacity able to contain the complete teletext magazine. It enables a different number of television lines to be used at the output from that of the input and, in the event of the input signal being switched off, the output is maintained. The input data signal updates the contents of the memory and the contents are then continuously available for the output. The converter can therefore increase the number of transmission lines used by the output system and the access time at the output of the converter can be very much faster than that of the input. The real time clock would normally be generated by the processor used in the converter so that the correct local time may be incorporated into the output page headers together with other station or service identification.
![]() Figure 6.2 Page exchange system for regional teletext services |
Regional teletext services can be provided in a number of ways depending on whether the pages are to be edited locally or at a central point.
A separate teletext system operating in parallel with the main system can provide a completely separate local service provided there is an unused FBI line available and a magazine number not used by the main service. Both teletext systems must operate in the parallel mode to prevent decoders being confused by the data signals from the two teletext systems. A control bit (C11) in the page header is set to '1' for serial magazine operation so that the decoder display clock is updated by the pages in any of the magazines. If this bit is set to '0' for parallel operation but the magazines (on the main system) are being transmitted in series, then the decoder can respond only to the data of the selected magazine. The display of clock time will then only be updated during the transmission of this magazine. A regular update of displayed clock time can be achieved by interspacing pages (or page headers), from the other magazines in the series.
The arrangement used for page exchange is shown in Figure 6.2. The signal from the video input is decoded, error checked and fed to the data store. This store provides the buffer memory for the main teletext pages and also the store for locally created pages. The system processor controls the read function under the control of the resident software. A data entry port is provided so that the editing terminal can communicate with the system, The editor creates the pages that are to replace others in the magazines. When these pages are entered into the memory, they must be given page and magazine numbers corresponding to those that they are to replace. As the input pages from the main programme are decoded, the page numbers are checked by the processor, which is then able to insert the local pages into the teletext output on an exchange basis.
As the pages of the main teletext service are not stored - only the current page being held in the buffer memory - the replacement pages must not require any more rows of data than those they replace. The local editor must therefore be able to check all the aspects of a page that is being exchanged. This is particularly important when additional rows or packets are used, as required by certain languages for example. It is also essential that the same number of television data lines are used at the output. These problems do not arise if the complete teletext magazines of the main programme are stored. Furthermore, the output can be maintained if the input signal is temporarily disconnected, and the number of data lines can differ from the input. Page exchange facilities allow a local or regional service to become operational quickly and the local facilities can be expanded later as required.
The editing terminal needs to contain a teletext decoder and to have a direct feed from the input video signal. The local editor can then capture any page from the input for examination at any time, independently of the local system. This facility is essential when the system does not store the complete input magazines of pages.
The video signal acts only as a carrier for the teletext data and the video programme content is not relevant except when there are teletext subtitles. In such cases, if the local video programme is different, the subtitles must be deleted from the teletext magazine, and possibly stored with the network video programme material for later use (Chapter 7).