[vdr] Re: vdr cuts recordings with ugly artifacts at cutting-points

["Wiljo Heinen" <wiljo@offeryn.de>]

>
> But I also see block artefacts at cutting points. I can get rid of them
> with PVAStrumento, but this creates mpegfiles, not VDR recordings...
>

Hmm.... maybe I should add an option to PVAS to output vdr recordings ? I'll
be thinking about that.

The reason for the artefacts (although cutting at I-frames) lies in the GOP
structure. Typically the TV stations send a GOP structure like IBBPBB.... .
The B-frames are bi-directionally predicted frames. Due to the nature of the
hardware encoders (i.e. it makes encoding easier) the order of the frames in
the GOP is often/mostly not the actual time-order. More clearly: though the
I-frame (full picture) cases first in the GOP it is the third to be
displayed. So the display order is B B I .... Now - as the B-frames are
bidirectional predicted they refer to the following I-frame (contained in
the GOP) as well as the preceding I-frame [or rather P-frame] (which is not
contained in the GOP).
In short: Typically the GOPs in DVB-transmission are not "closed", which
means there are frames referring to a previous GOP. Cutting at I-frame
borders now places are different (earlier) GOP as the "previous" one, which
results in artefacts, as the B-frames are now reconstructed on a wrong
reference.

What PVAStrumento does at cut-points is that it simply throws away pictures
referring to a "previous" GOP (which can be determined because the GOP is
then flagged as "not-closed"). Thereby avoiding artefacts, as the GOP after
the cut now starts (in temporal order) with the I-frame and is therefore
closed.

As I'm almost through with adding marks.vdr processing to PVAS, I will
consider adding vdr-output to PVAS - which should not be too different from
mpeg2-PS output. [Unfortunately there are more pressing things to add to
PVAS currently, like multi-audio and AC3....]

Regards

    Wiljo



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