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