How long is vertical blanking time?

Discussion in 'General TV Discussions Forum' started by Suso, Apr 22, 2007.

  1. Suso

    Suso
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    Please ignore this posting, and see my next posting. I messed up my numbers in the original posting; the valid numbers are in my new posting. DUH!!
     
  2. Suso

    Suso
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    I'm sorry! I screwed up completely the numbers on that previous posting. Here is what I meant to say:

    In some video cameras the image gets recorded progressively from top to bottom. Therefore, what you see in the upper part of the image happened earlier than what you see in the lower part of the image. This produces an artifact in which fast moving objects seem to be tilting backward. For instance, if you pan toward the right while shooting with one of these cameras, all the buldings in the background will show up tilting toward the right; vice versa, if you pan toward the left, all the buildings will show up tilting toward the left. My Sony HVR-V1P, a wonderful camera in many other respects, shows this effect.

    And here I have a question for you ... As I said above, the top of the picture gets exposed first, and the bottom edge of the picture gets exposed last. Then there is a short period of time during which nothing gets exposed, the "blanking time". Then the whole process starts again with the top edge of the picture. 0.0200 seconds (in PAL; it would be 0.0167 seconds in NTSC) is the total amount of time from the start of exposure of a field until the start of exposure of the next field. What I would like to know is, what fraction of that time corresponds to the period between the start of exposure of the top edge of the image and the bottom edge of the image, and what fraction of that time is spent waiting without exposing anything before the start of the next field. Is it 0.0192 seconds exposing and 0.0008 seconds without exposing? Is it 0.0180 seconds exposing and 0.0020 seconds without exposing? What is the proportion? I am pretty sure that these proportions will be different in different cameras. I am also pretty sure that the "blanking time" can't be predicted from the number of "blanking lines" in each frame (lines that contain no image data), because my understanding is that in my Sony HVR-V1P the number of blanking lines is supposed to be 45 out of every 1125 lines, or 4 percent, but some measurements that I have taken in the images indicate that the blanking time is about 12 percent. Does anyone have any advice in regard to where I might find a precise value for the "blanking time"?
     

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