I have an ATI 600 and Iím going to be capturing old home video VHS tapes (using VirtualDub with Lagarith). My desired end product are mkv files (to be placed on computers, streamed to HDTV, etc.). Iím currently trying to decide between capturing at 352x480 or 720x480 (thank goodness my ATI 600 doesnít support 704x480 so I have one less option to worry about .
My current tale of woe started when when I compared a screen shot of my 352x480 capture with my 720x480 capture. Iím comparing the videos after using avisynth to deinterlace them and ffmpeg to convert them to mkvs and set the AR to 4:3 (but I went back and checked the original capture AVIs and saw the same thing there as well when I force VLC to a 4:3 AR). I noticed that my 720x480 capture was just a tad skinnier than my 352x480 capture. To my untrained eye it looks like the 352x480 capture is automagically cropping the 8 pixels of padding on each side of the frame.
http://www.digitalfaq.com/guides/video/capture-understand-sources.htm looks like it was written by someone knowledgeable in the dark arts of capturing analog video with digital equipment. So when he said ďTRY TO NEVER CROP VIDEO FILESĒ I was ready to follow that advice. This would mean my 352x480 captures have an incorrect AR and I should capture with 720x480. Hooray, I finally made a decision!
Alas my joy was short lived since I soon noticed jagabo state
To further add to my analysis paralysis I then did the math and determined that if you have a 704x480 video with a 4:3 AR and add 8 pixels of padding to both sides (giving you a resolution of 720x480) you must use a AR of 15:11 on the padded 720x480 video to cause the inner 704x480 portion of the 720x480 video to have an AR of 4:3. When I compare my 720x480 capture with a 15:11 AR to my 352x480 capture with a 4:3 AR they do seem to have the same width.
Finally getting to my question, can anyone confirm that with my ATI 600 capture card if I want the end result to have the correct AR I have two main options:
- Capture at 352x480 (and have the 8 pixels of padding cropped or never captured)
- Capture at 720x480 and use an AR of 15:11 with my final videos
Iíve attached three files:
720x480.3s.15-11AR.png //my 720x480 capture with a 15:11 AR
720x480.3s.png //my 720x480 capture with a 4:3 AR
352x480.3s.png //my 352x480 capture with a 4:3 AR
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It looks to me like the device is using standard ITU aspect ratios. Crop to 704x480 (by integer multiples of 2, eg 6+10 or 8+8) and encode 4:3 DAR or 10:11 PAR. There's nothing wrong with cropping as long as you do it correctly.
352 carries the same area that 704 covers (note that 352 is exactly half of 704) but at half the resolution (obviously).
Your capture card is doing the right thing.
For example, resizing your 720x480 capture to 352x480 would be wrong Ė it would first have to be cropped to 704 and then resized to 352. It is then exactly 4:3, just like 704x480.
Thanks for the replies, I think I may actually be starting to understand this now. My card is correctly capturing at both 352x480 and 720x480. Therefore I simply need to decide if I think thereís enough (any) improved quality at the 720x480 resolution to justify the increased file sizes.
While using the 720x480 video with a 15:11 DAR (if I have my aspect ratio acronyms correct) works, cropping it to 704 is probably the more common way of dealing with this.
I assume a viable third option would be to use avisynth to convert the video to 640x480 (after cropping). After a quick search in the conversion forum based on this post https://forum.videohelp.com/threads/375770-Ripping-Encoding-old-4-3-TV-shows-%28Aspect-...=1#post2423048 I assume something like this would work:
crop(8, 0, -8, 0) Spline64Resize(640,480) # Spline64 (Sharp)
Also be aware that some playback devices only support square pixels. They will ignore the aspect ratio flags and display as square pixels. The media players built into many TVs are notorious for this.