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Old 19th May 2005, 04:55   #21  |  Link
Meat_PoPsiclez
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Quote:
Originally posted by SomeJoe
[B]Very true. I've seen a few 16:9 displays that do overscan, and then I've seen some that behave more like computer monitors where all of the 720x480 picture is displayed.
I have an oddity of a sd television, a pioneer "cimemascreen" rptv. It produces 720x480 in a 1.5:1 aspect ration with no overscan (less than a few pixels in each direction). Overscan corrected sources like videogames actually leave black borders on the sides, even though the tv has no aspect adjustments (being a pure analog tv).

Also, to my horror and dismay, a RCA Scenium 16:9 hdtv I had a chance to work with produces nearly 80.. 80 pixels of horizontal overscan in 1080i, and a considerable amount of vertical overscan, likely in scale. Apparently, in our age of high-def some companies feel that standards compliance (which hd standard specifies) is a useless prospect. When operating in 480i and p modes it produced very little overscan, seemingly similiar to a normal television.

I think it's best to try and consider the overscan during filming, so you can find a good result without too much (possibly) quality detrimental post work.
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Old 13th April 2006, 11:27   #22  |  Link
2Bdecided
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I'm glad this thread reached a sensible conclusion, because the earlier suggestion that 720 is equivalent to 704 "plus overscan" is simply wrong.

704 is approximately the width of the analogue active picture ( http://lipas.uwasa.fi/~f76998/video/conversion/ gives more accurate values). As explained on that page, samples outside this range (up to 720) are used when sampling analogue video signals to make sure pixels aren't lost at the edge when the sync isn't quite right, and to avoid single pixel transitions from black to picture causing ringing. The extra pixels are outside the 16:9 and 4:3 active picture area.

It seems to me there's no need to crop anything when going from DV to DVD, but there is good reason to crop down to 704 before resizing to a 1:1 PC resolution.

Cheers,
David.
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Old 12th February 2008, 20:29   #23  |  Link
Vetal
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Quote:
Originally Posted by North2Polaris View Post
Thank you for your explanation! After reading your post, I found another reference by Chris Pirazzi on Square and Non-Square Pixels at:

http://www.lurkertech.com/lg/pixelaspect.html

According to the Pirazzi article, in SGI libraries, pixel aspect ratio is specified as a fraction of vertical (y) pixel size divided by (x) horizontal pixel size. In “A Quick Guide to Digital Video Resolution and Aspect Ratio Conventions” that you cited above, the pixel aspect ratio is defined in x/y format.

I found the pictures in the Pirazzi article to be helpful, but I am still puzzled. According to Pirazzi, the actual pixel aspect ratios are defined purely in terms of the pixel sampling frequency of each video standard:

* Rec. 601 digital video is always sampled at 13.5 million pixels per second (for both 525 and 625).
* If you have a 525-line analog NTSC (ANSI/SMPTE 170M-1994) video signal which you want to sample square pixel, the industry standard is to sample at exactly 12 + 27/99 million pixels per second.
* If you have a 625-line analog PAL (Rec. ITU-R BT.470-3) video signal which you want to sample square pixel, the industry standard is to sample at exactly 14.75 million pixels per second.
* Therefore, we can derive from this that:
525-line Rec.601 pixel aspect ratio = 13.5 / (12 + 27/99) = exactly 11/10 (y/x)
625-line Rec.601 pixel aspect ratio = 13.5 / (14.75) = exactly 54/59 (y/x)

Thus, for 525-line video, the 11/10 ratio is used to convert square pixels to non-square pixels; and the inverse of that ratio, which is 10/11 or 0.909, is used to convert non-square pixels to square pixels.

Based on one of the practical examples in the “A Quick Guide to Digital Video Resolution and Aspect Ratio Conventions”, incorporating a DV clip that uses ITU-R BT.601 pixels and a resolution of 720x480 into a video project based on 640x480 industry standard square pixels, requires a horizontal scaling factor of 10/11 or 0.909.

Part of the problem in trying to understand these concepts is the definition of terms. In this case, one person’s “horizontal scaling factor” appears to be the inverse of another person’s “pixel aspect ratio”.

I find "horizontal scaling factor" to be a useful concept, now that I understand where it comes from.

What are the practical implications of this? We still need to take 8 pixels from both side edges and resample the 704x480 image to 640x480.

So why did I go through this? I guess that is what being a newbie is all about.
Wow, I'm overwhelmed with all that details.

I am using x264 and capuring with NTSC DV
If I want to see videon computer screen or on TV hooked to computer, while I want to preserve all video without cropping. Can I just set "-sar 10:11" when encoding and that's it?

I used 8:9 all the time before (720x480 => 640:480, 4:3). But 10:11 looks even better, but I don't know for sure, because a difference is in subjective range.

So which SAR is better for case above, 8:9 or 10:11?

Thank you
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Old 14th February 2008, 12:50   #24  |  Link
2Bdecided
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With the caveat that I've never used NTSC DV or x264, the "correct" pixel aspect ratio of NTSC DV is 10:11. Your camera may vary.

The software needs to crop to 704x480, and convert that cropped image to 640x480. If you want to keep the full 720 width, at the correct pixel aspect ratio, then you need about 654 pixels (!) in the output.

Cheers,
David.

Last edited by 2Bdecided; 14th February 2008 at 13:56.
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Old 14th February 2008, 14:34   #25  |  Link
2Bdecided
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I have a radical suggestion.

Given that...

1. It's very complicated: http://lipas.uwasa.fi/~f76998/video/conversion/

2. It's even more complicated than that, because Jukka Aho "conveniently" ignored the analogue half lines. If you base the calculations on the correct analogue picture dimensions, it's even worse!

3. The basic descriptor in the MPEG-2 standards give the display aspect ratio rather than pixel aspect ratio, so "officially" (and incorrectly) 720x576 and 720x480 really do equal 4:3 (or 16:9).

4. Industry standards simply state the pixel aspect ratios as 59/54 (4x3 PAL) and 10/11 (4x3 NTSC). The PAL one is within 0.04% of the correct values (4x3 PAL = 1150/1053 exactly) if you take account of the analogue half lines properly, but these are often ignored in the digital world.

5. Downconversions from 16:9 HD fill all the 720 width, even though it's not there in the original, so the aspect ratio is almost certainly being changed.

6. PC playback software follows the MPEG-2 flags, and displays 720x576, 704x576, 720x480 and 704x480 all at 4x3 (or 16x9), while DVD players connected to calibrated TVs will only display the middle 702x576 as 4x3 (or 16x9) - there are always 2 or 16 pixels outside of the active area.

Given all of this, it could be argued that to minimise aspect ratio errors across the full range of standards and display devices, the absolute "best" that can be done is to encode at 704x576 or 704x480 only (never 720, which is unpredictable on PCs vs TVs), and to treat that as the full active area.

If you do this, then the error for PAL is less than one video pixel WRT the correct aspect ratio (where 702x575 = 4x3 or 16x9). I haven't been through NTSC from first principles, since the "correct" vertical height of an NTSC image is not clear to me. However, the error, if you believe the industry standards, is exactly zero for 4x3, and less than one pixel for 16x9.


Therefore:

"PAL"
4x3 768x576 = 704x576
16x9 1024x576 = 704x576

"NTSC"
4x3 640x480 = 704x480
16x9 854x480 = 704x480


So there you have it. Stop worrying. Treat 704 as the full picture and you'll never be more than a pixel out.

2012 Edit: 704 pixels is not strictly valid for anamorphic 16x9 DVD and may cause problems on some old DVD players
details here: http://forum.doom9.org/showthread.ph...76#post1549376

Cheers,
David.

Last edited by 2Bdecided; 6th January 2012 at 19:16.
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Old 14th February 2008, 15:16   #26  |  Link
Vetal
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Quote:
Originally Posted by 2Bdecided View Post
I
"NTSC"
4x3 640x480 = 704x480
16x9 854x480 = 704x480


So there you have it. Stop worrying. Treat 704 as the full picture and you'll never be more than a pixel out.

Cheers,
David.
Thank you David,

One more question, what does it mean in procedural terms, "treat 704 as the full picture"?

a) Crop to 704x480 and apply SAR(10:11) ?
b) Downscale to 720x480 => 704x480 and apply SAR(10:11)?
c) Based on "your camera may vary", I should tape a square and find right AR, by applying ruler on my display. So its sizes on computer screen are equal. Ensuring display has 1:1 pixel AR, of course.

Thank you!

Last edited by Vetal; 14th February 2008 at 18:00.
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Old 14th February 2008, 18:34   #27  |  Link
2Bdecided
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Don't do (b)!

Cheers,
David.
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Old 15th February 2008, 00:09   #28  |  Link
Vetal
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Quote:
Originally Posted by 2Bdecided View Post
Don't do (b)!

Cheers,
David.
Thank David.

From what I got we are talking about the same thing:

Quote:
Originally Posted by 2Bdecided
If you want to keep the full 720 width, at the correct pixel aspect ratio, then you need about 654 pixels (!) in the output.
Is it come from

720/480 * 10/11 = x/480

Where x= 654.5.

So if I compress 720x480 with AR set to 10:11 inside mp4 container. And player will rescale it in windowed mode at 100% to 654x480. Or to any other scale while preserving AR, I.e. to full screen.

Because cropping (720=> 704) does not change SAR, it cuts off 8-pixel wide bands from sides. And after the player render the result, result would be the same, but with some information cropped from the sides, nothing more than that. I.e. it would crop 2*8*(10/11) = ~14 pixels. And it would give 640 pixels width (654 - 14). But those 2x8 pixels are not a noise, it is part of the image. Why should I crop them, that is my question.

Thank you

Last edited by Vetal; 15th February 2008 at 00:14.
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Old 15th February 2008, 13:20   #29  |  Link
2Bdecided
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If the player works correctly, and you're scaling on playback, then there's no need to crop.

It's MPEG-2 that I've had issues with; if the source is mod16 and contains only the active 4x3, it removes some of these issues. You may not have these issues with mp4 - I don't know, sorry.

Cheers,
David.
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Old 15th February 2008, 18:44   #30  |  Link
frank
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Quote:
Given all of this, it could be argued that to minimise aspect ratio errors across the full range of standards and display devices, the absolute "best" that can be done is to encode at 704x576 or 704x480 (never 720, which is unpredictable on PCs vs TVs), and to treat that as the full active area.
Agree.

Software dvd players (like PowerDVD) show 720x576, 720x480MPEG-2 pictures with wrong aspect ratio because they use it as active area. Don't trust them!
But 704x576, 704x480 handling is ok.

Not enough: A lot of new movies are encoded with this issue.
(All Harry Potters use 720x576 = 16:9). And so only the software players show the right AR on PC.
And look at the circle calibration tool on dvd Ratatouille, circle 720x576 PAL - same issue!
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Old 18th February 2008, 15:44   #31  |  Link
2Bdecided
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Originally Posted by frank View Post
And look at the circle calibration tool on dvd Ratatouille, circle 720x576 PAL - same issue!
Really? That's the first proof I've seen that this happens. It means none of these DVDs will play back with the correct aspect ratio on a "correctly" calibrated TV.

Maybe that's why they include a calibration tool - so you can adjust your TV to match the mistake on the DVD!

Cheers,
David.
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Old 22nd February 2008, 17:45   #32  |  Link
facialz
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Table of commonly correct and incorrect encoding. Incorrect encoding may be implied from wrong assumption about the shape of the active display area of the source.


Code:
        Source      IAR   FAR    PAR   Comment
        =========  ===== =====  =====  ===============
        PAL   4/3   5/4  15/11  12/11  Correct *
        PAL   4/3   5/4   4/3   16/15  Incorrect
        PAL   4/3  11/9   4/3   12/11  Correct *

        PAL  16/9   5/4  20/11  16/11  Correct *
        PAL  16/9   5/4  16/9   64/45  Incorrect
        PAL  16/9  11/9  16/9   16/11  Correct *

        NTSC  4/3   3/2  15/11  10/11  Correct *
        NTSC  4/3   3/2   4/3    8/9   Incorrect
        NTSC  4/3  22/15  4/3   10/11  Correct *

        NTSC 16/9   3/2  20/11  40/33  Correct *
        NTSC 16/9   3/2  16/9   32/27  Incorrect
        NTSC 16/9  22/15 16/9   40/33  Correct *
* Absolutely correct with respect to ITU-T H.264 [1]
and approximately correct with respect to a possible interpretation [2] of (analogue) PAL / NTSC standards, ITU-R BT.601, and all digital standards based on them (D1, DV,...).

PAR: pixel aspect ratio, aka sample aspect ratio.
FAR: frame aspect ratio, aka display aspect ratio, aka picture aspect ratio.
IAR: image aspect ratio, aka storage aspect ratio. For example, 720/480 = 3/2 and 704/480 = 22/15.




Reference

[1] ISO/IEC MPEG - ITU-T VCEG Joint Video Team (JVT): Draft ITU-T H.264 (2002E) and ISO/IEC 14496-10:2002(E), aka AVC standard. Table E-1.

[2] Jukka Aho: A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions. http://www.iki.fi/znark/video/conversion/

Last edited by facialz; 3rd March 2008 at 20:38.
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Old 9th March 2008, 17:03   #33  |  Link
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Table of PAL PARs, sampled under 13.5 MHz
Code:
    Width     Height     PAR      Error    Used by
    ========= ====== ===========  =======  ==============
    702       576      128/117    +0.174%  Aho, MeGUI
    690       566     1132/1035   +0.147%  SMPTE RP-187
    702+54/59 576       59/54     +0.043%  Sony, Apple
    702       575     1150/1053    0       MPEG-1, MPEG-2
    704       576       12/11     -0.111%  MPEG-1, ASP, AVC, Microsoft
    720       576       16/15     -2.330%  Avidemux
    720?      576?      1.066     -2.391%  Adobe

Table of NTSC PARs, sampled under 13.5 MHz
Code:
    Width     Height     PAR      Error    Used by
    ========= ====== ===========  =======  ==============
    711       487     1948/2133   +0.391%  MPEG-1
    710.85    486     4320/4739   +0.206%  Aho
    711       486       72/79     +0.185%  Aho, MeGUI
    710.85    485    38800/42651   0       ?
    704       480       10/11     -0.068%  MPEG-1, ASP, AVC, Sony, Apple, Microsoft
    711       483      644/711    -0.433%  MPEG-2
    708       480      160/177    -0.633%  SMPTE RP-187
    720       486        0.9      -1.067%  Adobe
    720       480        8/9      -2.289%  Avidemux


Width and Height are of the reference image, i.e. one defining the 4/3 FAR.

Errors are relative to the PAR based on the 52µs/575 PAL and (52+59/90)µs/485 NTSC reference image, respectively.

Last edited by facialz; 11th March 2008 at 14:49.
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Old 13th March 2008, 20:17   #34  |  Link
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Thank you facialz.

It's interesting that several of those are within one pixel (which is the point at which I think any reasonable person stops worrying!) but some are much further off.

Cheers,
David.
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Old 20th December 2010, 02:26   #35  |  Link
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In this page http://en.wikipedia.org/wiki/ATSC_Standards say something diferente at facialz. O_o
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Old 20th December 2010, 15:14   #36  |  Link
2Bdecided
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In this page http://en.wikipedia.org/wiki/ATSC_Standards say something diferente at facialz. O_o
What do you mean?

(The only PARs I can find on that page are for x.264/AVC, and for 4:3 it says 10/11, just like facialz's post from March 2008.)

Cheers,
David.
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