Welcome to Doom9's Forum, THE in-place to be for everyone interested in DVD conversion. Before you start posting please read the forum rules. By posting to this forum you agree to abide by the rules. |
13th May 2006, 00:01 | #1 | Link |
(schein)heilig
Join Date: Jun 2003
Location: Germany
Posts: 512
|
ITU-R BT.601 and PAR: How good is my knowledge?
Hi, guys.
After the latest update of my own site and (once again) reading A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions aspect ratios refuse to leave my head. I more and more get the feeling to be on the right track to thoroughly understanding the whole topic. But, of course, a couple of questions remain. Well... Let me first summarize the "facts" as I know them - values according to ITU-R BT.601. PAL For correct DAR, PAL needs following resolutions: 4:3 -> 768x576 16:9 -> 1024x576 A DVD's active area is 702x576 pixels. The original 768 or 1024 need to be squeezed into those 702 pixels. And knowing this, calculating the PAR is possible. Code:
4:3 768 128 PAR = --- = --- = 1.094 702 117 16:9 1024 512 PAR = ---- = --- = 1.459 702 351 4:3 -> 787.69x576 16:9 -> 1050.26x576 NTSC NTSC has these resolutions: 4:3 -> 648x486 16:9 -> 864x486 The active area here is 710.85 pixels. In reality of course only 711 pixels are possible. Anyway - calculating the exact PAR: Code:
4:3 648 4320 PAR = ------ = ---- = 0.912 710.85 4739 16:9 864 5760 PAR = ------ = ---- = 1.215 710.85 4739 4:3 -> 656.34x480 16:9 -> 875.12x480 So far, so good. Now I could as well - calculate NTSC with 711 instead of 710.85 pixels. - ignore ITU and assume the whole 720 pixels as being active area -> same calculations with 720 instead of 702/710.85. In the end I come up with this set of PAR tables: Code:
Calculated PAR according to ITU-R BT.601 | PAL | NTSC | NTSC (711) -----+-----------+-----------+----------- 4:3 | 128/117 | 4320/4739 | 72/79 -----+-----------+-----------+----------- 16:9 | 512/351 | 5760/4739 | 96/79 Usual "practical" PAR respecting ITU-R BT.601 | PAL | NTSC -----+-----------+----------- 4:3 | 12/11 | 10/11 -----+-----------+----------- 16:9 | 16/11 | 40/33 "Wrong" PAR without ITU-R BT.601 | PAL | NTSC -----+-----------+----------- 4:3 | 16/15 | 9/10 -----+-----------+----------- 16:9 | 64/45 | 6/5 Ok, and to give you some thoughts for a discussion:
*sigh* The day I'll finally understand 100% of the whole PAR-DAR-ITU-anamorphology, I'll throw a huge party! P.S.: I posted the same thing in the German forum.
__________________
Brother John When lost in BeSweet's options, have a look at the Commandline Reference. DVD nach MPEG-4 klappt nicht? Verzweifelt? Auf zum Encodingwissen! Last edited by Brother John; 13th May 2006 at 00:04. |
14th May 2006, 08:11 | #2 | Link |
Doom9ing since 2001
Join Date: Oct 2001
Location: Seattle, WA, USA
Posts: 2,002
|
The correct active area for both NTSC and PAL, according to ITU.601, is 704x480/576.
So the correct pixel aspect ratios for DVDs are: NTSC fullscreen 10:11 NTSC widescreen 40:33 PAL fullscreen 12:11 PAL widescreen 16:11 Those stretch the image to: NTSC fullscreen 640x480 NTSC widescreen 853x480 PAL fullscreen 768x576 PAL widescreen 1024x576 When resizing from full 720x480/576 resolutions to square pixel resolutions, one should use one of the two methods: 1. Crop away 8 pixels on each side, then resize according to the PARs listed above. Downside: tiny black bars on the side. 2. Resize according to the PARs listed above, then crop away any extra vertical pixels until your reach the square pixel resolution listed above. Downside: you sacrifice some vertical resolution. Similarly, when resizing from higher-than-SD resolutions to SD, one should preferrably resize like this (using Avisynth syntax and NTSC in this example): LanczosResize(720,492) Crop(0,6,720,480) Why? Well, even though only 704 horizontal pixels are active, it's desirable to use all 720 in order to take advantage of the full display area on devices with no overscan. But in order to do that, one must be careful to preserve the correct aspect ratio. If 704/480 is 1.46666, then 720/1.46666 is 490.91. We round up so we could crop by an even number. Similarly, when expanding the ITU.601 image to square pixels, we can use the whole image area, not just the active pixels. So anamorphic PAL 720x576 actually ends up being 1048x576. No, that's not quite 16:9, but 1024x576 is. The 24 extra pixels are the same 16 inactive pixels from the 720x576 source. Confused more? Last edited by zambelli; 14th May 2006 at 08:21. |
14th May 2006, 16:59 | #3 | Link | |
(schein)heilig
Join Date: Jun 2003
Location: Germany
Posts: 512
|
Quote:
One thing though: Why do you think ITU says 704 active pixels? I'm still a little lost between samples and megahertz when reading the recommendation itself, but the Quick Guide's conversion tabe claims 702x576 and 710.85x486 to be the correct active ITU frame. And I'm quite sure that the Guide is a trustworthy source.
__________________
Brother John When lost in BeSweet's options, have a look at the Commandline Reference. DVD nach MPEG-4 klappt nicht? Verzweifelt? Auf zum Encodingwissen! |
|
14th May 2006, 17:00 | #4 | Link | |
Moderator
Join Date: Nov 2001
Location: Netherlands
Posts: 6,364
|
Quote:
|
|
14th May 2006, 20:20 | #5 | Link | |
Doom9ing since 2001
Join Date: Oct 2001
Location: Seattle, WA, USA
Posts: 2,002
|
Quote:
Similarly, 710.85x486 is essentially the same aspect ratio as 702x480, so perhaps that's why it gets set to 704x480 as well. If I recall correctly, the extra 6 vertical lines in NTSC are used for blanking intervals. Since digital signals have no use for them, they're discarded. It all gets very complicated very quickly, so I think for all intents and purposes it's best to assume 704x480/576 are the active ITU.601 resolutions. |
|
14th May 2006, 20:38 | #6 | Link |
(schein)heilig
Join Date: Jun 2003
Location: Germany
Posts: 512
|
702 might be a strange number for codecs, and 710.85 even more, but they don't have to deal with it. The codec always gets fed 720 for a usual DVD. Ok, 704 is valid as well. But I've never seen a disc actually using it.
So why assume something that apparently is wrong? For practical purposes a "stick to the ITU PAR table" is sufficient anyway. No need to know about the backgrounds at all. And if you do set out on a quest to find the secret of the ITU-R , anything less than the whole truth isn't good enough, no matter how complex and difficult that truth might be. - At least that's how I feel.
__________________
Brother John When lost in BeSweet's options, have a look at the Commandline Reference. DVD nach MPEG-4 klappt nicht? Verzweifelt? Auf zum Encodingwissen! |
14th May 2006, 20:50 | #7 | Link | ||
Doom9ing since 2001
Join Date: Oct 2001
Location: Seattle, WA, USA
Posts: 2,002
|
Quote:
Quote:
Also, note that VCD resolutions are 352x240 and 352x288. Exactly quarter res. That's no coincidence. |
||
14th May 2006, 21:06 | #8 | Link | |||
(schein)heilig
Join Date: Jun 2003
Location: Germany
Posts: 512
|
Quote:
Quote:
Quote:
__________________
Brother John When lost in BeSweet's options, have a look at the Commandline Reference. DVD nach MPEG-4 klappt nicht? Verzweifelt? Auf zum Encodingwissen! |
|||
14th May 2006, 21:50 | #9 | Link |
Life's clearer in 4K UHD
Join Date: Jun 2003
Location: Notts, UK
Posts: 12,227
|
If you are into this kind of thing.... you might be interested to see what the good ol' BBC has to offer: -
http://www.bbc.co.uk/commissioning/t...turesize.shtml But remember, software based media players don't follow ITU output sizes when displaying PAL and NTSC anamorphic DVD's Cheers
__________________
| I've been testing hardware media playback devices and software A/V encoders and decoders since 2001 | My Network Layout & A/V Gear |
Last edited by SeeMoreDigital; 14th May 2006 at 21:52. |
14th May 2006, 22:23 | #10 | Link |
Moderator
Join Date: Oct 2001
Posts: 3,530
|
One thing to remember, ITU-601 describes how to digitize an analog standard, so, for NTSC, keep a copy of RS-170 or SMPTE 470 handy. In NTSC the active portion of a scan line is 52.6us, multiplied by the sample rate of 13.5MHz you get 710.1 pixels. ITU-601 uses 720 for a number of reasons, the best being that 720 is the lowest multiple of 16 greater than 710 - a consideration for mpeg. Digitizing more than the active region allows for misalignment of the blanking pulse.
NTSC has 525 lines, 485 (not 486 as many claim) are active, the others carry vits, CC, vitc, and vertical sync. 5 lines are lost when going to digital. The reason ITU-601 chose 480, besides being a multiple of 16, is that it allows a common sample rate for NTSC and PAL. 720x480x30 = 720x576x25 The differences in PAR are so small the average viewer cannot discern them, and are far smaller than the average misalignment of a crt. |
14th May 2006, 23:54 | #11 | Link | |||
Doom9ing since 2001
Join Date: Oct 2001
Location: Seattle, WA, USA
Posts: 2,002
|
Quote:
Quote:
Microsoft Windows Media Encoder 9 gets it exactly. Quote:
|
|||
15th May 2006, 00:50 | #12 | Link |
(schein)heilig
Join Date: Jun 2003
Location: Germany
Posts: 512
|
I'll have to get a little deeper into the analog side of the whole problem, but digital seems pretty clear now. Thanks for the contributions, guys!
Let me try a little summary: We have two "competing" sets of specs: ITU and DVD/MPEG, the former using PAR 521/351 etc. and the latter using 16/11 etc. Both have the same spirit but a slightly different focus. Of course it's impossible to say one is correct and the other one is not. And the difference is very small, anyway. We're talking about 3 pixels for a 16:9 PAL DVD. Additionally DVDs might be mastered according to any of the two. Hm. All this sounds like the most reasonable course of action for practical purposes (i.e. tutorial writing, backups) would be: Stick to the better known DVD/MPEG PARs, mention ITU values as advanced background info and strongly discourage any "generic" PAR because DVDs probably are not mastered with those values in mind. Thus using generic for a backup is very likely to result in a slightly distorted image.
__________________
Brother John When lost in BeSweet's options, have a look at the Commandline Reference. DVD nach MPEG-4 klappt nicht? Verzweifelt? Auf zum Encodingwissen! |
15th May 2006, 10:11 | #13 | Link | ||
Moderator
Join Date: Nov 2001
Location: Netherlands
Posts: 6,364
|
Quote:
Quote:
|
||
15th May 2006, 10:46 | #14 | Link | ||
Registered User
Join Date: Feb 2006
Posts: 823
|
Unfortunately PAR seems to be impossible to get exactly right due to the 2 following reasons:
1. Quote:
2. Quote:
IMO to the differences between ITU and non-ITU are too small to worry so much about. Even more so because even if you are following the correct way you are not guaranteed correct output anyway. |
||
22nd May 2006, 18:09 | #15 | Link | |||||
Squeeze it!
Join Date: Oct 2003
Location: Germany
Posts: 472
|
Quote:
Quote:
Quote:
Quote:
Quote:
Last edited by Inc; 22nd May 2006 at 18:18. |
|||||
23rd May 2006, 04:54 | #16 | Link | |
interlace this!
Join Date: Jun 2003
Location: i'm in ur transfers, addin noise
Posts: 4,555
|
Quote:
as for actual content, even theoretically compliant stuff is often out of aspect (old telecines tend to be vertically stretched and cropped heavily compared to new transfers). and stuff done digitally will just obey the digital video standards. it's up to the DVD player to make things correct then. i think so long as circles look round and squares aren't rectangles, it's all fine. remember also that TVs themselves are almost always wrong (even my HR trinitron with SDI inputs etc shows a different aspect in underscan mode). then consider the angle you're watching the TV from - only one person can watch from dead-straight on...
__________________
sucking the life out of your videos since 2004 |
|
23rd May 2006, 09:20 | #17 | Link | ||
Squeeze it!
Join Date: Oct 2003
Location: Germany
Posts: 472
|
Quote:
14.769Mhz (PAL TV) –––––––––––––––––– = PAR 1.09402:1 13.500Mhz (SAP/ITU) Two of the testet SAPs where mpeg4 supporting ones and so these ones also got an option for "generic" mpeg2 playback. In that case 720 was treaten by generic PAR. Quote:
Once I checked a Stream by exermining it's "Working Title" Intro. It was wrong but the main movie itself was correct. Last edited by Inc; 23rd May 2006 at 09:24. |
||
Thread Tools | Search this Thread |
Display Modes | |
|
|