SVCD - correct capture process for 480x480?

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Hello and thanks for this list, very helpful.

I was wondering if anyone knew of the proper way to capture or process video to match the 480x480 SVCD standard. This is obviously not a 4:3 aspect, and I understand that I could capture at 640x480 and then crop to 480 on width (but why? 160 pixels lost?).

What could a 1:1 aspect ratio be good for in this business? Is the answer in scaling the width to 480 and then the tv/dvd/whatever stretches the pixels? I didn't find anything conclusive on vcdhelper.com or anywhere for that matter. All help is appreciated.

stein

-- Stein (sk8boar@altavista.com), January 15, 2001

Answers

The answer to [what I think is] your question has something to do with the way a television displays a picture (as opposed to a computer screen). A computer screen has the same number of pixels in one inch vertically as one inch horizontally. A television, however, does not. As a test, one could take an image with the exact ratio of a television screen (4:3), let's say 320x240, and "squish" it to 160x240 and also make another "squished" version at 320x120. Don't trim the image, but actually "squish" it. Now you have two images, one compressed horizontally, and the other compressed vertically. Output each of the images to your television (assuming you have TV output on your video card) and see which one looks better. The one that was compressed vertically (160x240) will look a lot better. This is because television has a higher vertical resolution than horizontal resolution. So don't think of the SVCD image (480x480) being "stretched" to fit the screen (640x480) but think of it being "squished" (with no loss of information) to fit the screen (480x360) when it is displayed by your player.

-- Khad Young (webmaster@aphextwin.org), January 15, 2001.

480x480 is the res for SVCD MPEG2, and 720x480 is for DVD. Do note that both play correctly on your TV, and both are not 4:3 (square pixel-wise). Your TV ultimately does the stretching/squishing to get 4:3. To capture for SVCD, go ahead and capture anyway 640x480, 704x480, 720x480, nx480 (or whatever res full-screen your vidcap card/app allows you to). The 480 (576 PAL) part here is important because it signifies you want to create a full 480-line interlaced video; this is one prime difference between conventional VCD and SVCD: VCD uses only one field (240 lines) and repeats that field on playback to make a frame, while SVCD uses both original fields on capture and playback (480 lines) to recreate the original frame. This results in VCD having a coarser look especially on low-angled lines that have a pronounced staircase effect; SVCD looks closer to the original video. When you do encode your material to MPEG-2 for SVCD (with a version of TMPGenc that supports it, for example) it is at THIS point the resolution is extrapolated to 480x480; when you play it back the TV stretches it properly to 4:3. You can say the TV stretched it back to 640x480, but since we do not talk about pixels where analogue TV is concerned this is a kvetchy point here. You DO still see 480x480, and if you stick to pixels, then to fill a 4:3 space then indeed the pixels are no longer square. The reverse is true when you play back a true-blue DVD and 720x480 is displayed pn your TV: 720 is squished to fit a space that should be occupied by 640 square pixels, so what you see are no longer square pixels also. Either way, do not let these pixel definitions confuse you and make you think some pixels are gained or lost when capturing/displaying. NO, you DON'T crop your 640x480 captured video to 480x480; the MPEG-2 SVCD encoder does that for you. What the encoder does is extrapolate the 640 to 480 (okay you indeed lose some resolution here), and in your player, 480x480 is encoded to NTSC and sent out to your TV where it's stretched yet again to fill your 4:3 screen. Note that TVs stretch images willy-nilly. For example, conventional 4:3 programs look fat and stretched (and therefore incorrect) on 16:9 widescreen TVs, and we can choose certain display modes on that TV that either crops the top and bottom, or put black bands on the left and right of our originally 4:3 image for a correct look.

-- Mehmet Tekdemir (turk690@yahoo.com), January 18, 2001.

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