Packet 24 & 27 - Fastext / Linked Pages

Bytes in Packet 27 - Linked Pages
1-2 3 4-5 6 7-12 13-18 19-24 25-30 31-36 37-42 43 44-45
CRI
10101010		 Framing
11100100		 Magazine &
Row Address		 Designation
Code		 Link 1 Link 2 Link 3 Link 4 Link 5 Link 6 Display
Row
24		 CRC












Fastext is used to make using teletext alot easier. On every page there are 4 regularly used page links that can be selected by one of four coloured buttons on television remote controls. Fastext is a combination of packet 24 and packet 27/0. Packet 24 is used to display the coloured text relating to the corresponding button and packet 27 contains information about 7 linked pages and a simple checksum. The first 4 links correspond to the linked fastext buttons. The fifth I have yet to work out what this page relates to, as this doesn’t refer to a real page number. The sixth refers to the index page for the current area.

Packet 27 is Ham encoded, it is very easy to obtain the transmitted data by using only the binary digits that are in the positions

 

1

0

0

0

1

1

0

0

¯

¯

¯

¯

1

0

1

0

 

The best way to describe the format of linked pages is by an example.

Imagine the current page is 400. The first page link after being ham decoded will look as follows

0 1 F 7 F 3
Units Tens Mag Mag

The first two bytes are the page units and tens in that order.

Now, in binary 7 is:

0 1 1 1
a

and 3 is:

0 0 1 1
b c

 

The 3 bits labelled as a, b and c when rearranged as b c a (0 0 0) corresponding to the magazine number. This is then Exclusive ORed with the current magazine number (4 = 1 0 0) gives 4. So the link corresponds to the page 410

Last Updated 28 January 2001


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