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

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

Klaus Schmidinger wrote
> sequence would be no problem in VDR. Am I understanding this right that
> if the first GOP of a starting point of an edited sequence looks like
>
>   IBBPBPBBPB
>
> then what should be done is to remove the B frames after the I frame, but
before
> the first P frame, thus making the above example look like
>
>   IPBPBBPB
>
> Is that correct?
>

_Almost_, but not exactly. With most TV stations this is correct, but what
you really have to do is to remove all B-frames that have a temporal offset
that is lower than the first I-frame (which means that they will be
displayed before the I-frame although they are later in the stream).

In the above example you would ("normally") have

    I - with tempref 2
    B - with tempref 0
    B - whith tempref 1
    P - with tempref 5
    B - with tempref 3
    B - with tempref 4

(which means a display-sequence of BBIBBP .... here you can see why this GOP
structure makes live easier for encoders) and so on.

You then change that into

    I - with new tempref 0
    ( drop the 2 B-frames)
    P - with new tempref 3
    B - with new tempref 1
    B - with new tempref 2

I am not sure, whether you will have to actually change the temporal
reference values in a vdr-cutting situation (maybe the DVB driver can cope
with this), but for a correct cutting procedure you will have to change the
tempref values, otherwise you will have a GOP with a corrupt temporal
structure and DVD authoring programs might/will choke on this.

So - it's not only the IBBPBBP structure, that is significant, but also that
the B-frames come before the I-frame display-wise (which is of course
signalled by the closed-gop bit being set to zero in the GOP header).

rgds
    Wiljo



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