| 摘要 |
<p>Data is transmitted under the Teletext system (CCIR System B), in a packet format (P) of 42 bytes each. Each packet comprises a header (PR), a data body (DB), and a suffix (SU). The header comprises seven Hamming(8,4)-coded bytes, which are subdivided into a first field (MRAG) of 2 bytes forming a data channel address, and which can take the hexadecimal values 1F, 2F, ..., EF, FF, with exception of OF; a second field (PA) of 3 bytes forming a packet address, and which can take arbitrary values; and a third field (CON) of 2 bytes operating as control codes for the receiver functions, in which one bit is an indicator bit (L). The data body (DB) comprises 33 uncoded bytes, which are subdivided into the following fields: a fourth field (PT) of at least one byte, which is a data field if said indicator bit has a first value, and is a control field for desired functions, if the indicator bit has a second value; and a fifth field (DA) of at least 30 bytes, forming the meaningful data bytes of the packet. The suffix (SU) comprises two bytes forming a CRC error-checking code for the data body. <IMAGE></p> |
