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spanak 04-16-2018 02:02 AM

Understanding PAL aspect ratio?
 
2 Attachment(s)
Hi,

I have capture two VHS-C tapes and restored them to a pleasant watchable state. Tapes were eaten, so picture was jumping and scrolling, so this was my main concern. I haven't done much about colors, but I would take advices and comments.

Anyway, my main concern is aspect ratio. I would like to keep a remastered, interlaced copies of the tapes in .mkv, compressed losslessly with FFV1. So far, so good, I have no problem doing that. I also would like to set the correct aspect ratio, so that the player can display them properly, like the TV. I will also create DVDs from these files later.

I have captured with an ATI 600 USB at 720x576. As far as I understand the "active" video area is actually ~704x576. So what is supposed to be at 4:3? I think it is the inner 704x576, which means, I have to set the full 720x576 video to 15:11. If I do this, the picture looks too stretched horizontally, but it may be because of the fact, that I have edited (thus watched) the video at 5:4 for a month.

I am attaching two samples. Please tell me, what is the correct aspect ratio.

lordsmurf 04-16-2018 02:34 AM

Don't confuse SAR with DAR.
- Storage AR = 3x2 for 720x480, etc
- Display AR = 4x3 for 720x480 or 704x480, 4x3 for 720x576 or 704x576, etc

The actual video area usually is not 704x480 either. The exact measure varies. Remember, the source was analog, not digital. It wasn't measured in precise pixels. 720x576 is essentially 704x576 with an added matte. The matte was missing in the 704.

I really cannot tell which is correct here. Test patterns are needed, not clips of random footage.

spanak 04-16-2018 03:00 AM

1 Attachment(s)
Yes, I know it is not exactly 704 pixels. In my case, the usable width is 700 px, and there were between 1 or 2 more of fading into black, on each side. I am attaching an unaltered frame, if it helps.

What test patterns are you talking about? I do not have anything but home videos recorded with that camcorder. I can capture the test pattern that my TBC generates, but I am not sure it is going to be the same picture size at the same location within the 720 capture window. I'll do it, if it is going to be useful, though.

I am really trying to understand what is the proper DAR. I just need to know what digital resolution needs to be displayed at 4:3. Since there is no visual information beyond 704 px, and because this is a convenient digital width, I assumed this is it. Or it depends on the capture device, not the digitized video?

Aspect ratio is the only thing that has confused me so much.

sanlyn 04-16-2018 02:21 PM

4 Attachment(s)
The image you posted is a 720x576 PAL video frame. Like many capture setups, the equipment doesn't fully comply with some of the standards mentioned as to image placement, but that's not unsual. The image has 14 pixels of left black border, 4 pixels of right-hand border, and 14 pixels of bottom-border head switching noise.

http://www.digitalfaq.com/forum/atta...1&d=1523905847

The picture can be cropped (I used Avisynth) and the image centered as well as possible. I used Avisynth, which allowed me to crop off unwanted area without altering the core image, and replaced the discarded borders with new black pixels to restore the original frame size. The Avisynth methiod used was "Crop(14,0,-4,-14).AddBorders(10,6,8,8)". The result is shown below as 720x576.

http://www.digitalfaq.com/forum/atta...1&d=1523905898

Most lossless codecs don't honor DAR on playback, they simply play the frame as-is. So you have been watching a 5:4 image on your layer, not 4:3.The cropped and border-fixed image posted earlier will play this way at 4:3 (768x576) when encoded as DVD or SD BluRay:

http://www.digitalfaq.com/forum/atta...1&d=1523905898

The physical aspect ratio of the original 720x576 frame is 5:4, which is not a 4:3 image. VHS and VHS-C are designed for playback as 4:3 images fro your old 4:3 CRT TV. As far as rectangles go, a 4:3 image is slighter wider than a 5:4 image. Another way of stating the image ratios is that 4:3 = 1.333 to 1 and 5:4 = 1.25 to 1. What your lossless codec is playing for you is a square-pixel 5:4 image that isn't honoring the 4:3 display aspect ratio that the original video intended,

4:3 is the only image ratio that VHS and vHS-C can were designed to play as analog tape source, whether the image has extra borders or no borders and whether the core image fills the entire frame or not. The reason for capturing to the anamorphic format of 720x576 is becauase that is the format that will be required for DVD or Standard Definition BluRay authoring. You can also crop the 720x576 image to 704x480 (sorry, but a width of 702 simply will not play correctly and your DVD authoring program won't let you use it). ALso, some ornery equipment won't use 704 width exactly, but can use more or less than 704. It depends on the source and the capture gear. If you wanted square-pixel 4:3 for playback from FFv1, you should have encoded to 768x576 or to the more standard 640x480 (note that you would still have side borders and head-switching noise, and neither of those frame sixes can be used for DVD or BluRay).

The two AVI samples you posted are playing at a 5:4 image ratio because your FFv1 codec's DAR isn't being honored by most players. At 4:3 the image would appear as normally intended, not stretched. At 5:4 your image is actually "squished" horizontally. Objects are thinner than they should be. You can always tell VirtuaLdub or your media player to display at 4:3 if it doesn't do so automatically.

The attached m2v is your 15-11.avi 720x576 encoded with MPEG2 for DVD 4:3 DAR. No audio. Its physical frame ratio is 5:4. Its playback ratio (DAR) is 4:3.

spanak 04-17-2018 02:16 AM

sanlyn, FFV1 codec does store the aspect ratio and all the players that I use (vlc, mpv) do honor it. I have no problem with playback. Besides, I will NOT be using .avi containers, so saving the aspect ratio in the file is not an issue.

I would like to keep the video at 720x576, and not scale or crop it, as I am concerned with keeping quality as high as possible.

So, if I understand correctly, you are saying that the full frame size, be it 720x576 or 720x480, is supposed to be displayed at 4:3, not the actual video frame that is positioned inside, right?

Thinking (and calculating) some more, made me realize, that it doesn't matter in my case. Why is that? Well, because 720x576 has a ratio of 5:4, while the usable video is 700x560, but with the same aspect ratio (because I cropped 16px, instead of sanlyn's 14px). If they have the same proportions, adjusting them to 4:3, yields the same proportions again.

All this means that cropping proportionally save a lot of headache. :)

Thanks, guys, for pushing my thoughts into the right direction. Cheers!


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