Teletext Tutorial (6)
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Teletext Tutorial (6) |
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 The characters that make up the teletext page are transmitted in the Vertical Blanking Interval (VBI) of the television signal. Lines 6 to 22 in field 1 and 319 to 335 in field 2 are available to carry teletext data. Each character or control code is represented by a 7 bit code plus an error checking parity bit. If the teletext decoder detects a parity error the characters is not displayed. The data for one teletext display row together with addressing information is inserted in one VBI line. This grouping of data is known as a packet. Since there are 24 display rows or packets per teletext page, it takes 24 data lines to transmit a teletext page. Note that the first packet in a page is referred to as packet 0 or the header packet. The last packet in a page is packet 23.   Bits are represented by a two level NRZ signal. Synchronisation information is included at the start of each packet to indicate bit and byte positions. The first two bytes comprise alternating ones and zeros to indicate the presence of a dataline and establish the basic bit rate. This burst of synchronising pulses can be likened to the colour burst at the start of a normal TV line. The third byte is known as the Framing code which indicated and synchronises bytes in the data stream.
Addressing data indicating row, page and magazine numbers is sent with the display data in the packet. Carrying these addresses in every packet would require considerable bandwidth, therefore general display packets (1-23) only contain magazine and row addresses. The page address is contained along with magazine and row addresses in the header packet 0.  The magazine and row address group data (MRAG) is held in the fourth and fifth byte of every packet. Since transmission errors in the magazine and row addresses could severely disrupt the display, they are error protected using Hamming encoding. This permits single bit errors to be corrected albeit at the expense of additional data bits. Three address bits are used to indicate the magazine number - 1 to 8 and five bits indicate the row number 0 to 23. Note that five bits permits addressing up to row 31. These other rows or packets (24-31) are used for other teletext services and are discussed later.
The transmission of a page begins with the page header. Subsequent packets specify the rest of the page although these packets contain no page address data. Start of the next page is indicated by transmission of another header. Since the magazine address is carried in every packet, it is possible to interleave different magazine rows after a header. However there is a requirement that there is a video field break between a page header and any subsequent non-header packets in the same magazine.
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