The right frame size of still images for PAL DVDvideo

I have always thought (and learnt) that still images for slideshows and menus for DVD video (PAL System) had to be 768 X 576 pixels and then scaled to 720X576 MPEG 2 video file, but some WEB sites don’t think so. They demonstrate that 788 X 576 is the right frame size and not 768 X 576.
Any suggestions or comments are appreciated

http://www.bbc.co.uk/commissioning/branding/picturesize.shtml
http://www.vistavision.de/tutorials/palvideo/tutorial_pal_4x3.html


[/url]

Thanks for your answer real2reel. I made a few tests with 788 X 576 images with round subjects and circles as vistavision and BBC tutorials suggest and I found that they were perfect on TV screen and no geometrically distorted image. Instead 768 X 576 images looked a bit (nearly 3%) larger.
Most important, I can,t easily think that BBC site may be wrong .
I hope that if you real2reel or someone else did some tests with 788 X 576 square pixel images may report his results, if not, why not try?.

thanks

PS:

The various sizes of the frame have nothing to do with overscan; and much to do with pixel aspect ratio and active image area. In a 704 pixel image, the TV actually uses all of that image, and in a 720 pixel image, the TV uses almost of it.

There is of course overscan, but it is a seperate issue the CRT is actually creating the image area you don't see; it is just hidden because television CRTs are not very accurate at the edges. If the TV only "took" 720 pixels out of a 768 pixel image you would have distorted shapes.

Because the video standards are analog, the horizontal resolution is variable.

The BBC is using 768x576 as a figure because that produces a 4:3 image with a pixel aspect ratio of 1, which is the pixel aspect of a computer. This gives an image which "looks right" on a computer. "Pixels" are taller on TV. When the pixel aspect ratio is changed to that of PAL television (1.094, approximately), you get 702 pixels out of it (remember, approximately.. the digital standard is 704). Hence, your 704x576 image.

The 720x576 image has the same pixel aspect ratio, but slightly more image area horizontally (as demonstrated by the test card). Considering there is an 20 pixel difference (but wait! You say, there are only 18! Yes, but remember the PAR difference: 18 * 1.094 = approx 20). This is where the 788x576 figure comes from. This is the size required to produce a 4:3 image with a PAR of 1 that also has the extra image area of the 720x576 image.

So, in summary: 768x576 converts down to 704x576, 788x576 converts down to 720x576.

Something else to mention: Recent versions of Photoshop have a pixel aspect ratio feature that automatically stretches the image in real-time, but performs operations at the correct aspect so instead of mucking about with resizing images (which causes slight quality loss), you can work on a 720x576 or 704x576 image without quality loss. It will look only so-so on screen because the conversion is done at low quality, but the saved image will be of high quality.

You will have to create a custom aspect ratio, though, because Photoshop gets it wrong; 1.066 is not the correct aspect ratio. Doing the math, you'll find that they are calculating 768 / 1.066 = 720, which as I've demonstrated above is the wrong conversion. Set it up with 1.094 and you'll be set. All you have to do is create 720x576 images (or 704x576) and select your custom aspect ratio and the shape will be right for editing.

I completely agree with you Iantri and I’m sure many people in this forum will find your answer very useful.
It is surely important to be able to produce menus and slide shows with correct aspect in DVD video and your answer makes all this clear.
Thanks

Good. I just hope I got all the numbers right, I am in NTSC-land (Canada).