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 |
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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 |
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0 |
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1 |
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0 |
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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:
and 3 is:
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
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