Pixel Aspect & Stills

eman wrote on 9/1/2003, 8:34 PM
I am very confused about stills, pixel aspect ratios and regular 4:3 video sizes for computer.

I have a PAL DV at 720x576 1.0926 pixel aspect.

I want to export stills from it. Stills are exported at 720x576 also. This a 1.25 aspect ratio.

Now I know regular windows video has sizes such as 320x340 448x336 etc.. which are all 1.333... (4:3).

How can I encode the 720x576 video at a regular 4:3 resolution - 720x540 for instance.. but wouldn't that cut out 36 pixels from top and bottom ?
Why wouldn't stills come out at a 4:3 resolution ?

Can someone please shed some light on this? Highly confused here.

Comments

Chienworks wrote on 9/1/2003, 10:11 PM
That's because the DV frame isn't 4:3

Is there some reason you can't use the stills you're getting?
rmack350 wrote on 9/2/2003, 12:45 AM
A frame of video isn't really 4:3. Rather, the area of that frame that you are supposed to be guaranteed to see is 4:3. The frame itself spreads past that a bit.

Think of it this way: back in the days of tube electronics you could never be totally sure where a raster would come up to full voltage or drop down to 0. So the specs said that a TV should be able to display a certain part of the image and extra space should be left at the edges for the signal to rise or fall in.

As an example, try digitizing the signal from your VHS deck. Most likely you'll see black at the left and right edges. This is allowable slop and is generally masked off by the frame around the TV tube.

Now, the raster going from left to right gets sampled. In both PAL and NTSC it's sampled at a rate that yields 720 samples/line but the viewable area turns out to be about 704px. This represents the amount of area a TV is supposed to be able to display. I think it's actually a maximum.

I'm not working in PAL but it looks to me as if Vegas will save stills at 787x576. The clipboard tool will grab them at 720x576. When Vegas corrects exportes for PAR it always keeps the vertical dimension constant.

I'd say you can work on stills at 787x578 and you'll be fine.

Rob Mack