British Broadcasting Corporation

John P Chambers

A method of labelling a broadcast signal in which a predetermined pattern of special labels, preferably, a label for the next following program is transmitted repeatedly but irregularly at predetermined intervals prior to the program change to indicate that an event is to occur after a predetermined interval.

Method of labelling a broadcast signal including transmitting a sequence

A cathode ray tube video display device is adapted to display wide aspect ratio signals by reducing the vertical scanning amplitude of the scanning raster. The scan is collapsed in the vertical direction without reducing the number of lines used to display the active picture, leaving unscanned bands at the top and bottom of the display. The device can change between a normal raster scanning mode and such a collapsed wide screen mode automatically in response to an indicator in the received video signal.

CRT video display device with automatically adjustable scanning amplitude

A digital transmission system for transmitting, e.g., television sound has two packet type indicators indicating, e.g., mono/stereo transmission. Normally the same packet type indicator is transmitted for a long sequence of packets. To provide synchronizing information to enable, e.g., appropriate timing of a sound signal to the associated video signal, the packet type indicator for a single packet is changed to an indicator which is inappropriate to the packet concerned but is appropriate to another type of packet.

Multiplexed digital data transmission

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.

Broadcast teletext system

A cyclic redundancy check word is customarily placed at the end of a data block, occupying one or more of the last bytes of the block. This can impose an undesirable restraint upon the person composing the data block, e.g. the editor of a teletext page. The invention allows the check word effectively to be embedded anywhere convenient in the block. A location in the block is selected (one or more bytes--preset address 16) and all preceding bytes are cyclically encoded (encoder 18) to yield a first check word. The succeeding bytes are subjected in reverse order to complementary encoding (encoder 28) to yield a second check word. A third check word is formed from the first and second check words (encoding logic 32) and inserted in the selected location. Each bit of the third word is the logical product of the corresponding bit of the first word and selected bits of the second word, such that when the bytes of the complete block, including the third word, are cyclically encoded, the final check word has the required final form, e.g. all zeroes, to indicate compliance of the block with the cyclic redundancy check. The location can be selected by the editor but is preferably selected automatically by a search for a free location complying with predetermined conditions.

Cyclic redundancy data check encoding method and apparatus

Data receiving apparatus for receiving repetitively transmitted blocks of data stores a block of data (register 10), and compares (comparator 12) each received byte in DI with the corresponding stored byte in D(n). If the bytes match (M) a data selector 15 selects either byte for storage and sets a flag bit for that byte in a register 14 to 1. If the bytes do not match, the data selector selects the new byte when the existing flag bit is 0 and selects the old, stored byte when the existing flag bit is 1, in either event setting the flag bit to 0. In this way the probability is that each stored byte is an error free byte. A status circuit 16 signals when all flag bits are not 1 to indicate that suspicion nevertheless attaches to the block as stored.

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Tadworth, United Kingdom

John P Chambers