2. The teletext data signal

Bandwidth and data rate

The teletext data and video signals are hybrid in that both analogue and digital components are present. A 1 V p – p video signal has a black level of 0V, a sync level of -0.3V and a brightness component which can vary in level up to a peak white value of 0.7 V. The video signal therefore consists of a generated source of mixed syncs and colour burst, plus the colour video signal.

Digital data signals which originate in computer logic circuits have fast rise times and wide bandwidths and, (normally), binary levels of +5 V and 0 V. Before the two types of signal, logic and video, can be combined, the logic data must be organized into line-length packets, and adjusted in both level and timing. The data bit rate must be as high as possible to maximize the number of data bytes per line without exceeding the video channel bandwidth. The fast rise-time logic pulses will not pass undistorted through the 5 MHz bandwidth video channel and they must be pre-shaped so that their frequency spectrum is contained within that bandwidth. Data must then be inserted cleanly onto the video signal, using a fast electronic switch controlled by an accurately timed insertion pulse.

As previously mentioned, teletext information is organized in a page format consisting of 40 characters per row and 24 rows per page. The pages are transmitted cyclicly, that is one after the other in a continuous sequence. When the complete sequence or magazine of pages has been transmitted it is then repeated. The first row of a page is referred to as the ‘header row’ , or row ‘0’ . This row contains the magazine and page number, the service name, date and time. In order to capture a page for display on a receiver, the magazine and page numbers are keyed in by the viewer. When the requested page in the magazine is transmitted it is then captured by the decoder and displayed on the screen.

As the pages are transmitted sequentially it is unlikely that the page will be transmitted at exactly the time a viewer requests it. There is therefore a delay between when a viewer requests a page and when the page is captured and displayed. The average delay time is called the ‘access time’ and is a critical parameter in a teletext system. It must be as short as possible and is related to the data rate, the data format, the number of pages being transmitted and the number of FBI lines being used for teletext.

Non-Return-to-Zero (NRZ) coding was chosen as this provides the highest bit rate for the binary coded data signal. Theoretically the minimum bandwidth required is half the bit rate. A bit rate (clock frequency) of 6.9375 Mbits per second was chosen for the 625-line systems, which have video bandwidths of 5, 5.5 or 6MHz. The data signal frequency spectrum is centred at a frequency of 3.45875 MHz which is well inside the 625-line minimum video bandwidth of 5 MHz. The clock frequency corresponds to 444 times the horizontal line frequency of 15 625 Hz, but the data signal is not normally locked to the television signal. The bandwidth of 525-line television systems is normally 4.2MHz and for this system a clock frequency of 5.727272 MHz was chosen, corresponding to 364 times the horizontal line frequency of 15,734.264 Hz, and again the data signal is not normally locked to the television signal.

Figure 2.1 An approximate spectrum of a data pulse

The data pulse shape is chosen so that most of the energy is contained within a bandwidth of 5 MHz for 625-line systems (Figure 2.1) and 4.2 MHz for the 525-line systems. As a result of experience gained on video distribution networks feeding trans-mitters, a 100% raised cosine filter was found to be optimum for pulse shaping and is now standard. A typical data pulse at the input to a transmitter is shown in Figure 2.2.

Figure 2.2 Data pulse (eye display) at a transmitter input

The data pulse amplitude is specified [13] as 66% of peak white video for WST, as shown in Figure 2.3. This amplitude was determined after extensive tests involving different receivers to ensure that the data signals did not interfere with the normal reproduction of the picture or sound.

Teletext is a ‘one way’ transmission system utilizing television broadcast networks. Teletext information can therefore be rapidly transmitted to millions of receivers simultaneously. To maximize the information transmission rate it is therefore desirable to minimize the amount of data required for each page. Different data formats have been specified for different teletext systems [6,8,13,14], but any of these specified teletext systems can achieve similar results in such matters as the volume of displayed information and the quality of the graphics. However, there are significant differences between the systems in terms of transmis-sion time because of the different amounts of data required to be transmitted for a given page. There are also differences in the complexity of the receiver decoders.

Figure 2.3 Data levels

Teletext data format

There are basically two techniques for formatting the data for the pages that are to be transmitted in the FBI of the television signal: free-format and fixed-format.

Free-format

A page of data and graphics can be represented as a continuous string of digits using the normal ‘line feed’ and ‘carriage return’ codes to signify the end of each line. (This format is used when data is sent over telephone or data transmission lines.) A typical teletext page might contain some 8 K bits of data which could then be divided up into a series of 50-microsecond data blocks. These data blocks could be added to the fbi lines of a television signal at 50 Hz intervals, i.e. several blocks of data per field. Each block of data would need to be preceded by a data clock run-in together with some additional bits for ditional bits for byte synchronization. At the decoder the blocks of data would be stored so that after several field periods a complete page of information would be held in the decoder memory and could be processed for display. With free-format data there is no relationship between the position of characters on the screen and their positions in the data block. Positional information must therefore be transmitted as part of the data stream. Any interference or distortion resulting in any loss of information during the transmission will result in errors in the received data. The errors would therefore show up as either incorrect characters in the page or information appearing in the wrong position. In order to guard against these problems various forms of error protection must be employed and this very significantly increases the amount of data required for a page of information. Furthermore a data processor is an essential part of the decoder to enable the complex data stream to be satisfactorily decoded and displayed.

Free-format is used in the French ANTIOPE system which is used in France and in the North American Broadcast Teletext Specification (NABTS) [14], which was derived from ANTIOPE and TELIDON, but has yet to be implemented as a domestic service. The advantage of a free-format system is that the data can be readily supplied from external computers as there is no fixed relationship to the television system. In theory only the bit rate need be changed as no reformatting is necessary. The disadvan-tage of the system is that a very significant amount of additional data is required for adequate data protection to avoid poor reception seriously disturbing the display. More complex proces-sing is also needed to provide positional and control instructions in the decoder.

Figure 2.4 Data timing and run-in sequence

Fixed-format

The fixed-format system, as used by World System Teletext (WST) on both 625-line and 525-line systems, exploits the regular and defined timings of the television signal that carries the data to ensure that the characters are always displayed in the correct position on the screen. Very little positional information has to be transmitted and the decoder is correspondingly simplified.

One row of teletext data (40 characters) is transmitted in one television line period in the 625-line version. Each row starts with a ‘run-in’ data sequence to synchronize the decoder clock, followed by byte synchronizing bits for logic synchronization, and then the magazine and row number, as shown in Figure 2.4. This data is then followed by 8 bit codes corresponding to each of 40 characters in one row of displayed information. When no character is present a ‘space’ character is transmitted so that one row of information always has code corresponding to 40 characters present in the data stream. There is therefore a direct ‘one-to-one’ correspondence between the position of the character in the display and its location in the line of data, as illustrated in Figure 2.5.

Figure 2.5 Fixed Page format used by WST

A page transmission normally starts with the top row, row 0, and finishes with the succeeding row 0 of the next page. Row 0 is called the ‘header row’ and contains the magazine and page number, service name, date and time. After the header row, row 1, row 2, row 3 etc., are transmitted. The page-header and row formats are shown in Figure 2.6(a) and (b).

Figure 2.6(a) Page-header format (row 0)

Figure 2.6(b) Row format (rows 1-23)

As each row of data for a particular page has the magazine and page number as its address they can be transmitted in any order and mixed with rows from other magazines. Rows containing no information need not be transmitted (rather than transmitting a row of ‘space’ characters). The technique of transmitting only these rows which contain information is called ‘row adaptive’ transmission. This feature is particularly useful when subtitles are transmitted, as these normally only contain one, two or possibly three rows of information; and also for news flashes, which again normally contain only a few rows of information. When the complete page has been transmitted the following page header row is used as an indication to the decoder that a complete page has been received.

In 525-line systems the video bandwidth is less than that for 625-line systems, while the active line period is approximately the same (52 microseconds). The data rate is therefore reduced by approximately the ratio of 525/625. This means that only 32 characters per row can be transmitted in one television line period, but 40 characters are required to be displayed. In the fixed format system, the ‘gearing’ technique is used to overcome this limitation [12]. The last eight characters of each of the previous four rows are transmitted as a separate complete row. Each page of 24 rows therefore requires 30 data lines. As the field frequency of 525-line system is 60 Hz, compared to 50 Hz for 625-line systems, the overall transmission rate for the system is similar.

Character control codes (control characters)

Character control codes are required to augment the display of normal alphanumeric display characters in order to create special effects. These include colour changes, flashing, double-height or double-width characters, and graphic symbols. Character control codes use the same 7-bit format as the characters and are often referred to as ‘control characters’ . In fixed-format systems the control codes are normally inserted in front of the block of text to which they refer and the transmission of a control code occupies a character space. This does not normally impose limitations to an editor since the control codes are inserted in spaces at the start of sentences or prior to a graphics symbol. Control codes contained within the page in this way are referred to as serial or spacing attributes. In free-format systems a space in the text is not needed for insertion of control codes, that is, they are ‘non-spacing’ attributes.

Character code extension

The character code tables for use with fixed-formatted pages are limited to a font of 96 display characters. Certain languages, for example, Spanish and Arabic, require additional characters. Such characters are transmitted on additional data rows that do not correspond to one of the 24 display rows and are therefore not displayed. Initially these rows were referred to as ‘ghost rows’ , but they are now known as ‘data packets’ and have been assigned ‘packet numbers’ . WST has also been further developed to include high resolution graphics and other facilities, as outlined on pages 20-23.

Figure 2.7 Teletext 96-character code table

Error protection

One transmitted data line carries 40 8-bit teletext character codes for one display row, preceded by a 3-byte start sequence and a 2-byte row address. An 8-bit character byte comprises a 7-bit ASCII code and a parity bit, the logic value of which always results in an odd number of logic 1s in the byte. The 7-bit code can have a maximum of 128 binary combinations. The data bits are numbered bit 1... bit 7, and a byte is transmitted with least significant bit first and parity bit 8 last. The 128-code look-up table is shown in Figure 2.7 and the teletext code for any one of the 96 alphanumeric or 64 graphic characters can be found by combining the binary values of bits 1 to 4 on the left with the binary value of bits 5 to 7 at the head of the column. The 32 codes in columns 0 and 1 are control codes which switch the significance of codes in the other columns between alphanumeric and graphics and give some attribute, such as a colour change, to succeeding characters. Control codes are usually spacing and non-displayed characters, i.e. they occupy a character position in a row but are ‘displayed’ as blank spaces. (For editorial purposes, it is very often convenient to display control codes as two-letter mnemonics and this facility is available on editing terminals; see Figures 3.2 and 3.4.)

All teletext rows start with two bytes of ‘run-in’ . This forms a train of consecutive ‘1’ and ‘0’ which gives the maximum possible number of data transitions for excitation of the clock recovery tuned circuit in the receiver decoder. This establishes bit synchronism in the decoder. Byte 3 is the framing code, on recognition of which the decoder character counter starts up, thus establishing byte synchronism. Bytes 4 and 5 are Hamming codes, each containing four packet address bits and four error protection bits. The least-significant three bits of the 8-bit packet number is the binary-coded magazine number, identifying which of eight possible 100-page numbered magazines the row belongs. The most significant five bits are the binary-coded row address. Only 24 of the available 32 row-address codes are used for ‘level-1’ teletext (see pages 20 – 23). It is now common to express data row numbers as ‘packet X/Y’ , where X is the magazine number and Y the row number. All data lines start with the five bytes as described, followed by 40 teletext character codes, i.e. 45 bytes per line.

The header row carries a further eight Hamming-protected address and control bytes, leaving only 32 character codes for display, as shown in Figure 2.8.

Figure 2.8 Synchronisation and Hamming codes at start of page-header and row transmissions

Bytes 6 and 7 are binary coded decimal (BCD) page number units and tens (00-99). When the combined magazine number and page number on the transmitted header agree with a user-requested magazine and page number in the decoder, the page is written into the decoder’ s page memory and displayed.

Hamming-coded bytes 8, 9, 10 and 11 provide for page number sub-codes which can either be related to time of day as indicated in Figure 2.8, or can be used as an extended address to increase the basic 800 decimal numbered pages to some 2.56 million, or even more using hexadecimal numbers. The extended address is in BCD, byte 8 is minutes (0-9), byte 9 is tens of minutes (0-7), byte 10 is hours (0-3) and byte 11 is tens of hours (0-2). Control bits C4, C5, C6 take up the spare address bits in bytes 9 and 11. Eight further control bits make up Hamming coded bytes 12 and 13. The control bit functions are listed in Figure 2.8. C12, C13, C14, shown as unallocated, are now used to switch decoders to display a basic character set of 83 characters, plus one of three sets of 13 national option characters. The editor decides which national option set is displayed by transmitting a particuar combination of control bits C12, C13, C14. Character generators with national options are available for various languages or combined in a single LSI circuit.

Hamming protection codes make correct reception of the addresses, that is magazine and page numbers, more probable; without these the following data cannot be displayed. The parity bit check for individual characters normally ensures that should an error be received the character will not be displayed. In the event of two errors in one byte, the decoder will display an incorrect character. Since the teletext magazine of pages is transmitted cyclicly, when the page is next received characters that were received with an error on the first acquisition almost certainly will be received and displayed correctly. Should a single error be received in a character that is already displayed correctly, then that character will not be changed. In free-format systems Hamming protection is essential for positional and control information as well as for the page address, which is a significant overhead.

Access time and data rate

Access time is the time taken to display a complete page after selection by the viewer and it is very desirable that this should be as short as possible. To obtain the maximum throughput of data in a teletext system it follows that the data rate must be as high as practical, bearing in mind the constraints of the television channel. For 625-line systems the video bandwidth is at least 5MHz, (5.5 MHz for the UK television system I and 6 MHz for system D). When the teletext data standard was being evolved 625-line reception tests showed that the noise immunity of data transmitted over the television channel was not significantly less for data at a bit rate of just under 7 M bits per second than for data at a lower rate of 4.5M bits per second. A bit rate of 6.9375 M bits per second was chosen for the 625-line systems. The bandwidth of 525-line television systems is normally 4.2 MHz and a data rate of 5.727272M bits per second was therefore chosen. The teletext page is displayed in colour (on a colour receiver) but the teletext system and data signal is independent of the video colour standard (PAL, SECAM, or NTSC). The video signal simply acts as a carrier for the data, the decoder providing RGB output signals.

The data rate chosen for the 625-line WST enables one complete row of text to be carried in one television line period. The page is normally made up of 24 rows and it follows that approximately two pages are transmitted per second per data line used. When six data lines are used some 12 pages per second are transmitted. In this case the access time will be a maximum of about 16 seconds, with an average access time of about eight seconds, when 200 full pages are transmitted. Although the data rate is lower, 525-line fixed format systems have a similar access time as the field rate of 60 Hz is higher.

Higher levels of WST

As outlined in the introduction, WST has been further developed to cater for a wide range of future applications. These enhancements have been specified [13] as five levels, in a flexible manner so that the required features of any level can be implemented as required. However, current decoders exploit the various language capabilities of level 2 only. The main features of the various levels are outlined as follows.

Level 1

Ninety-six character font, double height, flashing, upper and lower case characters, mosaic graphics, eight colours, concealed characters, newsflash and subtitle pages.

Level 2

Level 2 introduces non-displayed rows, which are now called packets; the packet number corresponds to the row number. The teletext packet consists of 45 bytes. In every packet the preamble remains the same and is composed of a 2-byte run-in and' a one-byte framing code which ensure bit- and byte-synchronization respectively. Immediately following this is the two-byte magazine and packet address group (MPAG), formerly known as the magazine and row address group. These packets contain control data and are transmitted before the teletext pages (Packets 0 to 23) so that the additional control data for the decoder arrives before the actual page. This speeds up processing and avoids any necessity to change the page after initial display.

Packets 0 to 23 inclusive are as level 1. They relate directly to the displayed page and the 40 bytes after the preamble are dedicated to the definition of characters and their display attributes.

Packet 24 is used to display the ‘Fastext’ prompts at the bottom of the screen, on a 25th row. These prompts take the form of four colour-coded keywords, e.g. News (Red), Sport (Green), Weather (Yellow) and Finance (Cyan). The colours match keys on the viewer’ s remote control handset (see Chapter 5, pages 54 – 56).

Packet 25 consists of 40 display characters or attributes and overwrites the page header, row 0, on a decoder that supports this feature.

Packet 26 can support many modes but is primarily intended for extending a character set (typically from 96 to 128 different characters). This is achieved by using supplementary characters. The character overwrites the level-1 display at a particular row and column as defined within the packet. Fallback for level-1 decoders is defined by the editor, who ensures that a suitable character will be shown on level-1 decoders.
Whereas in level 1 each control character occupies a display space, in level 2 nonspacing control characters are carried in a packet. This can give more character spaces within the page for display use.

Changes to the character size in two dimensions are possible, give the ability to display characters both of double height and double width.

Packet 27 provides the ‘Fastext’ page links. Up to eight packet 27s are currently defined but a total of 16 are possible.

Packet 28 is allocated to define the display aspects of a particular page (apart from the header). A total of 16 packet 28s is possible; each packet contains a ‘designation code’ followed by typically 13 groups, each of three bytes of data. One mode of operation is to cause the decoder to select an extension character set or to switch from a Latin-based to a non-Latin one. A further use of packet 28 (with a different designation code) is to redefine pastel colours on a page basis.

Packet 29 is allocated to define the display aspects of an entire magazine.

Packet 8/30 is the broadcast service data packet. This packet is normally transmitted approximately once per second and carries a magazine and packet address group which is nationally equivalent to ‘magazine 8 row 30’ , although it is neither a part of magazine 8, nor a row of any page. It can therefore be received only by using a special decoder.

Packet 31 is used for carrying general purpose data transmission services.

Level 3

Dynamically redefinable (down loaded) character sets (DRCS).
Down loading using pseudo-pages.
Linking from pages for display to pseudo-pages, for down loading character sets.
Pattern transfer units (PTU). Up to 96 PTUs can be down loaded using two pseudo-pages.

Level 4

Alphageometric displays.
Introductory pages.
Links to pseudo-pages.
Pseudo-pages for overwriting.
Pages for reformatable data.
Presentation layer syntax (geometric display).

Level 5

Alphaphotographic displays (still pictures). Pseudo-pages carrying photographic data.

The data requirement for a level-1 page is 960 bytes and that for level 2 is limited to a maximum of 1920 bytes. A level-5 page may require 9600 bytes and hence the inclusion of such pages significantly increases access time.

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