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

Edward Wildgoose Edward.Wildgoose at FRMHedge.com
Fri Oct 3 10:32:22 EDT 2003

> > 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.
> First I'd recommend getting the transcoder to work with DVB with full
> reencode.  I don't have a DVB card, but this should theoretically work
> today (if not, it is an issue with the decoder that needs fixing).  As
> for honouring the cutlist and keeping the mpeg2 stream (like we already
> do for mpeg4), you need to be able to reencode key frames in mpeg2 format
> (because you need to mark a key frame after the 'cut').  I'm not aware of
> any legally free tools to do this, but someone had mentioned bbmpeg as an
> option (I've not looked into it).

This *would* be a great nice-to-have feature.  Does it make the problem any easier is we clip to the previous key frame before the cut point?  Why? Well I'm guessing that in practice the cut point is going to be used for commercial clipping and there is usually a large scene change around this time which will mean that the key frame is probably where we want it anyway...?

If this is no easier then please disregard...

> > 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.

> This is all true.  The DVB is a special case in that you have no control
> over what format you get the stream in.  Are there any controllable
> parameters? if not, then we can just have the 'Default', Live TV', etc
> profiles have a single switch in them (whether or not this program should
> be auto-transcoded).

That's right, there is nothing controllable.  I think this will be the same for HDTV as well?  Although perhaps for HDTV there will be a mix of MPEG2/4?

Basically it's just a bundle of mpeg2 streams all wrapped up in a transport stream to give some error correction.  The current myth code grabs a transport stream and records the normal mpeg from within the TS to give a fairly normal looking mpeg2 file.  I wouldn't be suprised if it couldn't be treated identically to the PVR mpeg files

For the UK at least over DVB-T (ie free to air).  The mpeg is usually in 720x576 with an embedded aspect ratio of 16:9 (implies that maintaining embedded aspect ratio is important)

Thanks Geoff.  I will have a play with the transcode stuff real soon.  If I understand the discussion correctly it seems that (as a DVB viewer) I just have one profile that I need to adjust, the MPEG2 option under the Transcode group.  Seems that I can ignore resolution, etc and basically the only configurable params are codec, bitrate and audio rate?  Is this about right?

Ed W

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