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

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

> If this is possible, VDR could simply cut one GOP before the
> actual cutting point and mark this GOP as "broken".
>
> In my format description in a GOP header 0x01b8 there are two
> flags: drop_frame_flag (byte4, bit7) and broken_GOP (byte7 bit5).
> But I don't know what they actualy mean.
>

broken_GOP (it's called "broken link" in the standard) is exactly for the
purposes you have in mind:

ISO/IEC 13818-2 says:

"broken_link - This is a one-bit flag which shall be set to '0' during
encoding. It is set to '1' to indicate that the first consecutive B-Pictures
(if any) immediately following the first coded I-frame following the group
of picture header may not be correctly decoded because the reference frame
which is used for prediction is not available (because of the action of
editing).
A decoder may use this flag to avoid displaying frames that cannot be
correctly decoded."

The question is, whether the decoder would honour the flag in our case.


The drop_frame_flag is only used for NTSC recordings (and furthermore
belongs to the timecode, which is not used in the decoding process):

ISO/IEC 13818-2 says:

"time_code - This is a 25-bit integer containing the following:
drop_frame_flag, time_code_hours, time_code_minutes, marker_bit,
time_code_seconds and time_code_pictures as shown in Table 6-11. The
parameters correspond to those defined in the IEC standard publication 60461
for "time and control codes for video tape recorders" (see Bibliography,
Annex F). The time code refers to the first picture after the group of
pictures header that has a temporal_reference of zero. The drop_frame_flag
can be set to either '0' or '1'. It may be set to '1' only if the frame rate
is 29.97 Hz. If it is '0' then pictures are counted assuming rounding to the
nearest integral number of pictures per second, for example 29.97 Hz would
be rounded to and counted as 30 Hz. If it is '1' then picture numbers 0 and
1 at the start of each minute, except minutes 0, 10, 20, 30, 40, 50 are
omitted from the count.
NOTE - The information carried by time_code plays no part in the decoding
process."

rgds

    Wiljo





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