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Sengstack
Known Participant
December 12, 2011
Question

720x480 -- SD and Widescreen not 4:3 or 16:9

  • December 12, 2011
  • 2 replies
  • 63385 views

Hi,

This is one of those topics that I've not seen answered definitively.

NTSC DV is 720x480 square pixels.

If it's NTSC SD, the final display is 640x480 -- a 4:3 "frame" or screen aspect ratio. The pixels are squished (taller than wide) such that the pixel aspect ratio is .9091. But if you multiply 720 by .9091 you get 654.552. What happens to those extra 14.552 pixels?

If it's NTSC widescreen, the final display is 853.333x480 -- a 16:9 "frame" or screen aspect ratio. In this case the pixels are stretched (wider than tall) such that the pixel aspect ratio is 1.2121. But if you multiply 720 by 1.2121 you get 872.712. What happens to those extra 19.37867 pixels?

Thanks to whomever clears this up or points me to a definitive answer.

Jeff S

      This topic has been closed for replies.

      2 replies

      Stan Jones
      Community Expert
      Stan JonesCommunity Expert
      Community Expert
      December 12, 2011

      I always found this chart interesting....

      http://lipas.uwasa.fi/~f76998/video/conversion/#conversion_table

      S
      SengstackAuthor
      Known Participant
      December 12, 2011

      The chart is interesting but I don't see how it addresses or, more directly, answers my questions.

      For example, the chart states that in DV (and D1), a sampling size of 720x480 has an actual picture size of 710.85x486. It doesn't explain why this is the case, but I'll accept it for discussion purposes.

      Multiply 710.85 times .9091 and you get 646.23, not 640. Why is the height 486? Finally, 646.23:486 is not a 4:3 frame aspect ratio.

      Same thinking holds true for widescreen: 710.85 times 1.2121 = 861.62, not 853.33. 861.62:486 is not a 16:9 frame aspect ratio.

      If 486 is the actual picture height, then 4:3 SD should be 648x486 and 16:9 widescreen should be 864x486.

      Jeff

      Stan Jones
      Community Expert
      Stan JonesCommunity Expert
      Community Expert
      December 12, 2011
      The chart is interesting but I don't see how it addresses or, more directly, answers my questions.

      Jim and Harm have answered your direct question. The chart (and all the narrative above and below) illustrates how complex the larger scenario is.

      I can do the simple calculations, but everytime I try to look at the bigger picture, I realize how complicated it is.

      This was one of my favorite articles, which includes discussion of production and clean apertures etc.

      http://provideocoalition.com/index.php/cmg_keyframes/story/par_for_the_course/

      Harm_Millaard
      Harm_Millaard
      Inspiring
      December 12, 2011

      Instead of 720, start with 704. The rest is to compensate for overscan.

        S
        SengstackAuthor
        Known Participant
        December 12, 2011

        But overscan applies to regions at the top, bottom and sides an NTSC screen. And it's a variable value depending on the TV set.

        Ann Bens
        Community Expert
        Ann BensCommunity Expert
        Community Expert
        December 12, 2011

        NTSC DV is 720x480 square pixels.

        No its not.

        The DV NTSC pixels have a pixel aspect ratio of 0.91 (nonsquare). DV pixels, which are always rectangular, are vertically oriented in systems producing NTSC video and horizontally oriented in systems producing PAL video.

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