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Projector Contrast Ratio and digital video dynamic range

Discussion in 'Projectors, Screens & Video Processors' started by mrwhippy, Jun 12, 2003.

  1. mrwhippy

    mrwhippy
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    I may be missing something here, but here goes anyway...

    24 bit colour digital video has 8 bits for each of the red, blue and green components of the signal. The dynamic range, or ratio between the quantisation noise floor and full scale is 2 to the power of 8 or 256. Therefore surely any projector with more than a 256:1 contrast ratio should be able to display 24 bit colour to its full depth? A projector with a much higher contrast ratio will be able to display a blacker image when all the red, green and blue 8 bit values are zero - but the moment any of these words is non zero you are back to the limited dynamic range of 24 bit colour, i.e. no improvement in grayscale reproduction?
    Basically my reason for asking this is that I am currently in a quandry about which PJ to go for - the Sony VPL-HS10 is my favourite at the moment, but it obviousely has only a 700:1 contrast ratio. The question is - given the 256:1 dynamic range of digital video (dvd and SKY etc), does this limitation (and it is the only limitation I am concerned about) really matter - apart from for the display of the black bars above and below a > 1.85:1 aspect ratio movie?

    Mark.
     
  2. calscot

    calscot
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    If you think about it properly You can reproduce 256 steps with a contrast ratio (cr) of 2:1 (or any ratio for that matter). Don't think the picture would be very good though.

    Say you have a projector which when "projecting" black produces 1 lumen of light. With a cr of 256:1, projecting white will produce 256 lumens - ie not very bright and more like grey. A projector with 768 lumens and a cr of 768:1 will produce a much brighter white (ie 3 times as bright) while producing the same black as the previous example. The result is a much better picture.

    Now most projectors have a similar output for white when used for a home cinema so the higher the contrast ratio means the blacker the blacks and the more easily we can resove the difference in all the shades. For example a 700 lumen projector with a 2100:1 cr will produce 700 lumens for white yet only 0.33 lumens for black.

    You will also have more degrees of difference between the shades which will allow you to see much more detail especially in the shadows.

    To see what your telly would be like with much less contrast, turn the brightness right up and the contrast right down.

    Cheers,
    Cal.
     
  3. mrwhippy

    mrwhippy
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    Thanks Cal,

    Nice explanation,

    Mark.
     
  4. Anders_UK

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    Very good explanation, although this is pure theory. Assuming a full-on and full-off measurements, what you really need is a contrast figure between black and white on the same picture. Usually done by the ANSI test pattern, this is a more accurate reflection on what the projector can do whilst in normal operation and most projectors regardless of full-on, full-off figures will only be around 100:1. Including image uniformity issues as well.

    Bear in mind though that this is just for the projector, then you have other factors to take into account like the FOV, resolution of the display, and the optics of the screen. Plus external issues such as ambient light levels, back scatter etc...
     
  5. Mr.D

    Mr.D
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    Ah its not a question of being able to resolve more detail its more that the greater difference in luminence level between intensity changes gives you an image that is percieved as being more pleasing : especially given video's fairly limited intensity range.
     

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