Re: [Mjpeg-users] bttv, streamer, yuv4mpeg, mpeg2enc, and field order

[Bernhard P. <sha...@ut...>]

Hallo

> I asked this question on the video4linux list and I'm not sure that
> everybody answering the question understood the question as well as the
> mpeg2enc tool.
> 
> My understanding is that in the US NTSC is broadcast bottom field
> first.  Thus, the TV will display the bottom field, then the top field.
> I am caputring from a US broadcast.  Now the bttv driver alledgedly
> delivers frames top field first, although streamer doesn't set the
> interlacing tag in the yuv4mpeg header.  Consequently, I set the
> interlacing flag to "top field first" in the yuv4mpeg header when I pipe
> the yuv file into mpeg2enc.  Now mpeg2enc has an option
> "--playback-field-order" which can be set to play bottom field or top
> field first.  Should I be setting this to bottom because bottom fields
> are broadcast first in the US, and hence should be played first?
That fild problem can only happen if you record at full size. For halfe
(VCD) size you usually only record one field. 

I would not expect the field order to be always top first. It could be
bottom first to. It might be that streamer only starts at a top field to
record. To be at the safe side, you have to fear that it can change
every time. 

Only use the mpeg2enc --playback-field-order option if playback look
fine on your computer monitor but has weird flickers when you play it
back on your tv using VCD/SVCD/DVD. Then "just" set the opposite from
what was used befor (sound easy I know) If you have to use that option
it means that something has mislabeld the stream as bottom or top first
when the stream was the opposit. 

One thing the mjpegtoos can't correct encode ist when NTSC switches the
framerate from 30000:1001 (about 29,97) to 24000:1001 (about 23,97).
That usually messes up the encoding. 

BTW: Have you read the mjpeg-howto section: 
Notes about "interlace field order - what can go wrong and how to fix
it" 
That should explain most of the problems that can happen. 

BTW2: I'll add a short description of the -z|--playback-field-order b|t
option to the manpage of mpeg2enc. Because it is missing. 

auf hoffentlich bald,

Berni the Chaos of Woodquarter

Email: sha...@ut...
www: http://www.lysator.liu.se/~gz/bernhard