Chroma sampling (and Cedocida YV12 option)

[ariga]

I'm quite confused (ignorant rather)... Here's what I THINK happens. Correct me if I'm wrong.

NTSC DV
YC Y Y Y
YC Y Y Y
C is the chroma info for pixel 1 only

PAL DV
YV Y YV Y
YU Y YU Y
No U info on line 1, no V info on line 2

MPEG 2
Yc1 Y Yc3 Y
Yc1 Y Yc3 Y
c1/c3 are the average vertical chroma of two consecutive pixels on two lines (2x2 block) ?

I'm guessing that the YV12 MPEG sampling option is needed to "copy" the chroma info in pixel 1 to pixel 3 in case of NTSC. Else an application would treat it as 4:2:0 and the pixels in col 3 & 4 would be displayed without colour ? Or is the chroma info calculated depending on the lines above and below it and assigning some weightage ?

Also can an application differentiate between MPEG2 YV12 and DV YV12 sampling? If not, then DV would be displayed with U info missing in one line and V info missing on the next !