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Old 26th August 2009, 08:15   #1  |  Link
an3k
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Why does DivX uses wrong PAR? / How SAR/PAR/DAR is correctly calculated

Hi everybody,

first i would like to explain SAR, PAR and DAR. I often read that PAR belongs to the Pixel Aspect Ratio of the Screen i want to use but this is completely wrong. It has nothing to do with the Display you use to watch a movie. No difference between PC-Monitor or Television!!

Lets begin:
SAR = Storage Aspect Ratio
PAR = Pixel Aspect Ratio
DAR = Display Aspect Ratio

SAR is calculated by Width divided by Height (eg. 1920/1080 = 1,7777777777777777777777777777778 which is rounded to 1,778 which is 16:9), thus SAR only specifies in which Aspect Ratio the Video is saved. Period.

DAR is the Aspect Ratio you want your video to be shown in. For example if you create a video with SAR of 1,250 (5:4 or eg. 720x576) and want it to be shown as 16:9 you have to set the DAR to 1,778 (16:9).

PAR is used to calculate the correct DAR by using the SAR.

Example:
We have a video with a resolution of 720x576, the PAR is 5:4 or 1,250. We want to show it in 16:9 or 1,778.

We only need two information to get the third. If we have SAR and DAR we just do DAR / SAR = PAR

If we have SAR (we always have this information) and PAR we just do SAR * PAR = DAR. This is what PC-Software, TFT-TV-Screens, DVD-Players, etc. do.

Open a DVD vob file and check the information GSpot gives you. You see that for the example above the PAR must be 1,422 but in Divx we only can set useless values like 16:11 = 1,455 (16:9 PAL) and 40:33 = 1,212 (16:9 NTSC). If DivX would change this we could create anamorph DivX.

I hope you understand what i wrote and can answer my question: Why does DivX uses such useless values?

@ DivX Developers: Just remove the "Input format" and "Output format" inputs and create a single one in which the user can enter a value like 1,422 or 1,066 (no dropdown!! manual input by user!!) and everything will be fine. The info in this field should be passed directly into the video, no need for the encoder to modify it. Thank you!

I always had my problems with those stuff but after i took a accurate look into GSpot i knew how it works. Yes, GSpot gave me all of that information.

Last edited by an3k; 26th August 2009 at 08:29.
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Old 26th August 2009, 10:16   #2  |  Link
GodofaGap
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I'm afraid you are only getting half the story here. DivX seems to be following ITU recommendations which state that for PAL only 702x576 belongs to the 4:3 or 16:9 anamorphic image and for NTSC this is 711x486.

I agree that it would be better if the option was more customizable and easier to use though.
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Old 26th August 2009, 12:20   #3  |  Link
an3k
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Quote:
Originally Posted by GodofaGap View Post
I'm afraid you are only getting half the story here. DivX seems to be following ITU recommendations which state that for PAL only 702x576 belongs to the 4:3 or 16:9 anamorphic image and for NTSC this is 711x486.

I agree that it would be better if the option was more customizable and easier to use though.
Thanks for the info. Didn't know that. I think you mean 704x576 cause then the whole calculation is correct:

702 / 576 = 1,219
1,778 / 1,219 = 1,459

704 / 576 = 1,222
1,778 / 1,222 = 1,455 = 16:11


But now the question is, why does the ITU recommend 704x576 while the whole world (as long as they use PAL ;) uses 720x576? Is there any advantage of 704x576? Both, 720 and 704 are multiples of 4, 8, 16.
Ok, only 704 is a multiple of 32 while 720 isnt - but which codec requires this?

I guess i install 3.11alpha again - best divx i've ever used ;)
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Old 26th August 2009, 12:37   #4  |  Link
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How about building a HTPC and use x264 instead?
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Old 26th August 2009, 12:40   #5  |  Link
an3k
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Quote:
Originally Posted by stax76 View Post
How about building a HTPC and use x264 instead?
I dont need a HTPC since my computer is connteced through HDMI 1.3 with my FullHD 52" TFT and x264 is MPEG 4 Part 10 aka. MPEG 4 AVC but we discuss MPEG 4 Part 2 aka. MPEG 4 ASP.

Sorry for being a bit aggressive but i love those 100% offtopic posts.
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Old 26th August 2009, 13:33   #6  |  Link
GodofaGap
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Quote:
Originally Posted by an3k View Post
Thanks for the info. Didn't know that. I think you mean 704x576 cause then the whole calculation is correct:
No I mean 702. 704 or the resulting 16:11 is a simplification.

ITU-R BT.601 defines that PAL systems have a 52 Ás active line lenght and a 13.5 MHz sampling rate. 52*13.5 = 702 samples per line.

Quote:
But now the question is, why does the ITU recommend 704x576 while the whole world (as long as they use PAL uses 720x576?
It's perfectly OK to use 720x576 as long as it is understood that 702x576 represents the actual 4:3 or 16:9 picture. In other words 720x576 is actually a little wider.

One of the reasons why normally 720 is used is to allow for inaccuracies in timing during analog to digital conversions.

It is however very difficult to know if a DVD is produced with ITU standards in mind or not.

Quote:
I guess i install 3.11alpha again - best divx i've ever used
And also doesn't have PAR

Last edited by GodofaGap; 26th August 2009 at 13:36.
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