Broadcast teletext system

Abstract

Each broadcast page contains a row such as row 29 which is outside the set of displayed rows. In addition to the standard initial bytes CR (clock run in), FC (framing code) and MRAG (magazine number of 3 bits and row number of 5 bits), there are 4 bytes allocated to a 16-bit cyclic rundancy check code CRC for the page and 6 groups, of 6 bytes each, NEXT 0 to NEXT 5. Bytes are Hamming coded and thus contain only 4 message bits. Each 6-byte group is a pointer to another page address made up of page tens and page units (4 bits each), hour code tens and units (2 bits plus 4 bits) and minute code tens and units (3 bits plus 4 bits). The three spare bits from the tens bytes for hours and minutes contain a number which is normally 0 but, if not 0, is added modulo-8 in the decoder to the current magazine number in MRAG to derive the magazine number for the `next` page. A decoder equipped with a multipage store can respond to the pointers to acquire a sequence of pages automatically, all available immediately to the viewer. A specific line, not related to any particular page, say magazine 8 row 30, can be transmitted say once a second and includes a 6-byte group addressing a title page. Whenever a receiver is switched on or to a channel carrying teletext, the decoder automatically responds to this line to acquire the addressed title page.