NTSC scanlines

Discussion in 'General Electronics Chat' started by vpoko, Apr 15, 2012.

  1. vpoko

    Thread Starter Member

    Jan 5, 2012
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    I'm a bit confused about NTSC video and hoping someone here might be able to explain this to me. I've always understood (and just verified) that NTSC has 525 scanlines. A book I recently read about the Atari 2600 ("Racing the Beam"), which doesn't have a frame buffer and hence forces programmers to use the horizontal and vertical blanking intervals for game logic, stated that NTSC had 192 scanlines. I don't doubt that the Atari's resolution would have been 192 lines instead of 525, but how does that work with old CRT TV's which have an electron gun making (I believe) 525 passes? I can understand that the gun could make several passes, drawing multiple, identical lines, but that would give the programmer additional horizontal blanking intervals, while with the 2600 the programmer can expect 192 horizontal blanking intervals per vertical blanking interval. Surely CRT's can't make dots of different sizes, can they?

    I realize this doesn't apply to LCD and plasma TV's, which have fixed pixels.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you provide a link to the actual statement that you citing?

    hgmjr
     
  3. vpoko

    Thread Starter Member

    Jan 5, 2012
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    I can't provide a link because it's in a printed book, but this is what it says word-for-word:

    "There are 192 visible scan lines per frame on an NTSC television." There's then a footnote to that sentence, which says, "NTSC is the television encoding system used in the United States, Canada, Japan, Mexico, and many other Central/South American and East Asian countries. The encoding standard used in most of Europe, much of Asia, Brazil, and about half of Africa is called PAL. A third major format, SECAM, is used in France, the other half of Africa, and the former Soviet Union. The development of PAL was necessary because North American NTSC television would not fit the 50 Hz frequency of European power grids. Because the Atari VCS does not automate its interface with the television, programmers would have to modify their programs to account for the 242 visible scan lines of a PAL television, compared with the 192 visible scan lines of an NTSC TV."
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    The key to NTSC is teh phrase "visible scan lines per frame".

    The 525 lines are actually divided into two frames. The two frames are interleaved. Each frame is equivalent to 262.5 lines. Due to vertical blanking approximately 70 lines in each frame do not get displayed.

    hgmjr
     
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  5. vpoko

    Thread Starter Member

    Jan 5, 2012
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    Thanks! I was thinking it might be related to interleaving but since the sentence referred to "192 visible scanlines per frame" instead of "per field", I figured that couldn't be. In retrospect, that was probably just sloppiness on the authors' part.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    To clarify, there are 262.5 scan lines per field (1/60 second), 192 of which are visible. It takes two interleaved fields to make an NTSC frame (1/30 second).
     
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