Aspect Ratio question

[McoreD]

Hi All,

I was following the Aspect Ratio Calculation Tool at http://www.haku.co.uk/ARCT.html

arpal=117/128;
arpala=arpal/1.3333;
arntsc=4739/4320;
arntsca=arntsc/1.3333;
arpalvcd=arpal;
arntscvcd=arntsc;
arpalsvcd=78/128;
arntscsvcd=4739/6480;
if(document.calform.standards.checked==false){
arpal=0.9375;
arpala=0.703125;
arntsc=1.125;
arntsca=0.84375;
arpalvcd=0.9166666;
arntscvcd=1.1;
arpalsvcd=0.625;
arntscsvcd=0.75;
}



and I was wondering where these values for example, (PAL lettebox) arpal =117/128 came from.

Can anybody please point me to a site that has more information about this? I've implented this calculations in one of my tools for local usage but I'd like to know the basis of it.

Thanks,
McoreD

[SeeMoreDigital]

Odd isn't it.... I can't say I've ever heard of "PAL Letterbox (4:3)" either!

That said: -

117
---- = 0.9140625
128


Which is very, very close to: -

11
-- = 0.916666
12

Personally, I prefer to use my own ARS Calculator (http://forum.doom9.org/showthread.php?t=107039).....


Cheers

[McoreD]

That's a nice tool SeeMoreDigital. I am looking at it.

Yes indeed it is odd. Now where does 11/12 come from? It's very irritating to have magical numbers that actually does work and not know where they come from.

I am familiar with calculating the PAR values, (what the ARS Calculator does). The ARS Calculator is going to extremely handy without having to manually calculate to input custom PAR in WME for cropped sources.
http://www.wmwiki.com/forum/viewtopic.php?p=2603#2603

After understanding the code logic behind ARCT, I am actually trying to understand where the numbers come from.

[Wilbert]

That said: -

117
---- = 0.9140625
128

ITU stuff:

Example 2: PAL DVD:
To determine the PAR of a PAL DVD, we first need to know the Image Aspect Ratio. From ITU-R BT.470-6 we know, that for PAL an active area of 52 µs x 576 lines results in a picture with 4:3 Aspect Ratio. A DVD has 720 pixels horizontally. From Jukka Aho's site we know that a DVD player has a sample rate of 13.5 MHz. This results in an active window width of 720 pixels / 13.5 MHz = 53.3333 µs. Vertically, 576 videolines are represented by 576 pixels; the height is not changed. This gives an Aspect Ratio of (4 / 3) * (53.333/52) = 1.3675. As you can see, the DVD window is not exactly 4:3, but a little bit wider. This results in a PAL DVD PAR value of 1.3675 * (576 / 720) = 1.094. (Exercise for the reader: do the same calculation for 704x576 PAL DVD and note this results in the same PAR.)

1.094 = 128/117 (approx)

Example 3: NTSC DVD:
From ITU-R BT.470-6 we know, that for NTSC an active area of 52.666 µs x 486 lines results in a 4:3 picture. Again, a DVD has 720 pixels horizontally, which results in a window of 53.333 µs. But now, the number of lines is also different from the standard: a NTSC DVD has only 480 lines instead of 486. The NTSC DVD window is a bit wider and also a bit less high than 4:3. This results in the following Image Aspect Ratio: (4 / 3) * (53.3333/52.6555) / (480/486) = 1.3674. This results in a PAR of 1.3674 * 480 / 720 = 0.9116.

0.9116 = 4320/4739 (approx)

etc ...

(from the analog capture guide: par.html)

[McoreD]

I am so grateful Wilbert. Thanks for your reply. That clearly explains where the numbers come from.

By the way, I was wondering, where would we continue such discussion in doom9? It doesn't seem belong to XviD forum. But I had no other choice because I didn't see a 'General Video Discussion' forum. But this kind of discussion would be rare and that might be why there is none.

The conventional PAR for PAL Anamorphic comes from
720x9:576x16 which simplifies to 45:64. So to conform to ITU-R BT.601-4 standards, for PAL Anamorphic, we use 117/128 instead of 45/64. I hope that's correct.

Thanks again.

Cheers,
McoreD

[SeeMoreDigital]

Yes indeed it is odd. Now where does 11/12 come from? It's very irritating to have magical numbers that actually does work and not know where they come from. It's the reverse of 12:11

3 576 11
-- / --- = -- = 0.916666
4 704 12


4 704 12
-- / --- = -- = 1.090909
3 576 11


Personally when calculating AR signalling ratios I prefer to use with fractions in the entire calculation process. Rather than converting fractions to decimals and then back to fractions


Cheers

[McoreD]

SeeMoreDigital, thanks for the clarification.

Yes indeed, I prerfer using fractions too. Now agian, howcome you are using 704x576 instead of 720x576. I am so behind.

[SeeMoreDigital]

... howcome you are using 704x576 instead of 720x576. I am so behind.Personally I don't... I prefer basing everything on 720 pixel widths, such as: -

4 720 16
- / --- = -- = 1.066666
3 576 15

3 576 15
- / --- = -- = 0.9375
4 720 16
And


16 720 64
-- / --- = -- = 1.422222
9 576 45

9 576 45
-- / --- = -- = 0.703125
16 720 64


Cheers

[check]

you'll find this interesting/illuminating: http://lipas.uwasa.fi/~f76998/video/conversion/