paulvdb is correct.
I edited the TS to give just the first minute and put this through DGAvcIndexNV - out comes 384Kbps 6 channel.
I then did almost the exact same thing except instead of starting at 0.00 sec I edited out the first 'I' frame (started at 0.76 sec). This time DGAvcIndexNV creates a filename with 192Kbps 2 channel.
So the switch over is occuring sometime during the first 'I' frame. I think I'm starting the cut too early when editing. I usually start the cut during the black bit just before the programme starts. I'll try cutting on the first frame with a picture in future.
I created 12 second versions to keep the file size down.
file1.ts (14.34MB - First 12 seconds)
http://www.mediafire.com/download.php?mny2mvn24jq
Creates audio file "file1 PID 157d 3_2ch 48KHz 384Kbps DELAY -1095ms.ac3"
file2.ts (13.38MB - First 12 seconds but with first 'I' frame removed)
http://www.mediafire.com/download.php?zfz4jedzyiw
Creates audio file "file2 PID 157d 2_0ch 48KHz 192Kbps DELAY -1087ms.ac3"
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I just replicated the same problem with another TS file.
This time I cut on the first picture (not the black picture) and created the TS file.
I then created the ac3 using DGAvcIndexNV - it created a filename with 384Kbps 6 channel.
The ac3 filesize was about 80Mbyte for 1 hour - a sure sign that the file is actually 192Kbps.
Media Info confirmed that it was 192Kbps 2 channel.
So I repeated what I did before - create 2 TS files from the start with the second having the first 'I' frame removed.
The first file created 384Kbps in the name - the second (with the first 'I' frame removed) created 192Kbps.
The switch over is definitely within the first 'I' frame period immediately after the first picture frame.
It's probably a bug or a feature of the new BBC HD encoders.
I'll carry on either using the Audacity or cut after the first 'I' frame.