The Real Aspect Ratio - not ITU

[jmac698]

Various guides go to great lengths to calculate the ideal aspect ratio to convert from DVD to square pixel xvid, etc. However I've always suspected they are completely wrong. Today I finally got around to proving it. I compared a dvd and hd version of the same material, and matched them. Here's what I used:

bilinearresize(720,480).shift(0,1).zoom(1,1.02)

I've always thought the editing software used in production determines the true aspect, obviously, and they are not following any standard. Here's what I've found:

Final Cut Pro uses 'NLE-PAR' 16:15
Many Adobe products use 'NLE-PAR' 16:15
Sony Vegas uses ITU PAR

So what does my example give?

[Ghitulescu]

ITU is just a recommendation. It's not mandatory. It's really at the discretion of the HW manufacturer whether to comply or not with the ITU-R provisions and how to do it.

[2Bdecided]

There's certainly professional equipment that converts HD to SD by resizing 1920x1080 to 720x576. Some of it doesn't change the colour either (Rec.601 vs Rec.709).

There are also plenty of DVDs which use generic PAR (as proven by set-up patterns on some of them).

But SD studio cameras used ITU PAR for decades. That's how they were set up.

Cheers,
David.

[Lyris]

So - can anyone provide some best practices for scaling 1920x1080 down to both 720x480 and x576 to obey ITU's aspect ratio recommendations?

[Sharc]

So - can anyone provide some best practices for scaling 1920x1080 down to both 720x480 and x576 to obey ITU's aspect ratio recommendations?
NTSC 16:9:
Spline16resize(702,480) #or a resizer of your choice
AddBorders(8,0,10,0)
PAL 16:9:
Spline16resize(702,576) #or a resizer of your choice
AddBorders(8,0,10,0)

[johnmeyer]

NTSC 16:9:
Spline16resize(702,480) #or a resizer of your choice
AddBorders(8,0,10,0)
PAL 16:9:
Spline16resize(702,576) #or a resizer of your choice
AddBorders(8,0,10,0)

Should the numbers above be 720 instead of 702?

[Midzuki]

Should the numbers above be 720 instead of 702?

"AddBorders(8,0,10,0)" ++ 702 == 720

Notwithstanding, me too still feels "702" sounds/looks weird :-/

[johnmeyer]

"AddBorders(8,0,10,0)" ++ 702 == 720

Notwithstanding, me too still feels "702" sounds/looks weird :-/Oh gee, I feel stupid. I guess the fact that 702 is a dyslexic version of 720 made me think it was a typo.

Thanks for being gentle on the old guy.

[wonkey_monkey]

I believe it should be 704 for NTSC (and therefore "addborders(8,0,8,0)"), and 702 for PAL (I tend to use 704 for both as it's mod 4)

David

[jmac698]

Whatever happened to the 710.85 number? I always had that one memorized.

[jmac698]

I'm not surprised if this is discussed before, http://forum.doom9.org/showthread.php?t=161143&page=3
I'm glad to see other people have made the measurements themselves, like I did with my capture card against various sources, and comparing versions of movies.

[DarkT]

This might help:

http://forum.doom9.org/showthread.php?t=134911
http://forum.doom9.org/showthread.php?t=155903

[scharfis_brain]

Whatever happened to the 710.85 number? I always had that one memorized.

This width is to be used with the formerly analogue full height of 486 lines of a NTSC source.

So 711x486 active image area would represent a correct 4x3 image.

but since there are only 480 pixels in height left, you would like to use 702 pixels in width for a correct 4x3 image.

[jmac698]

and that gives 702x480 - ok I believe it! I've seen videos 711 pixels wide, I guess they were cropped. Btw the analog pixels aren't just dots, the signal fades out over a few pixels at the edges, if you really want to capture all that.

[mandarinka]

I don't think that anything really prevents you from *not* using the black bars and just use the full 720x480 area. I mean, who will mind?

[jmac698]

Yes, I was going to say that too - for more than 16:9 (ie 2.4:1) it makes sense, just better quality, though it's *supposed* to be displayed cropped at the sides - it's still there though. And in the 16:9 case, it's vertically cropped on the dvd and displayed side cropped as well.

[Ghitulescu]

The BBC guides for importing digital images into real life videos indicate a size of 1050 x 576, whereas only the 1024 pixels (counting from the centre) should contain the real image (the extra may be black). The image must then be stretched to 720 x 576 in order to create an anamorphic video.


So, from HD to anamorphic DVD, one needs to add black borders up to a length of 1968 (24 extra pixels each side), then downscale to 720x576. Of course, colour corrections (R.709->R.601) are needed.

[2Bdecided]

So - can anyone provide some best practices for scaling 1920x1080 down to both 720x480 and x576 to obey ITU's aspect ratio recommendations?Yes, don't use 720, use 704. 704x576 is a legal resolution for DVB, DVD, etc etc.

Or scale to 704 and add 8 pixels of black down either size to give 720. That will give your encoder a harder job, and cause the aspect ratio to be incorrect in some software players (which will squash the full image, including the 8 pixels of black, into a 16x9 window), but it's your choice! ;)

The historic analogue 4x3 areas are 702x575 and 710.85x485. 704x576 and 704x480 maintain the same pixel aspect ratio (with an error of less than one pixel) and are mod16 values.

http://forum.doom9.org/showthread.php?p=1100187#post1100187

But just resizing to 720x576 (and hopefully flagging it as generic i.e. non-ITU PAR) is very very common...
http://forum.doom9.org/showthread.php?t=132378
...though this isn't correctly displayed by stand-alone DVD players feeding analogue TVs (unless the TV is set up intentionally wrong).

Hope this helps.

Cheers,
David.