Originally Posted by veedon
Maybe somebody who has greater technical knowledge than I have can explain how a "4:3 SD" picture, which to me should always be 640x480 (in its natural or native resolution) can be turned into a 16:9 image (by a technique other than simply stretching), yet still have different pixel counts or resolutions that are nowhere near the 1920x1080 that I associate with "full HD". Even 720p HD is far more pixels (1280x720) than in the various implementations of "widescreen SD".
Uh, of course 1280x720 has more pixels than widescreen SD. 720p is HD. SD is SD.
Here's a partial explanation of the crazy world of digital SD video:
The reason digital SD is so confusing is that it was kludged together as a way to transfer existing analogue standards (e.g. NTSC and PAL) to digital format. In the NTSC world, the image has a fixed number of lines, ~480 of which are used for the actual image and can be mapped to a digital vertical resolution, but the horizontal resolution is not fixed. This is why a CRT monitor can display any number of resolutions and have them look decent, whereas an LCD monitor can only display material at its native resolution at optimal quality, since the number of pixels on an LCD is fixed.
In order to convert NTSC to digital, the horizontal information is sampled a certain number of times, which was chosen as 720 for several different formats; however, not all 720 pixels are used for image information: some of them are used to carry extra information or as overscan, i.e. a margin of error in case the scanning process doesn't keep the image centered the entire time. As a result, NTSC only actually uses 711 pixels, not 720 pixels, which is why if you look at a raw 720x480 image, there are often black borders on the left and right. Because the overscan isn't particularly important anymore, 720 pixels is often cropped to 704 pixels, because 704 is the nearest value to 720 that is still divisible by 16 (a nice number that makes MPEG compression work more effectively).
So, we have either 720x480 or 704x480, neither of which are 4:3, but that's OK, because digital SD always uses anamorphosis in order to correct the aspect ratio upon playback. Anamorphic widescreen isn't the only type of anamorphosis: 4:3 uses it, too. Regardless of whether your video is 4:3 or 16:9, it's always stored at 720x480, which means the image is distorted. (If it's 4:3, the image is wider, so people are shorter and fatter than normal. If it's 16:9, the image is narrower than it should be, so people are taller and skinnier than normal).
The aspect ratio of the storage medium is sometimes called the Frame Aspect Ratio
, or FAR. When you play an SD video, the TV corrects the aspect ratio at the same time it resizes the image to fit your display. If you have a 4:3 image, the 720x480 pixels are upscaled and resized to 960x720 for a 720p display or 1440x1080 for a 1080p display. If you have a 16:9 image, the 720x480 pixels are upscaled to 1280x720 for a 720p display or 1920x1080 for a 1080p display. If you weren't upscaling, the resolutions would be 640x480 for 4:3 and 853.333x480 for 16:9 (but of course there's no such thing as .333 of a pixel, so the horizontal resolution has to be rounded to the nearest whole, even pixel, i.e. 854). The aspect ratio of the final image is often called the Display Aspect Ratio
, or DAR. The ratio between the FAR and DAR is sometimes called the Sample Aspect Ratio
, or the SAR.
An anamorphic video is stored in the FAR (720x480) and flagged with a SAR. When you multiply the horizontal resolution of the FAR by the SAR, you get the final horizontal resolution used during playback, which when divided by the vertical resolution, gives you the DAR.
For example, if you have an anamorphic widescreen DVD and want to process it for your HTPC, the video is stored with a FAR of 720x480, but only 711 of those horizontal pixels are used. You can then crop it to 704x480, but the image is still "squished", so we can save the video with a SAR of 40:33. This simply means that you multiply the horizontal resolution (704) by 40 and then divide it by 33, which gives us a final resolution of 853.333 pixels. If we then divide 853.333 by 480, we get 0.777, which is 16:9. Aha! We have restored the correct DAR!
In HD formats, the FAR and the the DAR match, i.e. the image is stored in the same way it is displayed without being stretched or squished at all. This means that the SAR is 1, and the image has so-called "square pixels". An image with a SAR that is not 1 is said to have "non-square pixels", but it's important to recognise that there is no such thing as a "non-square pixel" when referring to modern LCDs; the term is more of an abstraction used to describe the fact that the shape of the video frame in storage does not match the shape of the video frame when it's being displayed.
Returning to SD video, the main aspect ratio problem occurs when you have 16:9 material presented as 4:3 ("letterboxed widescreen"), so let's consider how that would work.
If you have a 4:3 TV (a CRT, or "tube"), your screen matches the 4:3 image area, so it doesn't matter whether your video is 16:9 letterboxed or 16:9 anamorphic. If it's letterboxed, the 4:3 image just gets mapped to your screen with black bars on it, and you see 16:9 letterboxed. If it's anamorphic, the image wants to be 854x480, but an SD signal can't carry that many pixels, so instead, it gets downscaled to 720x360, the vertical resolution is effectively padded with black bars on the top and bottom to reach the target resolution of 720x480, and then the image is mapped to your screen. The result? You get 16:9 letterboxed, so there's no difference between a letterboxed and an anamorphic widescreen source.
If you have a 16:9 TV, though, then this doesn't work the same way. If you have an anamorphic 16:9 source, the 480 vertical pixels get mapped to either the 720 or 1080 vertical pixels on your TV, and then the horizontal 720 pixels get stretched to either 1280 or 1920 pixels, which fills the horizontal resolution and corrects the image's aspect ratio.
If your source is not anamorphic, though, then you have a problem. The image is 720x480 pixels with only 720x360 pixels containing actual image; 60 pixels on the top and 60 pixels on the bottom of the image are black. The problem is your TV doesn't know that those pixels are useless; it only knows that the image is 720x480 and should be displayed as 4:3. As such, the 480 vertical pixels get mapped to either the 720 or 1080 vertical pixels on your TV, and the horizontal 720 pixels get stretched to either 960 or 1440 pixels, which fills the horizontal resolution and corrects the image's aspect ratio. The result, however, is a "windowboxed" 16:9 image: the black bars on the left and the right of the image are because it's 4:3, so those portions of the screen aren't used at all. The black bars on the top and bottom of the image are not unused portions of your screen, though; they're part of the image. That part of the image just happens to be black and uninteresting to watch, because it's a 16:9 image stored inside a 4:3 frame for convenient viewing on a 4:3 TV.
The only way to correct this is to use the zoom function on your TV, which makes the compression artifacts and the low resolution even more apparent than normal, especially if you have a large TV or sit close to it.
This is why "anamorphic widescreen -- enhanced for 16:9 televisions" DVDs became the standard when widescreen TVs came out, and people would always complain when a DVD was released as letterboxed 16:9, because not only would such a DVD require extra time spent on manual zooming, but that zooming would make image defects even more noticeable.
Unfortunately, many DTV stations show their programs this way, because this may very well be the way the content was originally produced. If they took a 720x480 4:3 image containing 16:9 material, cropped the borders to make it 720x360, resized it to 720x480, and set it as anamorphic widescreen, the image would not look any better (and it might even look worse, since it underwent an extra resizing step and got recompressed); you just wouldn't have to spend extra time using the zoom button on your remote control anymore.
Are you still reading?
Isn't digital video fun? If you really want the nitty-gritty on digital video, try reading this article
and see if your head explodes.