640x480 vs 654x480 for NTSC footage

[Axymeus]

Here's the situation, I have a NTSC disc detected by DGIndex as 654x480 display size (720x480 coded, 10:11 DAR). Until now I was pretty sure that I had to rip NTSC footage assuming a display size of 640x480 DAR of 4:3, but this is making me doubt. At first I thought this was a simple mistake but I've started noticing that this was very common among the many NTSC discs I have. Unfortunately I cannot find a good explanation for this difference. Should the footage be corrected for a DAR of 4:3 (what I believed was the expected display resolution of a standard television set) or should it be actually using a 10:11 DAR?

[Sharc]

Nothing wrong, assuming 10:11 is the PAR (Pixel Aspect Ratio) rather than the DAR (Display Aspect Ratio). 720x480 non-square (anamorphic) pixels are horizontally squeezed to 654x480 square pixels for playback, because 720x(10/11)=654. The player will do this for you when you play it back as 4:3 and you will see an undistorted 4:3 (DAR) picture.

[manono]

Yes, I agree. 720x480 resized to 656x480. But many 4:3 DVDs have a total of roughly 16 pixels of black along the sides. You crop them away and the video then gets resized to 640x480.

[Axymeus]

720x480 non-square (anamorphic) pixels are horizontally squeezed to 654x480 square pixels for playback.

That's what I don't understand. I'm sorry if I'm not being clear enough but I'll try my best. Televisions' resolution was 640x480, no other way around right? If the display size is coded as 654 then during playback of all those DVDs, it should add black bars above and under the picture to maintain the ratio on a screen limited to 640. And if most DVDs are coded 654, why aren't televisions natively 654x480, why the difference?

[davexnet]

The old TV's were analog, there was no absolute horizontal resolution, only vertical (the amount of lines) was fixed.

[DB83]

Well analog rvs were never exactly 4:3 since there is no such thing as 4:3 in the analog realm.

The analog to digital spec determined that the width of an image was 720 pixels with the active picture at 704 (some will argue 702) pixels. But I believe, as in the case of PAL, that 704 was adopted for dvd.


You do not appreciate the borders since they merge with others esp when viewed on wider screens. But the PAR only affects the active image not the whole.


(720-16) /11 *10 = 640 /4 *3 = 480

[Cornucopia]

DB83 is on the right track.

Analog doesn't have a fixed resolution, as it is a continuous signal. Many think this means "infitite" but it is constrained by bandwidth requirements (both with baseband wired CVBS and wireless RF). Because of this, the "effective" resolution is somewhere in the range of 540-800 pixels wide (of the visible image, usually more on the lower end) and 486 lines tall of the visible image (lines are discreet & fixed) for NTSC and 576 for PAL. Because of the need to find a common sampling rate for both NTSC and PAL, the digital sampling spec 601 defined the sampling rate at 13.5MHz. In doing so, it made the pixels' coverage area (aka "shape") turn out to be non-square. This was a departure from former early sampling attempts in the late 80s which did use square pixels. To achieve further commonality this new sample rate worked out to be close to 704 pixels wide for NTSC and close to 702 pixels wide for PAL, but since those were close enough and 704 was friendly to encoders that required mod-32 or mod-16 based blocks, 704 was chosen as the common width.
With the arrival of DV and DVD, MPEG influenced the sampling spec-makers to cover 720 pixels (both NTSC & PAL). For analog based systems being converted to digital, 720 included 8 pixels on either side that are black "guard bands". If analog is sampled using 704 and then black guardbands added, it will be 720 total, with the correct AR. If analog is sampled at 720, it will have a small (2%) error in the AR, which is usually unnoticeable.
But what about the 486? Those pro capture cards do capture at 486 NTSC, but that includes the 1/2 lines at top & bottom, and also a few of the VBI lines, such as closed-captioning. For consumers, this is unwanted, confusing, and possibly distracting if viewed on modern HDTVs, and as well the number doesn't play well with encoders either, so 480 was chosen.

If you cap analog 704x480 and resize to square pixels, you should have 640x480 (704 * 10/11). If you cap that same analog at 720x480 and resize to square pixels using the same 10/11, you would get 654.5454, most often rounded to 654. However, as stated, this still includes the "guard band" black bars, which if removed, would get you to 640 again. Others just cap at 720, convert straight to 640 using 9/8 and live with the 2% error.

The old analog TVs themselves were designed to be 4:3, in order to completely accommodate the old, ORIGINAL Film Academy aspect ratio. In the meantime the Academy ratio changed in order to accommodate the soundtrack. However, the TVs were already designed also with bezels overlapping the edges, covering the relatively unstable border signal, known as "overscan". So there was no way to know that the film wasn't 4:3.
Ironically, the success of TV scared movie producers into jumping onto the widescreen bandwagon.

All DVDs are coded as non-square (aka anamorphic), whether NTSC or PAL. Valid coding is 720x480, 704x480, 352x480, 352x240 (NTSC) and 720x576, 704x576, 352x576, 352x288 (PAL). That's it, regardless of whether it is 4:3 DAR or 16:9 DAR. After capturing, if you want it in square pixels, that's your choice of which figure to use, though for NTSC 640x480 is most common for 4:3 and ~853x480 for 16:9.


Scott

[Axymeus]

That's a very thorough explanation thank you. This particular movie has no black bars actually, the whole 720x480 area is used, meaning that part of the picture would be lost on old television if I understand what you said. Yet it would stand to reason that the correct aspect ratio would be 10:11 no matter, hm.

[Cornucopia]

If the full 720 is captured image, it may be assuming MPEG ratios as well, so you never know.

BTW, good professional movie/video producers will always show their important scene elements inside the safe area (action safe for objects, title safe for titles), even now.

Scott

[Sharc]

That's a very thorough explanation thank you. This particular movie has no black bars actually, the whole 720x480 area is used, meaning that part of the picture would be lost on old television if I understand what you said. Yet it would stand to reason that the correct aspect ratio would be 10:11 no matter, hm.

No tool I know of can really "measure" the true PAR (Pixel Aspect Ratio) of a video. If a tool reports a PAR (aka SAR) it is based
- either on the basis of some other known video parameters and the selection of the most likely PAR (aka SAR) from a set of standardized values (catalogue)
- or calculated according to the formula PAR=Frame Height/Width x DAR, for example PAR=480/720 x 4/3 = 8:9

In the best case - depending on the format - the PAR (aka SAR) can be read from the header/metadata of the video stream or from the container, but it may be wrong or conflicting there.

If in the OP's example the true PAR of the video would be 10:11 it should be displayed as 654.545 x 480 which is however a DAR of 1.3636 rather than exact 4:3 (the about 2% deviation mentioned before by Scott).

The PAR can be determined experimentally by means of the so called "circle test" plus the user's eyes (which tools don't have). In absence of a reference (circle or other known shape) it can even be difficult to conclude on the "true" PAR (aka SAR) experimentally.
See for the example here for a PAL 720x576 4:3 video:
https://forum.videohelp.com/threads/404828-SAR-PAR-DAR-some-practical-questions#post2649252

[manono]

This particular movie has no black bars actually, the whole 720x480 area is used...

Then it uses the Academy Ratio. Many films used it once upon a time. Warner Brothers DVDs, for example, often fill the entire DVD with picture when releasing films on DVD from a certain period.

[jagabo]

This particular movie has no black bars actually, the whole 720x480 area is used, meaning that part of the picture would be lost on old television if I understand what you said.

It's far worse than that. Old analog CRT TVs overscanned the image. You usually didn't see anywhere from 5 to 10 percent of each of the edges because they was hidden behind the bezel. This was because CRTs were prone to all kinds of distortions (AR, centering, pincussion, barrel, trapezoid, parallelogram, etc.). And the distortions varied with age, temperature, brightness of the picture etc. Hiding the edges of the frame made those distortions less obvious.

no black bars... it would stand to reason that the correct aspect ratio would be 10:11 no matter, hm.

No. The pixel aspect ratio would more likely be 8:9 in that case.

The DVD spec refers to the MPEG 2 spec regarding aspect ratios. The MPEG 2 spec is perfectly clear: the full frame constitutes the specified aspect ratio unless there is a sequence_display_extension indicating otherwise (very rare). So PAR 8:9 is the DVD spec for 4:3 DAR. The confusion arises because the ITU spec for digitizing analog video specifies that the 4:3 frame is captured at 704x480 (PAR 10:11) with 8 pixels of padding at the left and right to make a 720x480 frame (so the full 720x480 frame is about 2 percent wider than 4:3). Unfortunately, most DVDs made from analog tapes are captured according to the ITU spec at 720x480 and not corrected for the difference between the ITU spec and the MPEG 2 spec when written to DVD. Hardly anybody in the industry cares. And nobody notices the 2 percent DAR error.