Scanning Lines & Resolution

Discussion in 'Archived Threads 2001-2004' started by SeanA, Mar 3, 2003.

  1. SeanA

    SeanA Second Unit

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    O.K.... I am a newbie here, having just purchased my first widescreen HDTV (Sony KV-34XBR800). I am still frustrated in trying to understand some of the real basics of HDTV. I have read numerous topics now about scanning lines and horizontal vs. vertical resolution, but I still don't understand what the relation is between scan lines and lines of resolution (if any). For instance, the specs for a particular 30" widescreen Samsung HDTV state that it has 1080i vertical scanning lines (native mode) but only 800 lines of horizontal resolution. I don't understand, even after reading the FAQ's on similar topics. Can someone help me through this and also answer the following:

    1) Are "scan lines" in the vertical or horizontal orientation, or no orientation at all ? (When I think of scanning, I imagine lines being "refreshed" over a certain increment of time and in a wave either up/down or across the screen horizontally).

    2) Is "vertical" resolution the # of vertical or the # of horizontal lines, and vice-versa ?

    3) Will the number of horizontal resolution lines and the # of vertical resolution lines be related in terms of the screen format ? For instance, a 4:3 TV with 1080 vertical lines should have 810 horizontal lines [ 1080 x (3/4) ] ?

    4) Must an HDTV have a minimum of 1080 lines of either vertical or horizontal resolution to truly be HDTV ?

    I think you can see how confused I am ?!?! [​IMG]
     
  2. Allan Jayne

    Allan Jayne Cinematographer

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    1. Scan lines go in the horizontal direction.

    2. Vertical resolution is the number of dots or stacked parallel line segments in a column the system can resolve, horizontal resolution is the number of dots or upright parallel line segments in a row the system can resolve.

    3. The number of lines of vertical resolution is determined by standards, for example 480 for NTSC. The number of lines of horizontal resolution depends also on standards but is much more affected by the quality of equipment. For sources that were digital at any time, there are horizontal resolution numbers that go with vertical resolution numbers such as 1920x1080 and 720x480. If the TV cannot make a spot small enough (almost none can make it as small as 1/1920'th the screen width) the resolution is less. The 440 maximum pixels across for (analog) NTSC broadcast and 565 across for (analog) laserdisk are system limitations, not standards.

    4. The TV industry accepts that a TV is HDTV if it can put 1080 scan lines in 1080 unique positions on the screen and put a single spot in any one of 1000* possible positions across any one scan line. An above average consumer grade HDTV can make the spot about 1/800'th the screen height (about 1/1200'th the screen width) in size. It can do HDTV but it cannot resolve all of HDTV. Currently the industry also allows an HDTV to have a 4:3 screen where the 1080 scan lines are more or less fixed to be evenly spaced up and down the entire screen and where a 16:9 TV picture is altered to occupy the middle 810 scan lines in order to be in a 16:9 shape.

    One other note: The 480 lines of vertical resolution for 480 scan lines is true for computer generated video sources. For live sources and material transcribed off of films, the 480 scan lines may or may not reproduce closely spaced details 1/480'th the screen height depending on the exact position on the screen. (A slowly moving grid of stacked alternating dark and light lines each 1/480'th the screen height in thickness will go in and out of focus in a 480 scan line system, even progressive, while if the lines were each 1/336'th the screen height they will seem to most people to be in focus all the time.) The accepted vertical resolution of non-computer generated material for a given number of scan lines is about 70% of them, or 336 lines for 480 scan lines. The 70% factor (Kell factor) also applies in the horizontal direction to video that was at any time in digital form with fixed pixels.

    Video hints:
    http://members.aol.com/ajaynejr/video.htm

    * HDTV is roughly defined as having at least approximately one million pixel positions. 1080i, fully resolved at 1920x1080, is approx. two million pixels.
     
  3. SeanA

    SeanA Second Unit

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    Thanks Allan,

    I appreciate the time you took to explain. I think things are now starting to piece together for me. You have definitely cleared up most of my confusion. The odd thing with the terminology is that horizontal resolution does not refer to horizontal lines but vertical lines running horizontally (or across the width of the TV), and vice-versa.

    The only thing I am still struggling with is the relation of top to bottom (vertical) scan lines or resolution to side-to-side (horizontal) resolution. I read somewhere that HDTV's are suppose to produce "square" pixels where analog TV's have traditionally produced "rectangular" pixels. If this is true, than it seems that there would have to be a direct relationship between vertical and horizontal lines of resolution in an HDTV... based on the format (i.e. 16:9 or 4:3). I feel like I am being cheated if my 16:9 HDTV with 1080 scan lines does not in turn give me 1920 lines going across the width of the screen. [​IMG]
     
  4. Allan Jayne

    Allan Jayne Cinematographer

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    It so happens that the standard for 1080i and 720p HDTV calls for pixels that are square given a 16:9 shaped picture. If a show was 4:3, the same 1920 or 1280 pixels respectively across are there, squeezed closer togehter so they are no longer square. Except for the standards already chosen, there is no rule stating that pixels have to be square, and therefore no implied relationship between vertical and horizontal resolution that applies generally.

    DVD also uses the same 720 pixels (some disks use 704 pixels) across for both 4:3 and 16:9 pictures.
     
  5. SeanA

    SeanA Second Unit

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    Thanks again Allan,

    You are really helping me to understand all of this. I hope I am not wearing out my welcome, but I would just like to know how you interpret the specification data for the Samsung 30" widescreen HDTV I mentioned in my opening post ???

    The specification data, word for word, from the Best Buy website is as follows:

    Samsung TXM-3096WHF
    Vertical Scanning Lines (native mode): 1080i
    Horizontal Resolution: 800 lines

    If the horizontal resolution refers to lines from "top to bottom", than it falls short of 1080 and the specification of 1080i scanning lines would seem to be false. If it refers to lines "across" the screen, it is way short of what it should be... either 1920 (for 1080i) or 1280 (for 720p). This all just makes me think that some manufacturers are doing a little "smoke & mirrors" thing and not truly providing an HD TV even though it is advertised as such.
     
  6. Allan Jayne

    Allan Jayne Cinematographer

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    The correct way of measuring horizontal or vertical resolution requires specifying some distance, and the official standard for TV screens is the number of dots across the largest circle that fits in the screen. Unfortunately lots of manufacturers use the entire screen width for horizontal resolution and don't say so.

    If you are lucky, 800 lines for a 16:9 screen means 800 dots across the first 9/16 of the screen or about 1420 all the way across. (The smallest spot achievable is such that if light and dark spots alternate at a pitch of 1/1420'th the screen width they will be distinguishable.) Normally 1080i HDTV will never produce evenly spaced spots at a 1/1420'th pitch although some digital to analog methods may produce this. Picture details continue to be aligned according to a 1/1920'th pitch grid from left to right and a 1/1080'th pitch up or down. It is just that if consecutive side by side pixel positions at a 1/1920 pitch are light, dark, light they will be blurred together.

    If the 800 lines refers to the entire screen width, that would mean that the smallest spot achievable is about 1/800'th the screen width, but could still be aligned with any of the 1920 possible horizontal positions.
     

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