Teletext Tutorial (7)
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Teletext Tutorial (7) |
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Normal level one teletext page transmissions comprise 24 packets, numbered 0 to 23 inclusive, which form the displayed page image. Each packet corresponds to a row, numbered 0-23, and the data for each row is contained within the appropriately numbered packet. Additional packets of data can, for example, extend the size of the displayed page to include a 'status row' for user prompting. This is contained within packet 24. Other information can also be sent as part of the data for a page although that data does not appear on the resultant screen display. For example data encoded within packet 27 controls the response of the decoder to user actions in selecting other pages. Packet 8.30 is often transmitted although it is not associated with any page. It contains data which controls or informs the decoder at a general level about the transmission being received. Many ghost packets have been defined to support enhancements to teletext displays. These enhancements such as higher display resolution, additional colours etc. are defined under Lever 2, 3 etc. in the WSTS. To date, attempts to implement these enhancements have not been successful. For this reason details of the associated ghost packets will not be covered in this guide.
The original Level 1 teletext specification defined 24 display rows labelled packet 0 to packet 23. The WSTS specification defines an additional display or status for user prompting. This is packet 24. Suitable equipped teletext decoders use data in packet 27 to react to prompts in the packet 24 display row. See packet 27 for more information about user prompting.
There are 15 variants of packet 26 identified by a designation code within the packet. The packet contains 13 data groups comprising 3 bytes each. Each data group contains a specific screen address or location and data relating to that location. Identification of screen locations is required in various teletext applications e.g. for rewriting additional foreign characters symbols required for Spanish teletext.  Another example is the packet 20 preselection packet required for Programme Delivery Control. It contains related data for each programme listed on the TV schedule page, identifying channel, programme date and start time and also the cursor position of the programme information on the page. When the viewer uses a remote channel handset to select a programme, the cursor position for the selected programme is linked or mapped to the appropriate packet 26 preselection data. This data is then used to programme the VCR timer and contains unique programme label and channel information. When the programme is actually transmitted, the information is transmitted in the recorder control packet 8.30 format 2 (see later). A match between the preselection data and the recorder control data sets the VCR into record mode.
Packet 27 instructs the decoder how to react to user page selections made in accordance with the displayed information contained within packet 24.  Packet 27 contains information linking the current page to six linked page numbers. The packet contains six page addresses of which four are assigned to four coloured page request keys on the TV set remote control unit. The most common colours are Red, Green, Yellow and Cyan. Typically these four keys correspond to four coloured menu boxes displayed at the bottom of the screen as packet 24. Selection of one of the coloured request keys causes selection of the corresponding linked page, the number or address of which is stored in packet 27. The fifth page address is reserved for identifying a page which the user might most want to see after the current page, for example, the next page in a sequence. The sixth page address is associated with an Index key on the TV remote control unit and contains an address for a page which routes the user to an index or start of a section.
Similar in structure to packet 26. Used to define level 2 and 3 pages which defines higher resolution pages. Higher level teletext information is not covered in this guide.
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Guide to packets (1)