The editing terminal used for creating teletext pages allows text, numerical information and graphics to be entered by the editor to create the pages simply without being concerned with any special teletext system requirements. The page being created is viewed on a colour monitor and the input keyboard is arranged in the same fashion as a normal typewriter but with additional keys for special effects such as colouring, double-height characters, etc. The editing terminal also includes a wide range of word processing features such as word wrap and tabulation to facilitate page creation.
Special software is therefore necessary in the terminal to simplify the editorial tasks. In some designs of teletext systems, much of the editorial software forms part of the main teletext system processor. This arrangement simplifies the editing terminals but continuous communication with the system is then essential and furthermore, the editorial tasks impose a very significant overhead on the system processor. This can result in a slow response when a number of terminals are in use and a high rate of output is required to insert data onto several FBI lines. The alternative arrangement, made possible by 16-bit microprocessors, is to increase the processing power of the terminal so that it functions as a self-contained work station. Such terminals only need to communicate with the teletext system processor when pages are being entered or changed or when system instructions are being given.
![]() Figure 3.1 Editing Terminal Keyboard showing layout of special keys |
The keyboard layout of an advanced design of a stand-alone editing terminal is shown in Figure 3.1. The keys for colouring, tabulation, insert and delete are contained in the top row and many have appropriately coloured key cap marking. The text and numerical entry keys are arranged in the normal QWERTY layout. This area of the keyboard layout may be changed for different language requirements. The 12 cursor command keys are grouped in a separate area, as are those for the mosaic graphics cell, the editing grid and the page swap keys. The transmitted page consists of 24 rows of 40 characters per row, but the first row, row 0, is reserved for the service name, page number, time and date. This information is generated by the system processor and is not controlled by the editors. The 24th row at the bottom of the editing screen does not form part of the transmitted page and is often used for editorial commands to the system, as described later in this section. A typical editing terminal display is shown in Figure 3.2.
![]() Figure 3.2 Editing terminal display showing command row (row 25) |
The basic character font used in teletext has a capacity for 96 character shapes but many languages require a larger number to produce an acceptable display. For example, Spanish requires 128, and 131 are required for the Czechoslovakian language. The basic code table is used for the 96 most used characters and the additional ones are sent as a page extension packet. Packet 26 has been allocated for the purpose. The complete sets of upper case character shapes used for the Spanish and Czechoslovakian languages are shown in Figure 3.3 The terminal software automatically generates and sends the extension packet, with the page information, to the teletext system.
SPANISH BASE96 CHARACTERS ! " ç $ % & ' ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ? ¡ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z á é í ó ú ¿ a b c d e f g h i j k l m n o p q r s t u v w x y z ü ñ è à ▮ EXTENDED CHARACTERS ö ï â ô õ í ë û Ñ Ã ↑ É a ó ò Ú Ã · ê ã Ü ä î Á Õ Ç È → o # Ï Ò CZECHOSLOVAKIAN BASE96 CHARACTERS ! " # ů % & ' ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ? č A B C D E F G H I J K L M N O P Q R S T U V W X Y Z ť ž ý í ř é a b c d e f g h i j k l m n o p q r s t u v w x y z á ě ú š ▮ EXTENDED CHARACTERS ó ŕ ĺ ô ä Ó β Ú § ° À ň Č Ď Ě Ľ Ñ Ř Š Ť Ž É Ô Ů Í Ä ö ü Ö Ü Ý Ŕ ď Í Ĺ |
![]() Figure 3.3 Extended character sets |
Latin-based languages are written left-to-right but languages using cursive script are often read right-to-left for text and left-to-right for numerals. Furthermore, in Arabic, the character shapes depend on the position in the text. There are isolated, medial, initial and final forms. A total of 120 characters are required to cover the various forms to provide an acceptable display. The Arabic terminal therefore uses a font of 120 characters and special software that automatically changes the shape according to the location of the letter in the text. Normal page display codes are used for 96 of the characters, the extra 24 being sent in the additional packet 26.
Special control codes are required by the decoder in the receiver to cause colour changes, double-height characters, flashing of words etc. These codes are produced by the terminal software, the required code being generated when the appropriate key is pressed. For example, if a word is required to be in red, then the red control key is pressed immediately before the word and the text is then automatically displayed in red until a colour change key is again pressed. The appropriate colour control code is generated by the terminal in the space preceding the word. Such control codes are usually displayed in receivers as blank spaces but an editing terminal has a facility to display a two-letter mnemonic to represent the code, if required, to assist the editor. The mnemonic chosen by the terminal designer usually represents the control code action, e.g. GG = Graphics Green; HG = Hold Graphics etc. The page shown in Figure 3.4 has the control codes displayed and is the same page as that shown in Figure 3.2. The page on the editing terminal is normally displayed as far as possible in the same way as it will be seen on a domestic receiver.
The editing terminal is normally able to store several pages in RAM, typically six or seven. This storage facility enables an editor to review a series of continuation pages off-line from the teletext system very rapidly. Graphics or text can also be copied from page to page. The area of text or graphics to be copied is marked using the cursor command controls. The area so marked is identified by a dotted line or background colour change. This area can then be moved on the page, duplicated in another part of the page or copied onto a separate page. The editorial term for this technique is cut and paste.
![]() Figure 3.4 Editing terminal display showing control characters |
Mosaic graphics are created using a special block of keys. Each graphics character is made up from a 2 x 3 cell format, so that 64 different shapes are available. The required shape is obtained by pressing the appropriate keys and repeating as necessary. When a graphics key is used in conjunction with the column mode key, vertical lines of graphics are automatically produced. All the necessary teletext control codes for colour and graphics shape are generated automatically by the terminal software without any editorial involvement. To assist the editor in laying out a page, a 23 x 40 grid can be displayed on the editing monitor. However, to produce a complex graphics picture, such as a map, using the keyboard only, is a very time consuming task and such pictures are normally produced using a graphics digitizer.
The functional diagram of an editing terminal is shown in Figure 3.5. The keyboard contains a processor together with the appropriate software that is associated with the various key functions. Different keyboard layouts may be used for different languages and the appropriate software would be contained in the keyboard processor. The keyboard is linked to the electronic unit by an RS232 data link operating at 9600 baud. The editorial processor would normally have the resident software contained in EPROM to avoid the use of mechanical disc drives and floppy discs being involved with the operation of the software. The RAM memory used for storing teletext pages, and other data used during the editing processes, is battery backed to prevent the data being lost in the event of a power failure. The processor also drives the display circuits which produce the RGB and sync signal for the colour monitor. The EUROM LSI circuit, widely used in television receiver decoders, is often used for the display circuit function to help ensure that the appearance of the page being created by the editor corresponds to that on a typical receiver.
The disc controller and associated disc drive is an optional feature that is often fitted to editing terminals. An editor can then produce a large number of pages off-line and stored on floppy disc, in advance of a sequence of events (as in a sports programme, for example).
The inclusion of a teletext decoder in the editing terminal is also an option. This allows an editor to capture a page off air for checking purposes; alternatively, pages from another channel may be used to verify information, or possibly as an alternative data source.
Additional I/O ports are normally provided on an editing terminal. A printer can then be used to make records of any pages or information that is used. A remote data input would allow an editor to receive information on particular topics, such as financial information, from remote data sources. A side-by-side display mode is available on some terminals so that the external data can be displayed, in addition to the teletext page, to simplify editing. The digitizer input, previously mentioned, enables a digitized teletext page of graphics to be fed directly to the terminal. There is also an RS232 feed to the teletext system processor. Terminals are normally connected directly to the system, but modems are used when the editing terminal is located at some considerable distance. An example is an editing terminal located at an airport or sports event.
![]() Figure 3.5 Functional diagram of an editing terminal |
The editor communicates with the teletext system processor and provides instructions to it by entering commands onto the command row, row 24 (Figure 3.2). This row does not form part of the transmitted page but is simply used to display the commands to the editor or to enable the system to respond. Depressing the command key immediately moves the cursor to the start of row 24 ready for the editor to type in the instruction.
After a page has been created to the editors satisfaction, an instruction would be typed in on row 24 to cause the terminal to transfer the page from the terminal store to the teletext system. The command row contains the page number and any other instructions to the teletext system concerning that particular page. For example, if the page is part of a rotating or extended page set, then the display duration will have to be entered by the editor. Other system commands enable an editor to examine the magazine structure of the pages being transmitted and to enable or 'disenable pages from the transmission as necessary. Pages can also be updated or modified off-line from the system.
On editing terminals that are equipped with a teletext decoder, the instructions concerning which magazine and page should be captured are entered on the command row. Some commands (in general, destructive commands) must be entered twice to prevent an editor inadvertently erasing pages. In general, any executive commands to the teletext system processor which affect the transmission of pages have safeguards to prevent unwanted changes. To prevent unauthorized editing when a number of editing terminals are used in conjunction with the system, barring may be included, so that only certain terminals can communicate with particular pages.
Communication to the teletext system processor is normally an RS232 feed at 9600 baud. This is a suitable speed for passing data to the processor without making the serial nature of the communication obvious to the editor. The page data structure of simple pages is sent to the processor in a serpentine form: reading from top left to the right and then starting again at the next row, and so on. Each row starts with a prefix code (row number) and has a CRC checkword following the 40 bytes of data. This enables the processor to determine the location on the page and to check the validity of the data.
Pages that have non-displayed rows or packets associated with them, for different language character sets or Fastext, have these rows sent first, normally in the order they are to be transmitted. A special prefix is always used on the last row of the page to indicate page end. This facility is used for row adaptive transmission, which is normally only used for subtitles and news flashes. Update information can also be sent in this manner to specific pages, which avoids sending a complete page. Pages at this stage do not contain the full teletext header (row 0), as this is added by the teletext system processor as previously mentioned. A normal teletext decoder cannot be used to decode pages for viewing at this stage, but the decoding process is undertaken in the editing terminal with special software.
Graphics digitizers (sometimes called graphics cameras) are used to produce complex pictures such as maps or pages for educational type programmes. The graphics digitizer is able to create the teletext page directly from a picture or drawing. The functional diagram of a mosaic graphics digitizer is shown in Figure 3.6. The picture to be digitized is positioned under a video camera, which is normally fitted with a zoom lens and mounted so that the distance between the camera lens and the picture can be varied. A source of controlled lighting is also provided. The video signal from the camera is clamped to hold the black level constant at the input to a slicing circuit; the slicing level is variable by the editor. The slicing circuit produces an output which is high for components of the video signal which are positive with respect to the slicing level, and low for those below. A monitor is provided to enable the editor to check the picture from the camera in the preview mode and can be switched to view the two-level or digitized video signal. This allows the editor to adjust the slicing level to produce the optimum picture.
The final teletext mosaic graphics page consists of 80 horizontal by 72 vertical cells. The next step is to convert the digitized video signal into a signal which is divided into this cell matrix. The 625-line television picture has approximately 290 active lines per field. The graphics digitizer uses 288 of them so that each cell corresponds to a row embracing four television lines. The digitized video signal is fed to a sampling gate which effectively chops the row into 80 discrete segments, each segment corresponding to one cell. The output from this circuit is fed to three memory circuits which are enabled sequentially by the timing generator. The stored outputs of these, together with a direct output, are fed to the graphics cell generator, which uses majority logic to determine whether the average of the four inputs is high or low (i.e. white or black).
Each graphics symbol in the final teletext display consists of a two-by-three matrix and there are 64 different combinations, depending on which of the cells in the matrix are black or white. Each of these combinations is designated by an ASCII code (see Figure 5.1). The state of each cell in each graphics character is registered and the state of each cell is fed from the register to the code generator to produce the appropriate ASCII code. Each character code is entered into the page store. The output signal is a serial bit stream of ASCII characters which is fed to an unsolicited input of the editing terminal. The editor is then able to use the terminal to make fine adjustments to the graphic shapes using the keyboard, and add the appropriate colour information together with alphanumeric script to complete the page. A graphics decoder is also fitted in the digitizer so that the editor is able to view the final teletext graphics page on the monitor before sending it to the editing terminal.
![]() Figure 3.6 Functional diagram of a mosaic graphics digitizer |
It is not normal practice to use a graphics digitizer with each editing terminal in an editing suite, but to switch the digitizer between terminals. Alternatively graphics pictures can be stored on a floppy disc before being transferred to the editing terminal for the final page preparation. The graphics digitizer is able to operate in real time so that the editor can move the picture and adjust the camera field of view to optimize the final teletext presentation on the monitor, using the internal decoding facility.
High-resolution graphics needed for level 3 require the picture to be analysed into the much finer resolution of 480 x 250 pixels.
The video signal from the camera is clamped and sliced to produce a digitized or two-level video signal. This signal is then sampled on a one-pixel-wide sample basis. The television lines are sampled progressively, the sample position moving to the right by one sample period each field, as illustrated in Figure 3.7. The line samples are effectively about 64 microseconds apart. When the complete picture has been sampled and stored in memory it is then processed by the software.
![]() Figure 3.7 Level-3 sample structure |
The picture being digitized is stationary and therefore a monochrome camera can be used in conjunction with coloured filters to produce the three digitized images of the RGB video signal. This enables the software to process the three digitized images directly into the teletext format complete with colour information. A complexity that occurs with level 3 is that the complete image cannot be digitized for transmission because the standard allows only a certain number of DRCS characters per page and therefore only part of the full screen image can be processed. Software algorithms are used to make the best fit with the smoothed graphic shapes and so reduce the number of DRCS characters required to construct the final page. The output circuit of the digitizer is arranged to display the graphics page on a colour monitor. The monitor can also be switched to display the monochrome video signal or the two-level digitized video signal so that the editors optimization of slice level, position and orientation of the image is facilitated. When the editor is satisfied, the page will be finally processed and transferred to the editing terminal via an RS232 link, to enable alphanumeric information to be added as required.
Teletext is primarily designed for the rapid dissemination of information, and the main requirement of the editor is an adequate supply of editorial material. The editing suite will therefore require agency news feeds and be linked to a range of external information sources. The layout of an editorial suite is therefore not restricted by any technical requirement other than data communications.
External information sources can be linked directly to a teletext system, without editorial involvement, provided the appropriate page format and communication protocol is used so that the remote source emulates an editing terminal. When reformatting of the information is required, the necessary software is usually incorporated in the data management facility of the teletext system.