720x480 widescreen pixel aspect ratio wrong?

john-beale wrote on 11/28/2005, 11:29 AM
I'm confused about why the Vegas project preset for "NTSC DV Widescreen" sets a pixel aspect ratio of 1:1.2121. DV is 720x480 pixels, and the DV widescreen aspect ratio is 16:9. If you do the math, you find that (16/9) / (720/480) = 1.18518...
So where did 1.2121 come from?

As a concrete example, if I convert my 16:9 HDV footage to 720x480 "DV Widescreen" using the Vegas preset of 1.2121 pixel aspect, I get thin black bars at right and left on the image (this is hidden by TV overscan, but is visible on a PC display). But if I manually type in a 1.1852 pixel aspect ratio for my 720x480 project, the resulting image fills the screen perfectly.

Comments

Chienworks wrote on 11/28/2005, 11:42 AM
1.2121 works for 704x480. But, as you say, if the image is 720x480 then why not use 720x480 to fill the screen?
Coursedesign wrote on 11/28/2005, 11:54 AM
720x480 (720x486) fills a 4:3 screen when 0.9091 pixel aspect ratio (standard NTSC DV) is used.

720x480 (720x486) also fills a 16:9 screen when 1.2121 pixel aspect ratio is used.

If you convert DV to square pixels you get about 655x480 pixels (for use in for example old versions of Photoshop).

Chienworks wrote on 11/28/2005, 12:14 PM
720x480 (720x486) also fills a 16:9 screen when 1.2121 pixel aspect ratio is used.

But it doesn't! 720x480 at 1.2121 is 16.3634:9, not 16:9
Coursedesign wrote on 11/28/2005, 12:41 PM
See MIR Aspect Ratios and Frame Sizes.
john-beale wrote on 11/28/2005, 12:46 PM
Thanks, that explains it. I'd forgotten about the 704x480 resolution. I've never used 704x480 and I've never seen a DVD that did, but it is indeed part of the DVD spec. Still, I can't understand why Vegas uses the 704x480 pixel aspect ratio for DV at 720x480. This seems like a simple mistake to me.
Laurence wrote on 11/28/2005, 1:00 PM
HDV renders into 704x480 without any cropping. I've started using this format for rendering HDV to SD.
john-beale wrote on 11/28/2005, 1:09 PM
How very strange. According to http://www.mir.com/DMG/aspect.html the Rec.601 standard says that 1.2121 is indeed the pixel aspect ratio for SD widescreen, and if you have 720 pixel scanlines, you're supposed to crop 8 pixels off each side before displaying! Maybe someone can explain why that's the standard, though I'm not sure it's relevant to me now.

Basically, I have HDV at 1440x1080 and I want to convert it to 720x480 for a widescreen DVD. Logic would dictate that you take 1440 and divide by 2.000 and get 720. But (according to Vegas, apparently following Rec.601) I "should" take 1440 and divide by 2.04545.. to get 704 pixels, and then add 8 extra black pixels on each side. And those extra black pixels, by the way, are visibly displayed onscreen by common playback programs like WinDVD. I think I will do it the "wrong" way and actually use all 720 pixels, so my finished product doesn't have the odd black sidebars when displayed on a PC screen.