Can someone explain 240p and NTSC to me?

Discussion in 'Other Consoles & Oldies' started by Fishaman P, Nov 15, 2013.

  1. Fishaman P
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    Fishaman P Speedrunner

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    Not the literal terms; those are a piece of cake.

    What I mean is display and capture. NTSC is 525 (effective 480) lines vertically, and half of those lines, alternating, are displayed every 1/59.94 second. That's fine and dandy; I get deinterlacing for 480i just fine.

    Now, stop me if I'm wrong here.
    What's tripping me up is 240p. The TV is still updated at 59.94 fields per second, except the lines untouched by one pass are turned black. This creates scanlines. If you take the updated fields 59.94 times per second, that's the original image.

    However, when capturing videos of 240p material on a PC, you capture at 29.97 fields per second. I'm also told that you can deinterlace to get the lossless source. I simply don't see how that can be true. Aren't you getting half black during that time? How can you recover all the original data? Or is it just not lossless?
     
  2. Foxi4

    Foxi4 On the hunt...

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    I think you may be a bit confused here.

    NTSC video runs at 29.97 frames per second (originally 30, decreased after adding colour) and displays 59.94 fields per second. 240p means 240 lines progressive and 480i means 480 lines interlaced.

    Now, the way interlacing works is that it displays two frames at the same and interlaces them. As you said, the TV alternates when refreshes the lines - this way it effectively doubles the resolution without having to update a larger number of lines per cycle.

    The reason why you capture less fields per second in 480i is because the picture is composed of two frames displayed at the same time. In 240p, there is only one frame on-screen, in 480i, two frames are interlaced with each other. Upon de-interlacing the image, the number of captured frames will double and with it the fields per second rate.
     
  3. Fishaman P
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    Fishaman P Speedrunner

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    I think I get it, but I'm still confused on how capturing 29.97 frames per second for 240p can get the entirety of the original picture back. Wouldn't every second frame be dropped, since the last frame's info is overwritten with black?
     
  4. Foxi4

    Foxi4 On the hunt...

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    Frames aren't fields. :)

    In progressive mode, one frame consists of one field. In interlaced mode, one frame consists of two sub-fields, one is the updated one and one is the non-updated one.

    Field (Video)
    Frame (Video)

    Mind you, I'm no expert so perhaps someone with more experience will pop up to describe this in detail.
     
  5. Fishaman P
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    Fishaman P Speedrunner

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    Well, in 240p, it's essentially:
    [line 1/240]
    [black]
    [line 2/240]
    [black]

    And so on, right? So in 240p, each field is either black or the entire original frame?
     
  6. Foxi4

    Foxi4 On the hunt...

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    This is exactly the reason why you capture fields at half the rate - you don't want the intermediate states of "blackness" composed into frames, so you capture at half the rate.

    [line 1/240] <--- Captured!
    [No new, updated line...] <--- Ignored.
    [line 2/240] <--- Captured!
    [No new, updated line...] <--- Ignored.

    Simple. :) (at least that's how I understand it....)
     
  7. Fishaman P
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    Fishaman P Speedrunner

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    This sounds like the answer, but it's not making sense in my head.
    Here's what I'm thinking, for a 29.97 capture rate:

    Frame 1
    [line 1/240] <--- Captured!
    [black] <--- Ignored.
    [line 2/240] <--- Captured!
    [black] <--- Ignored.
    ...

    Frame 2
    [black] <--- Ignored.
    [line 1/240] <--- Ignored. (29.97)
    [black] <--- Ignored.
    [line 2/240] <--- Ignored. (29.97)
    ...

    Frame 3
    [line 1/240] <--- Captured!
    [black] <--- Ignored.
    [line 2/240] <--- Captured!
    [black] <--- Ignored.

    I think a corrected diagram would put this to rest.
     
  8. Foxi4

    Foxi4 On the hunt...

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    Think of it this way - the video is running at approx. 60 frames per second and you are capturing at the speed of approx. 30 fields per second, what are you capturing?

    30 frames of raw footage rather than 60 frames of raw footage in-between of black frames.

    A picture of a standard piece of video prior to dividing into fields:
    [​IMG]
     
  9. Fishaman P
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    Fishaman P Speedrunner

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    Actually, that picture is of 3:2 pulldown for TV shows. I'm interested in game consoles.

    So... wouldn't capturing 29.97 fields per second result in 29.97 frames per second?
     
  10. Foxi4

    Foxi4 On the hunt...

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    It is, but I'm trying to show you why you'd be getting the "original" image.

    I worded that in an unfortunate manner, what I meant was that the actual image is in one field, the other field is just black bars which is different than with interlaced video where you have two frames on-screen at the same time.

    NTSC television runs at 29.97 FPS, this is entirely normal. You have 29.97 frames, each consisting of two fields, one field being useless to you since it's either black (240p) or the frame you don't want (480i).
     
  11. Fishaman P
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    Fishaman P Speedrunner

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    I'm still getting nowhere with this. If you capture 29.97 frames per second, you'll get one half black and the other half is a full video frame. Capturing 29.97 fields per second just gets you the full video frames, but still only 29.97 per second, not 59.94.

    That above statement must be false, but I don't see how.
     
  12. Foxi4

    Foxi4 On the hunt...

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    Why would you want to capture blank fields?
     
  13. Fishaman P
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    Fishaman P Speedrunner

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    What? When did I mention that? When I said that capturing the full frame would get you half black?
    That's my point; that is why interlaced capture is best (?).
     
  14. marcus134

    marcus134 GBAtemp Advanced Fan

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    1. I'm not sure if it's well understood here but progressive doesn't mean 60 fps, it means that the entire screen is refreshed for each frames and can have whatever refresh rate people want them to have.

    2. deinterlacing of ntsc media can result in both 30 or 60 fps video depending on the type of deinterlacing method chosen.
    http://en.wikipedia.org/wiki/Deinterlacing
     
  15. Fishaman P
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    Fishaman P Speedrunner

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    1. Yeah, I get that. Here, I'm talking about retro consoles that output ~60FPS at 240p.

    2. I want to know how lossless deinterlacing for the above occurs. In other words, getting the original 60FPS after deinterlacing, without any interpolation or loss of detail.
    I've read this is possible (and done by, say, AmaRecTV), but I don't get how.
     
  16. Foxi4

    Foxi4 On the hunt...

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    I don't understand why you'd treat 60FPS video from a console as lossless material when the retro consoles in question produce 24-odd frames per second with the odd exception, like the Sega Saturn.
     
  17. Fishaman P
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    Fishaman P Speedrunner

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    Odd exception? The odd exception is the console that doesn't produce ~60FPS.
     
  18. Foxi4

    Foxi4 On the hunt...

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    Even some NES games produced 30FPS, Nintendo 64 games are anywhere between 15 and 25, the Gamecube is almost exclusively 30FPS with some exceptions, the PS2 is 30/60FPS.

    Only really old systems are strictly 50/60FPS because they try to match the TV's refresh rate, anything post that point simply doubles frames to keep up sync-wise.
     
  19. Fishaman P
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    Fishaman P Speedrunner

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    Well, the consoles are capable of 60FPS output.
    I know the games I'm trying to capture are 60FPS; e.g. Super Mario Bros., Sonic 3 & Knuckles, Zelda: Ocarina of Time (menus are 60FPS, game is 20), Smash Bros. 64 and Melee (latter is 480i, so that's different).

    Well anyway, the source is 240p @ ~59.94 frames per second, the capture is 480i @ 29.97 fields per second. How do you get the original data from that?
     
  20. Foxi4

    Foxi4 On the hunt...

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    That already is the original data straight from the TV.

    In 240p, you're running approx. 60 frames per second interlaced with black lines to make up for the resolution, in 480i, you run the video slower, at approx. 30FPS, but with two "original frames" on-screen at the same time, they're interlaced with each other.

    The amount of actual original frames is exactly the same - to get the 240p @ 60FPS video, you de-interlace the 480i video so that you isolate the two frames from one picture.