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Contrast Ratio can be deceiving

Discussion in 'Projectors, Screens & Video Processors' started by kballs, Dec 28, 2004.

  1. kballs

    kballs
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    I've made an intuitive realization that contrast ratio while a useful number to consider can be a deceiving measurement in any display device's specs. Here is why:

    I got a new Panasonic AE700 and it is rated at ~2000:1.

    In some high-dynamic scenes with dark darks and light highlights all in the same scene (Soul Calibre 2 is a good game for this), the contrast is excellent, the whites are very bright and the blacks are very dark looking.

    In other low-dynamic scenes that are darker, you really notice the gray "black" and a lot of non-black pixels fall into the gray (see the opening scenes in The Bourne Supremacy). You could possibly adjust for this, but then it might throw off the shadow detail in brighter or more dynamic scenes. I don't mind this too much in dark room (since it is much less pronounced) but with increasing ambient light the issue grows (as it would with any display technology other than electronic paper which gets worse in a darker room :)).

    On some direct-view CRTs I have seen a similar problem with blacks... they have a very dark black, but then the other dark shades fall into black as well (are darker than they should be so the actual black isn't much darker, so it might as well be gray like a LCD).

    I think the reason for this issue is that contrast measurement is a simple ratio that assumes that the light output will be linear, when in most cases you lose the linearity near the bright and dark ends. It is kindof like rating speakers by frequency response: 20hz-20khz! :rolleyes: It's more useful to look at the frequency response graph to see if there are any nasty dips or spikes in there.

    I would much rather see a light output graph than a simple contrast ratio number when comparing display products.
     
  2. hornydragon

    hornydragon
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    ambient light is critica land most contrast measurements (panny plasma especailly) are done in near total dark rooms
     
  3. Gary Lightfoot

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    Most contrast ratio figures are misleading anyway - a projector like the AE700 can achieve the 2000:1 figure, but the colours will be way out - this is an uncalibrated figure (hardly any quote calibrated figures). The lamps tend to be red deficient, so the image will look blue/green. To make the colours more natural looking, the green and blue contrast settings are reduced (and the image brightness with it) till they match the red. This will greatly reduce the contrast figure. That's why you often see filters and ISF calibration mentioned. Adding a filter to opticaly correct the lamps colour imbalance can allow a greater contrast ratio at the expense of some overall brightness.

    As Hornydragon says, to achieve this figure,you will need a fully light controlled room, as the blackest you will achieve on a screen is only as black as the room and it's lighting will allow. Reducing the measured black a small amount amount can make a large difference in the contrast ratio, so even if a pj is capable of a good CR figure, any light in the room at all will greatly reduce this. This is especialy so with ANSI contrast where white and black is displayed in a chequerboard pattern. Having light coloured walls means that the white will reflect of the walls and any other light coloured surfaces and reduce the black level, that's why dark room colours are prefered.

    Doing a basic white and black level calibration is essential if you want to be seeing all the detail your pj can display in the light and dark areas of the image. If this isn't done, you will get large areas of black where some grey shadow detail should be, or bright blown out areas of sky where cloud detail will be for instance. Use Avia or Video Essentials to set this correctly.

    HTH

    Gary.
     
  4. PJTX100

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    Hi Gary, sounds like good advice, would the THX calibration thingy on some DVD's suffice?...PJ

    PS thanks for the help you've (unknowingly) given me so far on things like making ceiling brackets etc!
     
  5. Gary Lightfoot

    Gary Lightfoot
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    The THX thingy is OK, but tends to only work well for the disk it's on, and can't really be used as a blanket calibration for all disks. It should get you into the ballpark so to speak though and is better than nothing. Idealy you should use Digital Video Essentials or Avia. I use Avia but it's designed for NTSC and not PAL, so on a normal DVD player you'll need both unless youi can set the NTSC to 0ire (it may be referred to as something else within the players menu if it's there). DVE should be fine though.

    Glad to have been of some help with the pj bracket. :) A little DIY can save a lot of money sometimes, and projects like that add some extra satisfcation to the hobby.

    Gary.
     
  6. cyberheater

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    I think one of the problems is gamma related.

    I noticed that my CRT TV definitetly has a different gamma profile then my projector. I can get quite convincing blacks but the stuff that are near blacks should be a bit brighter.

    I'm still tweaking and have found some different gamma ramps in the service menu of my PJ that are yielding good results.

    It's a pity you can't get the machine to do a self ramping profile. Surely it wouldn't be that hard. Get a light sensor on the machine. Start firing out different colours and the machine could self calibrate for it's environment. I should patent that idea :)
     
  7. Gary Lightfoot

    Gary Lightfoot
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    Good point. Incorrect gamma can have a similar effect to a raised brightness level and make the image appear washed out.

    Marantz has a self calibrating pj so I think thaye've beaten you to the patent. :)

    To properly calibrate a display, you need a colorimiter or similar device that can see colour in the same way as the human eye. Using a light meter with RGB gels will balance the colours correctly, but they won't necessarily be to D65. That's why you can pay a fair bit for proper meters and calibrating software like Colorfacts.

    Gary.
     

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