Minor confusion over frame sizes.

farss wrote on 8/30/2004, 4:18 PM

OK this sounds like a real dummies question, I know.
PAL is 720x576 right, except the preview window is 787x576? So that's the true PAL DV frame size right. A bit more is needed to cover the horizontal blanking perid when coming of analogue source and the possibility of jittery syncs. I'm fine so far.
Now when I capture from say VHS or SP I'll get some black bars at the sides, makes sense. But cut that with DV material and I start to see a small problem. The effective frame sizes are different. Normally this isn't a problem, most TVs and monitors will mask this out. But with the advent of LCD monitors you can now see the whole frame and having the edge of the frame jump in and out is rather distracting. I know there's a few ways I can fix this, mask or zoom. But zooming in on the frame incurs a quality hit due to interpolation, yes I know Vegas does an excellent job of this but better it didn't have to. Masking bits off of all the footage seems a bit dim also.
Also I'm finding telling the graphic artist to make the stills 720x576 means they don't fill the frame. Again, easy enough to fix but again interpolation is involved. Telling him 787x576 seems to solve that problem but that's not what most seem to work to.

Anyone got a better handle on this than me?

Bob.

Comments

PeterWright wrote on 8/30/2004, 5:30 PM

Not a hugely strong handle Bob, but 787 x 576 with square pixels comes out as 720 x 576 once the DV pixel aspect ratio gets to work, so it's always best to create graphics at that larger size to avoid distortion.

Regarding the side black bars - I get these from one of my DV cameras and the only way to remove them is pan/crop. Previously with TV playback I didn't worry but as you say some viewers can now see "everything" so I have to do some trimming sometimes - ah well, new camera next year ....

Chienworks wrote on 8/30/2004, 5:39 PM

The actual PAL frame size is 720x576 pixels. However, the pixels are rectangles, not squares, and they are slightly wider than they are tall. So if you were to fill the same frame with square pixels big enough to fit 576 high you would need just about 787 pixels across. So, when your graphic artists create 720x576 images they are correct, as long as they set the 'Pixel Aspect Ratio' to 1.0926. If, however, they create images with square pixels (aspect ratio 1.0), then they will have to create them as 787x576. Both are correct, both end up the same shape and fill the frame nicely.

The problem you seem to have occasionally is that someone will give you a 720x576 image set to PAR of 1.0. Vegas will assume this is correct, adjust it to fit on the PAL frame with a PAR of 1.0926, and end up squashing the image slightly so that it ends up being only 659 pixels wide. In this case, in order to correct it, you must tell Vegas to use the correct PAR. Open up the properties window for the image event, click the Media tab, and change the Pixel aspect to 1.0926 (PAL DV). This should fix the problem. Contrariwise, if someone gives you a 787x576 image that is set to 1.0926 PAR and you end up with black bars at the top and bottom, do the same thing except set the Pixel aspect to 1.0000 (Square).

farss wrote on 8/30/2004, 5:55 PM

Thanks guys!
Hadn't quite factored in the PAR aspect of it. That bit of it makes sense now.
Still not 100% happy with handling stuff from analogue source. Guess I'll need to do some more in depth digging. Just to show you how messy all this can get I just did a job for a TV network. Only had VHS copies to work with but however made the master tapes left line 21 visible in some of it. Now you don't see that on the average monitor or TV but when it's captured and burnt to DVD you may end up seeing it. These are only viewing copies so I'm not fretting over it but it does show how easily you can come to grief.

BTW, anyone doing work from DigiBetacam, there is a switch on the A500 to turn line 21 off, not exactly easy to find but it's there.

Bob.

Chienworks wrote on 8/30/2004, 6:58 PM

If you are worried about seeing edge goobers and don't want to crop/zoom to avoid them, you could create a mask to black out the edges instead. True, you'll end up with a black border around the image when the full frame is seen, but most people won't notice it, and it would look better than the goobers.

farss wrote on 8/30/2004, 7:44 PM

That's what I tend do do anyway, particularly on some of the really cruddy VHS stuff I'm given, can't see much point wasing bandwidth on the garbage on the edge.

Bob.