Wednesday, 9 July 2014

Decoding Freesat, Part 2

As I mentioned in the last post, I'm reverse engineering the Freesat transmissions in order to extract the channel numbers so I can automatically update my MythTV system to use sensible channel numbers.

I've now managed to figure out most of the important bits:  Transport stream 2315 is broadcast on 11.428GHz Horizontal, at 27.5 Mbaud, FEC 2/3 and contains a stream with PID 3002.  This stream transmits a carousel of service description tables (SDTs) and bouquet association tables (BATs).  The SDTs aren't especially interesting, so I'm ignoring that for now.

Freesat tailor their channels to groups of consumers by grouping them into bouquets - each of the four countries (England, Wales, Scotland and Northern Ireland) get three bouquets - one for standard definition receivers, one for high definition receivers and one for G2 (second generation) HD receivers, so 12 bouquets in total for the time being.  (But more about regionalisation later).

Each BAT is packetised into one or more sections, and there is one BAT for each of Freesat's bouquets.  To collect all the information, you just keep watching the carousel until you've seen all of the sections for all of the BATs.

A BAT consists of a header, zero or more "descriptors" (lumps of data that have an ID that identifies the type of data they hold) and zero or more "transport streams".  Each "transport stream" in the BAT contains zero or more descriptors that contain information relating to a DVB transport stream (i.e. satellite transponder).

The top level descriptors in the BAT include standard descriptors (bouquet name, country availability, private data specifier descriptor) and some non-standard ones:
Descriptor IDDescription
0xd4Region table
0xd8Category table

I haven't investigated any of these except for the region table.  0xd5 - 0xd7 appear to be binary data.  0xd8 looks like a list of (ID, language, category name) tuples, but I'm not sure what the category IDs are referenced by; the category names are stuff like "Entertainment", "News", "Shopping", etc.

0xd4 is the one that's of interest to me - The bouquets are geographically pretty coarse, and Freesat tailor the channels to much smaller regions.  So the south of England gets BBC One South on channel 101 whilst the East Midlands gets BBC One East Midlands on channel 101, etc.  Descriptor 0xd4 contains a list of the regions that are served by the bouquet (I don't understand why bouquets are used at all for regionalisation though, since I can't see a reason for not handling it all through this fine grained regionalisation system?)  The data in this descriptor is a bunch of variable length chunks concatenated together, with the header of each chunk containing its size so the next chunk can be found. The data format of the chunks is:

Offset (octets) Length (bits) Description
0 16 Region ID
2 24 Language
5 8 Length of region name
6 Variable Region name

"Language" is a three letter text string and is always "eng" at the moment.

Now, as I mentioned above, the BAT also contains a list of transport streams, with a bunch of descriptors in each.  So looking at the descriptors within a transport stream, as well as a few standard descriptors there is descriptor ID 0xd3, which maps service IDs to channel numbers.  This contains a bunch of variable length chunks concatenated together, with the header of each chunk containing its size. The data format of the chunks is:

Offset (bytes) Length (bits) Description
0 16 Service ID
2 16 Unknown
4 8 Length of remainder of the chunk
5 Variable LCN/region mappings

The "LCN/region mappings" data is a concatenated set of fixed length subchunks as follows:

Offset (bytes)Length (bits)Description
0.512Logical channel number
216Region ID

So, we can select the appropriate bouquet (e.g. if we're using an HD receiver in England, we would choose bouquet 272, which is England HD) and pick our region (such as region 15 - "E Midlands/Central E").  In theory we can filter down the data to get a list of channel numbers and what transport ID and service ID (i.e. what channel) they are assigned to.

There are a couple of gotchas:

Firstly, one service can be assigned to multiple channel numbers.  The BBC regions are all typically available on 9xx channel numbers, but your local region is on 10x as well.

Region number 65535 appears to be a fallback or default region.  So, for example, in the England HD region, service ID 10060 (ITV 1 London) is assigned to channel 103 in regions 1, 18, 27, 31 and 38 and channel 977 in region 65535.  Region 1 is London, the other regions don't appear in the region list so I assume they are legacy IDs.  So in this example, if you're in London then ITV 1 London appears on channel 103, but if you're anywhere else it is on channel 977.

Region number 0 is a complete unknown...  See below!

Open questions

Region 0 - I can't figure it out at all.  It only seems to be used for BBC One (logical channel numbers 101 and 108), and it seems that multiple channels can end up assigned to a single channel number in region 0.

For example, looking at the Wales HD bouquet (274), BBC One London, BBC One West Midlands and BBC One South are all assigned to channel 108 in region 0.  Channel 108 isn't assigned in any other region.

Similarly, the Wales SD bouquet (258) assigns these same three channels to 101 for region 0.  We clearly can't just ignore region 0 because there's no other way to assign a channel to 101 in this case, but I can't see how set top boxes can choose between the three assigned channels.  Also, I note that the SD version of BBC One Wales (service ID 10311) isn't listed in the BAT at all - do SD Freesat receivers in Wales no longer get a regional BBC One?

Even more confusing are bouquets 272 and 280 (England HD and England G2), which seem to have BBC One Scotland HD (service ID 8901) assigned to channel 108 in region 0!

It would certainly be interesting to look at a branded Freesat decoder and see what channels appear on it; unfortunately I don't have one.


  1. Good to see this being worked on.

    I think it may be inadequate for you needs but is the "Your Channel Choice" pdf from helpful for channel numbers?

  2. Actually, on further investigation it seems that I can ignore the region 0 stuff because there is always another region (or the wildcard region - 65535) assigning that channel number. I had a bug in my code which meant I didn't spot that originally. Still curious what region 0 is all about though.

    Anyway, my code is available through Subversion at if it is any use to anyone.