interlaced video question for you XV experts

[Billy Biggs]

Benjamin Herrenschmidt (benh at kernel.crashing.org):

> > In MPEG-2 there is a flag called "top_field_first" which indicates
> > the parity for each encoded frame.
> 
> Well, from memory, there is something about the fact that the "top"
> line is either on the first or the second field of a frame that is
> different between PAL and NTSC, no ? That is, on one of them, the
> second field starts in the middle of the screen above the first field
> since scanning isn't exactly horizontal, or something like that ?
> 
> I don't remember much of this, I wasn't on the HW side though :)

  Yeah, NTSC has the funny half-scanline.

  In most digital video standards though, you only have like 720x480
images instead of the "full" 720x486 which would include the half
scanline.  But, I don't see how which one starts the frame determines
the order that you should play them back.  It's really orthogonal and
something you'd decide when you're storing the media, not playing it
back.  I believe you of course that there are storage formats which
store NTSC and PAL differently like that.

  -Billy