interlaced video question for you XV experts

[Benjamin Herrenschmidt]

On Thu, 2005-03-17 at 21:18 -0600, Billy Biggs wrote:
> Benjamin Herrenschmidt (benh at kernel.crashing.org):
> 
> > Which field comes first in a frame is different between PAL and NTSC
> > if my memory is correct. 
> 
>   AFAIK, this is a convention and it depends on how it is being stored,
> and has nothing to do with the actual output (you have a 4-field and
> 8-field sequence for the colour carrier in NTSC and PAL respectively,
> but in digital video you don't usually care about that).
> 
>   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 :)

Ben.