[mythtv] Freesat (UK)

David Matthews dm at prolingua.co.uk
Thu May 15 11:32:00 UTC 2008

Stuart Auchterlonie wrote:
> James Buckley wrote:
>> Although not exactly on topic, is there any chance that patches to
>>  get Freesat's EPG would make it into 0.21-fixes. 0.22 could be a 
>> while, and the new EPG will make a lot of people's lives easier. I
>>  know it's not directly a fix for a bug, but neither was Schedule 
>> Direct (although I can see that the new EPG is not quite as 
>> critical as SD was).
> Well lets just get them working in head first....

This is the situation as I understand it. I did ask on the BBC
backstage mailing list about whether the technical details for Freesat
were available but the reply was that currently the manufacturers of
set-top boxes had agreed exclusive access to them. That means that the
details are limited to what can be worked out from the on-air information.

The Freesat information seems to be transmitted on non-standard PIDs
presumably to avoid conflicts with existing Sky boxes.

There is an SDT transmitted on a non-standard PID of 3841.  I've only
looked at this with dvbsnoop but presumably it would be possible to get
MythTV to scan it and I guess it would require a patch that allowed the
user to set a non-standard PID in mythtv-setup. It might be useful since
it seems to contain the "default authority" for the CRIDs and may also
contain the channel numbers. It seems to have "Channel 4" in there where
the default tables contain something like "8350".  For the moment,
though, it's possible to insert the data manually into the database.

The EPG is transmitted as EIT on PID 3842.  It contains CRIDs that are
similar to those on Freeview and, for BBC, identical. The text is
compressed and I've largely worked out the compression by comparing the
Freesat and Freeview data. It seems to be based on the ATSC compression
scheme but uses non-standard compression tables. There are two tables,
one for programme titles and the other for the description. They are
Huffman encoded with the encoding for each character depending on the
preceding character. That means that it's very difficult or impossible
to get a complete table because it's not just a matter of seeing every
character but seeing every combination of characters.  Nevertheless I've
got most of the titles and text to decompress correctly now so it is
possible to use this as an EPG but with some of the titles and text

What is the attitude towards having a patch that would be known to be
incomplete and would have to be updated periodically?  Should I create a
ticket anyway so that people can try it out? It's definitely going to be
work-in-progress and just at the moment I don't have much time to do any
more on it.


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