Monday, 7 July 2014

Decoding Freesat, Part 1

To watch/record TV, I use MythTV connected to a satellite receiver.  Unfortunately, MythTV's handling of channel numbering is a bit bonkers... So I've been doing a bit of reverse engineering of the data transmitted by FreeSat to try and automagically pull out the local channel numbers and update the MythTV channels database...  Information on the internet seems thin on the ground, so this is what I've figured so far:

The "FreeSat Home" transponder is the interesting one (transport stream ID 2315).  This is located at 11.428GHz Horizontal, with a symbol rate of 27500 and FEC 2/3.  PID 3002 on this transponder transmits a bouquet association table (BAT).

For each transport stream, there is an entry in the BAT, containing a descriptor tag 0xd3 and an associated lump of data.  The data is a set of variable length chunks concatenated together, with each chunk containing a length value so the offset of the next chunk can be calculated.

The chunk format appears to be:

Offset (octets) Length (bits) Description
0 16 Service ID
2 16 Unknown
4 8 Length of remainder of the chunk
5 4 Unknown
5 + 1 nybble 12 Local channel number
7 Variable Unknown

I haven't been able to figure out how the channels are selected by region - for example, local channel number 101 is allocated to BBC 1 London if you're in London, BBC 1 Wales if you're in Wales, etc. but I haven't found this information in the BAT yet. Compare:

BBC One London BBC One West
Service ID 18 9d (6301) 18 c5 (6341)
Unknown 1 81 f9 (33273) 82 01 (33281)
Size 08 (8) 08 (8)
Unknown 2 d (13) d (13)
LCN 3 b6 (950) 3 c5 (965)
Unknown 3 ff ff f0 6c 00 00 ff ff f0 6c 00 00

Edit: "Unknown 2", "LCN" and "unknown 3" appears to be an array mapping LCNs to regions:

ITV 1 London ITV 1 Granada
Service ID 27 4c (10060) 27 60 (10080)
Unknown 1 83 f3 83 f2
Size 18 (24) 0c (12)
Unknown 2 d (13) d (13)
LCN 067 (103) 067 (103)
Region 00 01 00 07
Unknown 2 d (13) d (13)
LCN 067 (103) 067 (103)
Region 00 12 00 27
Unknown 2 d (13) d (13)
LCN 067 (103) 067 (103)
Region 00 1b 00 2b
Unknown 2 d (13)
LCN 067 (103)
Region 00 1f
Unknown 2 d (13)
LCN 067 (103)
Region 00 26
Unknown 2 d (13)
LCN 3d1 (977)
Region ff ff

Descriptor 0xd4 in the BAT seems to translate the 16 bit region IDs into human readable strings (is it me, or does 16 bits sound a bit excessive for region IDs?)

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