[MEncoder-users] Re: field order mysteriously changing

[Dieter]

In message <441658BE.9090908 at pacifier.com>, "Scott W. Larson" writes:
> Dieter wrote
> 
> >Do you think the broadcaster was randomly changing the field order?
> >Or Tivo?  Perhaps different cameras had different field order?
> >Or something else ???
> >  
> >
> Broadcasters do change the field order in 1080i. I have no idea why but 
> I discovered it the hard way. If you watch the top_field_first flag 
> during a 1080i broadcast, you'll see it switch back and forth. Naturally 
> displaying the fields in the wrong order will make motion look awful.

I was hoping the top_field_first flag would be useful for automagically
correcting field order, so I tried watching it in ffmpeg, converting
mpeg2ts to dv:

In libavcodec/mpeg12.c mpeg_decode_picture_coding_extension()
I turned on the printf for top_field_first.  Some files have
top_field_first all 0, some have it all 1, and some have
*interesting* changes, like being 1 for awhile, then changing
to an alternating pattern, with 1, 2 and 3 samples:

top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=1
top field first=0
top field first=0
top field first=0
top field first=1
top field first=0
top field first=0
top field first=1
top field first=1
top field first=1
top field first=0
top field first=1
top field first=1
top field first=0
top field first=0
top field first=0

I have trouble believing that the field order is supposed to be
changing this frequently.