Do computers correctly display DV and DVD video?

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    • #39438
      Avatarnate3po
      Participant

      DVD and and DV NTSC video is set to a resolution of 720×480 (with lower resolutions possible for DVD that we can ignore for the sake of simplicity). Of course these are not square pixels and if you display 720×480 square pixels on the computer (from a DVD or DV source) we see a physical image that is too wide if we’re displaying a 4:3 video or too narrow if displaying a 16:9 video. My editing program has an option for the preivew image where I can "Display Square Pixels." One assumes that this selecting this option allows me to properly view a video with non-square pixels on a square pixel display (namely the computer monitor). But the resulting image is 655X480 for "4:3" and 873×480 for "16:9". Now obviously the physical aspect ratios of the display are not actually 4:3 and 16:9 respectively.
      I found a website that stated that these so called 4:3 videos are actually slightly wider and that a display is suppose to crop 8 pixels from each side in order to display 704×480 rectangular pixels that will give us a physical aspect ratio 0f 4:3 for the video frame.
      But do computers really do this. I tried recordind 4:3 DV video and viewing it in my computer. Windows Media Player will display my video as if its 4:3. When played at full screen fills the entire 4:3 monitor’s display. But I compare the WMP display with the display in my editing program. It appears that WMP displays every pixel. So does this mean that Windows media player is displaying video that is slightly compressed horizontally when it comes to 4:3 DV and DVD videos?

    • #170441
      AvatarAnonymous
      Inactive

      DV and DVD video, for all practical purposes, are 720X480. The pixel aspect ratio determines if its 4X3, 16X9, or something else. DV video uses a 0.9 pixel aspect ratio so the image is a bit narrower than the square 1.0 pixel. Letterbox is 1.3 (I think).

      Anyways, a television does crop off a small portion of the video edges. This is called "Overscanning". The reason is due to older analog video, especially with VHS tapes. The video sync signal tells when to start the picture scan and when to jump to the next line. Analog signals fluctuate ever so slightly so in order for the next line to start at the correct time, a tiny portion of the line being drawn does NOT get completed. This typically happens on each line. The result is a black "noisy" stripe along the right edge of the video picture. Overscanning hid this. Digital video corrected the problem rather than hide it.

      When you play your DV video, its displaying every pixel. No cropping. If your video was captured from an analog source, you’ll see the black stripe. Most software media players read the pixel aspect ratio code in the file and displays it accordingly. From time to time you may encounter a video file that the ratio is all out of whack. Its not a display problem, its the pixel ratio code embedded in the file. Who ever made the file your viewing set the pixel aspect ratio incorrectly. (Unless the file is partly corrupt and the player couldn’t read the pixel aspect ratio information).

    • #170442
      Avatarnate3po
      Participant

      Actually widescreen as a pixel aspect ratio (par) or 1.2 (close enough to what you guessed, though). But if you calculate the display aspect ratio (dar) using with the 720×480 resolution and .9 par the resulting dar appears to be to be 1.36:1 instead of 1.33:1(aka 4:3). And calculating the widescreen dar with the 1.2 par the resulting dar appears to be 1.81:1 instead of 1.78:1(aka 16:9). So, when we view DV and DVD video on the computer – being able to see the entire frame without the overscan we get on TV – it should appear slightly wider. But when I play DV footage in WMP and play it full screen it will actually fill the entire 4:3 monitor. So my question is am I viewing video whose frame is slight compressed horizontally. In other words, is it true that the DAR of DV and DVD video actually 1.36:1 and 1.81:1 instead of 1.33:1 and 1.78:1 respectively, and if so does this mean that media players are actually displaying these videos incorrectly (technically speaking)?
      BTW, thanks for explaing how the line does get complete on analog video. I had noticed black edges before but never understood it.

    • #170443
      AvatarAnonymous
      Inactive

      If your viewing 4X3 video on your computer monitor "Fullscreen", and the par is 0.9, the physical "squareness" of the monitor will distort the image horizontally. Monitors don’t use the overscanning feature, (but some graphic cards will allow it). Therefore, it will "squish" the picture horizontally. Some software players may even compensate for it by letterboxing the 4×3 picture. The black bars would be very narrow and barely noticeable, thus preserving the true aspect ratio of the picture. Check your playback softwares setup menu and see if you can set it up that way. I don’t think Windows media player has this feature but I may be wrong. Also check your graphic cards set up. My ATI Radeon X1300 Pro allows for overscanning, but its currently off.

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