How does 480i get to be 240 TV lines?

Discussion in 'Amateur Video Production' started by N Hamilton, Jul 12, 2003.

  1. N Hamilton

    N Hamilton Guest

    I've always heard that NTSC signals have 720x486 (or 480 if it's DV) image
    sizes and that the image that shows up on consumer TVs has only about 240
    horizontal lines. Currently reading Poynton's Video book confirms this but
    doesn't mention what happens to the 486 lines that we started with.

    When the signal gets to the TV and the CRT gun "paints" the screen with
    lines does it just paint every other one of the 480 lines to get the
    approximately 240 lines? If not how is the 480 mapped to the 240?
     
    N Hamilton, Jul 12, 2003
    #1
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  2. N Hamilton

    RGBaker Guest

    No, Poynton says you have abut 240 lines of horizontal resolution ... which
    is measured by counting the number of vertical lines that can be resolved
    across a picture width equal to the picture height.

    The vertical resolution is fixed, as defined. The horizontal resolution is
    variable, and is measured.

    GB
     
    RGBaker, Jul 12, 2003
    #2
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  3. N Hamilton

    David McCall Guest

    Another candidate for an http://groups.google.com search.
    Lines of video and lines of resolution are not the same thing. The way to
    get to 240 lines of video would be to chuck half of your resolution. That
    is called deinterlacing. A lot of people do that to their video for use on a
    computer, and some do it to just get a more "film like" video look. The
    staccato look you get by chucking every other field looks more filmic to
    some.

    Approximately 240 lines of resolution is about what you can expect from
    an ordinary VHS tape, but it is a sort-of arbitrary measurement. It refers
    to the maximum number of lines you can clearly count when shooting an
    appropriate chart. It's the "clearly" part that is a bit arbitrary.

    At any rate, it has been covered from every angle, so do the google search
    if you want to learn about this in excruciating detail (from multiple opinions :).

    David
     
    David McCall, Jul 12, 2003
    #3
  4. N Hamilton

    N Hamilton Guest

    If I have a 480 line video and display it on NTSC, and can see only 240 or
    so lines on the TV monitor, which of the original 480 lines am I seeing?
     
    N Hamilton, Jul 13, 2003
    #4
  5. 480 and 240 are numbers referring to completely different things, so
    the question has no answer. TV lines and video resolution are
    completely different measurements.

    NTSC has a fixed vertical resolution, 525 total scan lines, of which
    approximately 486 are potentially visible. DV ignores six of those,
    but on most TVs you can never see all of the lines anyway, so that
    doesn't matter -- on a TV screen at least.

    Since vertical resolution is fixed, there is no point in specifying
    that when measuring camera, broadcast, display, tape, etc., sharpness.
    Instead, a measurement called "TV lines" is used. The lines are
    measured in a square placed on the full screen size. 240 TV lines
    means that you can clearly see 240 lines on the test pattern. The
    usual test pattern has a range of graded lines on it, and you simply
    look at it and see when they blur into indistinguishable grey areas.
    This measurement is somewhat subjective, but in a pre-digital
    instrument era it was the only way to judge which device was sharper.

    But the full picture isn't square, is it? The full frame size is
    larger than the test pattern. The digital frame size is fixed in DV
    (always 720x480); D2 tape format (not used too much now) allows
    704x480 or 752x480). A PC monitor shows 640x480 frame for the same
    image as either of the video standards, because PC pixels are square
    while video pixels are rectangular (but on an actual TV, the physical
    matrix is usually square, but most TVs cut off enough of the picture
    shown so it all works out right).

    Note that frame size and picture resolution are different things.
    If you have a low resolution camera, it still captures to a DV frame
    size of 720x480, even though the image sharpness is well below that.
    The TV line measurement, OTOH, is based on the visible output, and it
    can be far less than that of the frame. Capture VHS, for example
    (generally limited to TVL of about 240), and you aren't going to
    magically increase the TV line resolution to that of the frame size.
    But you'll still get 720x480 frames out of the capture.

    Since TVL measure a square pattern, and the full frame size is
    rectangular, you need to add about one third to it in order to get the
    approximate full frame resolution needed in order to capture the
    image. But this isn't exact either. 240 TVL is about 320 resolution,
    except that *lines* are composed of an alterating black/white pattern.
    You need two pixels (at mininum) in order to see a line. So a 640
    pixel display can show, at most, the same as 240 TVL. In a square
    pattern, full screen, you can see at most 240 distinct lines.

    Video has no limit to the number of lines possible to resolve,
    though the NTSC standard makes it hard to go past about 240 on
    broadcast. Cameras often boast TVL much higher than 240 -- real CCD
    resolution is often much higher than usable output.

    Remember, in all cases, TV lines *never* measures vertical
    resolution. If a given camera/recording medium only records
    non-interlaced video, you'll have only 240 pixels of significant image
    data, even though the TV output will of course still be 480i --
    because TV can't put out or use any other vertical signal.



    If you want some more confusing info about the different between
    frame size, TV lines, and various digital formats, check out this
    site: http://www.uwasa.fi/~f76998/video/conversion/
     
    Jeffery S. Jones, Jul 13, 2003
    #5
  6. N Hamilton

    FLY135 Guest

    That's because the statement...

    "the image that shows up on consumer TVs has only about 240 horizontal
    lines"

    has no relationship to the 486 scan lines. The 240 you refer to is image
    resolution as measured across a scan line. It is the "detail" that is in
    the scan line. Terms like "modulation transfer function" and "line pairs"
    are used in measuring horizontal resolution. It has absolutely nothing to
    do with the number of scan lines (i.e. 486).

    Although it focues on photography, this might give you an idea....

    http://www.normankoren.com/Tutorials/MTF.html

    Measuring video resolution is a completely different concept than computer
    and captured video resolution. But it is often confused. Basically the
    horizontal resolution specs for video equipment is good for comparing
    different types of equipment and many times is a theoretical optimum for the
    technology. The actual details of the resolution measurement is probably
    not meaningful to the average videographer.
     
    FLY135, Jul 13, 2003
    #6
  7. N Hamilton

    N Hamilton Guest

    So on my Sony TV, how many (roughly) horizontal lines can I see if I get
    real close to the screen?

    Seems like if the image resolution is 240 across a vertical scan line, and
    if the aspect ratio is 4:3 (ignoring square vs rect pixels).
    then the resolution in a vertical direction, (i.e. how many horizontal lines
    are visible) has to be roughly the same, around 200-300.
    So am I missing something?

    I realize there are other factors, perception, gaussian pixel intensities,
    etc. but I'm trying to understand where the original 480 lines
    end up on the TV. Is it 240 lines in one field, then 1/30 second later
    another 240 lines for a total of 480 but some of the lines are fading
    while others are being refreshed?? So if I get real close to the screen, I
    can see only 240 lines not 480?

    Each time I think I understand this, I gets more confused.
     
    N Hamilton, Jul 13, 2003
    #7
  8. N Hamilton

    RGBaker Guest

    Roughly 480
    The scan lines are HORIZONTAL scan lines, there is no 'vertical' scan line
    ...
    You are confused; the answers in this thread are correct, reread them.
    There are roughly 480 horizontal lines of information per frame, and a frame
    is displayed in two interlaced halfs per 30th of a second. The variable
    resolution is the number of unique pixels per horizontal line, which is
    measured as HORIZONTAL resolution by the number of VERTICAL LINES that can
    be resolved across a width of the tube equal to the height of the tube, i.e.
    3/4 of the total width of a normal TV. Unlike photographic resolution, the
    number of lines resolved is counted as both the line and the gap between the
    line, i.e. if you have 540 alternating black and white vertical lines in 3/4
    of the tube width (the absolute maximum possible using any of the digital
    standards) it is described as 540 lines, though in photographic terms it
    would be described as 270 lines. In practice a resolution chart is
    carefully designed and engineered as converging lines & the resolution is
    revealed as the point in which the converging lines merge & are no longer
    distinct. This is more difficult than it sounds, as the lines may continue
    to appear to be resolved as lines beyond the resolution maximum -- but if
    any of the lines have merged with adjacent lines than the maximum has been
    reached.

    The reported resolution is the horizontal resoluion, the number of vertical
    lines your set resolves across 3/4 of the picture width. The vertical
    resolution is never reported, tested or compared -- it is fixed and
    absolute; in a tube television the scanning gun paints the tube 480 (or so)
    times horizontally descending vertically until it reaches the bottom, jumps
    to the top and repeats. In practice most sets do this in an interlaced
    fashion but this is not to be confused with the reported resolution of the
    image.

    GB
     
    RGBaker, Jul 13, 2003
    #8
  9. N Hamilton

    FLY135 Guest

    I have no idea how many you will see. It's kind of hard to count and some
    of them will be off the top and the bottom of the screen (overscan).
    What you are missing is that the resolution you are refering to does not
    have anything to do with pixels. The horizontal video resolution isn't
    specified that way. Think of horizontal resolution like the bass and treble
    response of a stereo. It's analog in nature and varies with the frequency
    bandwidth of the circuitry and the shadow mask of the CRT. Resolution of
    computer outputs are measured in pixels (discrete elements). But the
    resolution of a video display (even your computer display) is not a discrete
    measurement. That's why the video resolution measurement of camcorders and
    TV's is more of relative importance, because the specific number doesn't
    really mean much. It's like a stereo that has 20Khz vs 25Khz frequency
    response. You know which is better but the specific number doesn't
    translate into anything all that specifically meaningful. For example a
    decent computer monitor should have a bandwidth of about or greater then
    200MHz.
    Yes, except replace 1/30 with 1/60.
    It's pretty simple. All NTSC TV displays (except HDTV modes and DVD
    progressive scan) have ~480 *scan lines* (much better terminology than
    vertical resolution). In fact it is erroneous to say a TV has 480 lines of
    vertical resolution. They are scanned out 240 lines per 1/60 second and
    each of the two fields is slightly offset in the vertical so that they fill
    in. The phophers in the crt are designed to persist enough to create the
    illusion of a solid 480 lines. However hard edges of dark and light will
    defeat the persistance enough to show flicker and belie the interlaced
    nature. This is rarly evident with natural scenes but can show up with
    computer graphics and text.
     
    FLY135, Jul 13, 2003
    #9
  10. N Hamilton

    N Hamilton Guest

    Thanks guys I finally get it now.

     
    N Hamilton, Jul 14, 2003
    #10
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