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Similar to this post, I've sent some video tapes off to be digitized. In this case, they are mini-DV tapes which should be 720x480 resolution (more or less). The resulting MP4 files do claim to be 720x480 but the images don't look right. I uploaded one such movie here, it's a little hard to tell but even for dancers they aren't shaped quite like that. The video makes them look more like elves than humans, which is probably better than looking like hobbits, but still not right. If I play this MP4 using the standard Windows player, pause it, and grab a screenshot, the resulting image is 1118x832 which is an aspect ratio of 1.34 which explains the elvishness. But why?

I was not familiar with the concept of pixel aspect ratio until I started reading up on DV format but I'm guessing that's what's happening here. The Wikipedia page on DV says there are "different pixel aspect ratios for fullscreen and widescreen video" but doesn't say what those values are.

Since Windows is stretching this video when it plays it, it makes me think that there is a pixel aspect ratio embedded in the MP4 file, but the standard Windows "Show Properties" doesn't display it. Or maybe Windows always plays videos using a 1.33 pixel aspect ratio or something?

What I'm thinking, is that since DV is recorded with 480 scan lines, and the resulting image is 480 pixels high, they don't need to re-digitize the video. I could drop the MP4 into my video editing software (Sony's Vegas Movie Studio) and hit it with a "compress vertically" deform, but it seems like that's going to end up blurring the video. I have all the right pixels, it's just that my pixels aren't the right shape (right?) Is the pixel aspect ratio stored in this MP4 file in a place where I could adjust it and get my video to look right?

EDIT

Initially, ffmpeg reports this for my video :

Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 720x480 [SAR 10:11 DAR 15:11], 2886 kb/s, SAR 109:120 DAR 109:80, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)

A DAR (display aspect ratio) of 109:80 is 1.36 which is basically correct for full screen DV, so I'm guessing my video is actually wide screen (16:9 = 1.77777). If I run ffmpeg using this command:

ffmpeg -i input.mp4 -aspect 1.7777777 -c copy output.mp4

I get a video which ffmpeg describes like this:

Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 720x480 [SAR 10:11 DAR 15:11], 2886 kb/s, SAR 32:27 DAR 16:9, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)

which looks almost OK - it says DAR 16:9 but then also says DAR 15:11 which is 1.36 and not what I want. And sure enough, if I play this video using Windows 10 "Movies & TV", the screenshot shows that the video is still at the 1.36 DAR, not the desired 1.77.

Is there an ffmpeg command that will produce what I need? Or is the Windows 10 Movie Player unable to actually handle the DAR setting?

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You are correct. The pixel aspect ratio is wrong. DV has a fixed horizontal resolution, but can display 16:9 frames by stretching out the pixels. The display resolution is set in metadata, so it is possible to make the file play back properly without re-encoding by changing the metadata.

The free, open-source tool ffmpeg can do it. This would be the command you would use:

ffmpeg -i infile.mp4 -c:copy -aspect 16/9 outfile.mp4
  • Awesome! Before I make any changes, ffmpeg reports "Stream #0:0(und): Video: h264 (Main) (avc1 / 0x31637661), yuv420p, 720x480 [SAR 10:11 DAR 15:11], 2995 kb/s, SAR 109:120 DAR 109:80, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)" – Betty Crokker Aug 7 '20 at 16:15
  • Why are there two values reported for both Display Aspect Ratio and Sample Aspect Ratio? – Betty Crokker Aug 7 '20 at 16:17
  • Sample aspect ratio is the shape of the pixels. Display aspect ratio is the shape of the frame. The two are related, but not the same: DAR = SAR * X / Y where X and Y are the number of pixels across and down. – stib Aug 8 '20 at 2:21

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