[mythtv] DVB, (new) recording profiles, and transcoding

Mike Wilcox mike at trouble.org.uk
Thu Oct 2 04:25:12 EDT 2003


I've been looking into a few things related to the MPEG2 stream 
generated by the DVB cards, and come to the conclusion that I need to 
try to get the transcoder to run on these files.

My first aim is to be able to get the transcoder to run on the files 
without re-encoding the stream, but merely honouring the cutlist. After 
that, I want to experiment with some other things. In turn, I think this 
means I have to do some work to help correlate between the DVB capture 
side, the recording profiles, and the transcoding profiles.

But I'm a little confused, what with the changes to the recording 
profiles, plus automatic selection of profile groups in the GUI based on 
card types, plus a bit of extra confusion because the DVB code re-uses 
the code and/or term "MPEG2" from the PVR-X50 stuff in some places, but 
not in others.

If I've got things understood correctly, then, when altering the setup 
of recording profiles...

Selecting a group of "Hardware DVB Encoders", I should get the 4 
profiles. If I choose "Live TV", I should then be able to set the 
parameters with which I wanted "Live TV" to be spooled into the disk 
with (including capture card settings, and any software codec settings). 
If I select one of the other 3, I should be able to set the parameters 
for which recordings are made (same capture card settings and software 
codec settings), then each individual recording may specify one of those 
3 profiles.

Selecting a group of "Transcoders", I should be able to see all the 
possible file encoding types for recordings originally captured using 
the profiles above, and for each one, specify new software codec 
settings for the transcoder to turn the recording into.

I hope I've got those right... The rest is based on this...

So, for capturing from DVB cards, we ought to be able to see/change some 
  setup data for the capture card, plus any additional software 
transcoding we do between the capture card and the disk.

These parameters might include, for example, aspect ratio, resolution, 
but wouldn't include channel tuning parameters, or parameters to 
determine whether to output the whole TS or individual PES.

As I see it at the moment, there aren't really *any* parameters to set 
for capture from the hardware - we basically get the stream in whatever 
MPEG-2 format the broadcaster chose to send it. And right now, there 
aren't any parameters to set for any intermediate software transcoding 
before the stream hits the disk either - though I can see the need for 
some later, perhaps in transcoding from the PVA/PES format of the DVB 
transmission through some error-correction and/or AV-sync code.

Today, the setup for the DVB capture cards still allows us to set 
parameters as though its a V4L device, and I guess this should change.

Similarly, when we want to setup the "Transcoders", we need to allow 
transcoding from the DVB-MPEG2 into the existing RTjpeg and MPEG4 
codecs, but we also need to allow a specification that the transcoding 
should not re-encode either video or audio (or probably both), or to 
force certain settings if the stream is not already of that format.

Today, we have two profiles for the transcoder: "MPEG2" and 
"RTjpeg/MPEG4", but they appear to only allow to transcode towards the 
RTjpeg and MPEG4 codecs - I don't see how you can configure the MPEG2 
transcode to *not* re-encode (but I've read in the lists that it does. 
Is this true?)

Of course in deciding whether re-encoding is required for DVB, we won't 
be able to compare all the capture settings with some transcoding 
settings, as we have just taken the stream as the broadcaster sent it. 
Recordings will be at different resolutions (certainly), with different 
audio settings (possibly). I think we really will need some settings 
that state "don't change".

Are my thoughts/conclusion right sofar? Am I heading down the right path 



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