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A book confused me

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Joined: 22 Sep 2006
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PostPosted: Sat 28 Oct 2006, 3:19    Post subject: A book confused me Reply with quote

Hi, I'm reading the book "A Practical Guide to Video and Audio Compression" by Cliff Wootton. In the book, there're such words:

"In the U.S. TV standard with 525 scanning lines, only 480 of them contain picture information. Each line has 640 samples. This is commonly referred to as NTSC, but that merely describes the encoding of the color information. European TV has 625 scanning lines but only 576 of these are active picture lines. While these will also be sampled 720 times for each line, that image must be stretched using interpolation to 768 × 579 so as to achieve square pixels. The 640 × 480 raster must be stretched to 720 × 480 to achieve the same effect. Square pixels are desirable because they make processing of any effects or overlays more straightforward."

What confused me is " as for NTSC, each line has 640 samples. " many other books tell me it should be 720 samples.

and "640 × 480 raster must be stretched to 720 × 480 to achieve the same effect (square pixels)." Let's do a simple computing. 640/480 = 1.333, that's 4:3 in square pixels! if 720 x 480 in square pixels, then 720/480 = 1.5 . That's not 4:3.

by the way, the 768 × 579 mentioned in the book is definitely not 4:3. It should be 768 x 576.

what's wrong here? is there any hidden information?
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PostPosted: Mon 30 Oct 2006, 3:08    Post subject: Reply with quote

Looks like the author is confused not just about the numbers, but also about some of the underlying concepts. In analog video there are no "pixels" as such, so the number of samples per line is a somewhat "flexible" concept. But, as far as a "standard" can be enforced, that standard is 720 samples per line.

Unlike the number of pixels per line, the number of lines for broadcast video is exactly defined: 525 (480 or 486 visible) lines for NTSC, 625 (576 visible) for PAL.

In the case of native digital formats (such as DV), both the number of lines and pixels per line is exactly defined (and there are no "non-visible lines", as such).

Video pixels are not square, they are rectangular. On a standard PAL TV, a resolution of 720x576 will result in an image with a 4:3 aspect ratio. On a 16:9 (widescrren) PAL TV, the same 720x576 pixels will result in a 16:9 image. What changes is the shape of the pixels (they become wider), not their number. Same for NTSC, but with a different shape.

See the second entry in the DVD-HQ FAQ for more information.

When displaying video on a PC screen, where the pixels are usually square, it's traditional to keep the vertical dimension (number of lines) and adjust the number of pixels per line so that the image retains the correct aspect ratio (which is not always exactly 4:3, but it is very close). This is because vertical resizing of interlaced images can be a bit tricky.

In other words, 720x480 (NTSC) is usually resized to 640x480, and 720x576 (PAL/SECAM) is resized to 768x576. This is assuming the video is 4:3. If the video is 16:9, it's resized to 853x480 or 1024x576, respectively.

But this should only be done for display. The actual file should always keep the "official" resolutions.

The text you quoted says "Each line has 640 samples [...] While these will also be sampled 720 times for each line". In other words, the 640 there could be just a typo (hence the "also"). But saying "European TV" when he means "PAL and SECAM" really doesn't help the author's credibility. Most of the world uses PAL or SECAM (Europe, Africa, nearly all of Asia, Australia, and most of South America).

Also, the 525 and 625 line refences have to do with broadcasting, not digital video capture or storage. So if he's mentioning them in the context of video capture, he probably missed the point.

I don't know that particular book or author but I have seen more than my share of apparently "authoritative" books where it's clear that the author has no idea what he's talking about. Don't assume something is correct just because it got published. And don't assume an author understands digital imaging (let alone video capture and compression) just because he has experience with analog video.

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