Contrast ratio question, please enlighten me on this subject

Discussion in 'LCD & LED LCD TVs Forum' started by Nielo TM, Aug 3, 2006.

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  1. Nielo TM

    Nielo TM
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    First of all, I like to thank for taking your time to read my post.

    I recently stumbled upon this subject, which left puzzled. I know that CRTs CR is lower then that of LCD and PDP because pure black isn't need to achieve high contrast. I also know that that the human eye can only obtain contrast ratio of 100:1 and has 1,000,000:1 dynamic range.

    I recent found out that higher contrast doesn't mean more colors and that colors are determined by the display. For example, a consumer LCD can display maximum colors of 8bit as where CRTs are capable of displaying unlimited colors.

    Any information that you provide is greatly appreciated.
     
  2. neilmcl

    neilmcl
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    What was wrong with your other recent contrast ratio thread?
     
  3. Nielo TM

    Nielo TM
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    They are flawed. I need to make sure my information is correct before advising others.

    Before I taught, I understood it but now I'm back to square one lol
     
  4. andrewfee

    andrewfee
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    Put simply, contrast ratio is the difference between the darkest and lightest area of the picture.

    There are two ways of measuring the contrast ratio of a display - on/off contrast, which is the one used most often, and ANSI contrast, which is the more accurate of the two.

    On/Off CR is basically measuring a full-black screen, and then a full-white screen. Some manufacturers may even be taking this one step further and measuring black at the lowest light output setting the display can do (e.g. backlight, brightness and contrast turned down to 0) and then white at the brightest it can go. (everything maxed out) So you might have, for example, 0.2cd/m2 for black and 500cd/m2 for white - this would produce a contrast ratio of 2500:1. (500 divided by 0.2) This is the reason Plasmas have ridiculously high contrast ratios, because they can be upwards of 1000cd/m2 max brightness before all the layers of glass/protective & anti-reflective coatings go on, but may only be about 300cd/m2 max by the time they get into your home. As you can probably guess, this isn't a good way of measuring contrast ratios, and gives you results that mean you can't compare between manufacturers. (or even products from the same manufacturer but different generations sometimes, as they might have changed how they measure things)

    Dynamic contrast ratios are meaningless, as they may help improve the perceived contrast ratio, but can't go beyond what the display is capable of. (and this usually results in an awful picture)

    ANSI contrast is the way things should be measured, as it takes what the screen is really capable of into account because it is made up of 8 black patches, and 8 white ones in a 4x4 checkerboard pattern. The readings of all 8 black patches are taken and averaged, as are the white ones, and then the contrast ratio is worked out from this. This takes variations across the screen into account (e.g. uuneven backlighting) and gives a much more accurate reading, as the settings have to be at a fixed level.(e.g. turning down the backlight makes both white and black darker and vice-versa)

    In this scenario you might have (for example) 500cd/m2 brightness and 0.5cd/m2 blacks, giving a 1000:1 contrast ratio, or 0.2cd/m2 blacks with a 200cd/m2 brightness (also 1000:1) rather than the inflated 2500:1 that the display will never be able to reach when actually watching something. This way of measuring is especially important with Plasmas (iirc) and CRTs, as their black and white levels fluctuate depending on the overall brightness of the scene. If you have something that's all dark onscreen on a CRT, then blacks will be very dark, but add a bright object to it, and everything onscreen gets lighter.

    This is all well and good, but there's something else you should take into account - ideally, the maximum brightness output of your display will be around 100-200cd/m2, depending on the ambient light in the room. E.g. in a completely dark room, you want around 100cd/m2, in a room with low lighting you might want 125cd/m2 (which I use in mine) or in a bright room you might want as much as 200cd/m2, but you should never really need any higher than that. This tends to drop the contrast ratio even further.

    One way many manufacturers increase their contrast ratio is by increasing the light output of the display without improving the black level. Say you have an LCD that is capable of 0.5cd/m2 blacks and 500cd/m2 whites, and another that is capable of 0.5cd/m2 blacks with 600cd/m2 whites. The first one will have a CR of 1000:1, and the second, 1200:1 - but when set to a "usable" brightness level of, say 150cd/m2, you might find that they produce exactly the same contrast ratio. Sharp are great, because every LCD they've put out for a number of years has had a 450cd/m2 maximum brightness output, yet they have increased the contrast ratio from 800:1 to 1200:1, which should mean that they have only improved the black levels. (or that they have changed their method of measuring it, but Sharp seem pretty good in this regard)

    It's true that contrast ratio has nothing to do with the amount of detail in the picture, or the number of colours it produces - it's only a measure of the difference between black and white on the set. You could have a TV that turns everything below 10% grey solid black with a 10,000:1 CR, and another that can show the difference between black, 1% grey and 2% grey with only a 1000:1 CR. (this is another reason to avoid dynamic contrast systems, as they typically throw away shadow and highlight detail to "improve" things)

    It's my understanding that LCDs are indeed "only" 8-bit devices and that anything higher will only affect the image processing inside, with the ouput then downsampled to 8-bit to be displayed on the screen. This means that RGB values are limited to 256 steps. This is why, on most LCDs, you want to leave the brightness, contrast, and colour controls at their "default" values. Typically this would be 100 contrast, 50 brightness, 50 colour. If you have an 8-bit display, changing any of these is likely to cause banding in the image - lowering contrast tells the screen to make the bright pixels darker (so 255 might become 245, for example) which would then mean than there can only be 245 steps in a gradation, rather than 255, so the range is being compressed and is likely to merge some values together, which means banding. The same applies to adjusting brightness & colour. This is why it is important to have an LCD with a "proper" backlight control, allowing you to set it low enough (or even adjust it at all) without having to resort to lowering contrast to make the screen dimmer. This is why I prefer Sony LCDs, as the last two BRAVIAs (V series) have allowed the backlight to drop to around 70cd/m2 at its lowest setting with power saving enabled, which is lower than anyone should ever need.

    Higher bit processing (I believe Sony LCDs have 14-bit colour processing) means that you should be able to adjust things more without causing banding, but in the end it is still downsampled to 8-bit for the display. You have to remember though, that with digital sources, they're are encoded in 8-bit RGB anyway - in theory, anything higher should only makes a difference if you start adjusting things. (unless they're upsampling, but I won't go into that)



    With regards to the human eye, the dynamic range is the range of absolute brightness and darkness where you can still see subtle differences in brightness. It's really just like the on-off contrast ratio measurement - it basically means that in a very dark room, once our eyes have fully adjusted, we can see very, very subtle differences at very low light levels, and the same for bright things. Our eye can only see around 100:1 contrast though, but it's constantly adjusting, which is why the dynamic range is so huge. Think of it this way - if you're in a dark room, and walk out into a bright one, things seem very, very bright - almost completely white. This is because your eye's 100:1 was adjusted for a dark room. Once it adjusts to the bright room, going back into a dark one seems pitch black until you give it a few minutes to adjust again.

    This is a crude attempt at explaining it:

    Imagine that this bar shows off the entire 1,000,000:1 dynamic range the human eye can see - from the very darkest, to the very brightest.

    [​IMG]

    We can only see a contrast ratio of 100:1 though, which means we can only see a "slice" of this dynamic range. Lets say that this red box is the contrast ratio we can see when we're in a fairly dark room:
    [​IMG]
    Anything below the light-level our eye had adjusted to is pitch-black, and anything higher is a blinding white.

    Then lets say this is it on a bright day outside:
    [​IMG]

    We can see detail on brighter objects, but anything below a certain brightness is black, and there can still be objects that are blindingly bright. (looking at the sun, for example)

    Our eye is constantly adjusting to the range of light depending on what we're looking at though, so it's never fixed at any one range, but takes longer to adjust to bigger changes. (which is why walking out of a dark room to a bright one takes a few seconds for your eyes to adjust) Of course, unlike the pictures above, we basically see a small greyscale in that 100:1 gap made from the range it's currently adjusted to, it's not a sudden transition to full black and full white as shown.


    This is why it's not a good idea to be watching a bright TV in a dark room, as you could be watching a dark scene in a movie and due to the low light, your eye has adjusted to it, and is making out all the subtle details, but if it then cuts to a bright scene, or something bright flashes, your eye moves that 100:1 CR up the scale to adjust for it, and then back down when it gets darker again. This means that it can be adjusting an awful lot, causing eye-strain. With a light on in the room and/or a properly adjusted TV, the range is much lower, and far less straining on the eye, as it isn't having to make such big changes. (e.g. going from an image that may only be 10cd/m2 to 500cd/m2 rather than say 50cd/m2 from the room light to 200cd/m2 from the TV)




    Long story short, just pick a TV that looks good to you, and don't worry about the technical side so much. Technically a plasma should have a higher contrast ratio than an LCD, but from my experience, this is only when comparing in a completely dark room, where you can see the black level improvement. With the 9th Gen. Panasonic I had, as soon as you put a light on, the image started losing a lot of contrast, with blacks turning grey pretty quickly. (even just a 9 watt light on made things worse than current LCDs) In the same situation, the Sony BRAVIA KDL-32V2000 was totally black in anything other than a completely dark room, but was also able to be set bright enough to be seen perfectly in a very bright room on a Summer day. (under which conditions the Plasma was virtually unwatchable)

    If you're asking about this to help decide on a new television, I'd probably go with one of the BRAVIAs, although the new Sharps look like they could be quite good on paper. (I haven't used one of the new Sharps personally - although last year's GD7 looked good on paper but turned out to be pretty awful - then again last year's BRAVIA wasn't very good either, and the new ones are spectacular)
     
  5. LV426

    LV426
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    Now that is the best bit of advice I've seen for a long time.................:thumbsup:
     
  6. andyb_68

    andyb_68
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    This thread is excellent, now I know, I dont know enough to make a technical decision and will indeed follow your advise about [buy what you like and sod the technical mumbo, jumbo]

    Very informative, thank you

    :clap: :clap: :clap: :clap:
     
  7. Downinja

    Downinja
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    Great post andrew, very informative.
     
  8. dacco

    dacco
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    An excellent post until you spoilt it by offering your own personal point of view. Where did you get the bulk of the post, minus your “cut a long story short bit”?
     
  9. Nielo TM

    Nielo TM
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    Wow, thx andrewfee for that very useful information cos that's exactly what I was looking for. I greatly appreciate that you took your time to explain it, especially when its 5 in the morning.
     
  10. Nielo TM

    Nielo TM
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    What’s you problem? He is right about the Panny and the BRAVIA. Nemours users reported that the net Panasonic has floating blacks and unnatural image due to dynamic contrast and image enhancing algorithms.


    I've seen the BRAVIA in action and it’s stunning. The new 40inch "S series’" CR is very close that of 50" Pioneer PDP. The new BRAVIAs also use S-PVA LCD panel and CCFL backlighting (V series only), which in my option and other as the best for viewing compressed videos. The S-IPS used by LG.Philips is poor as it has twinkling effect, low contrast, poor viewing angles, strong afterglow etc... However, high quality S-IPS has outstanding color and viewing angles but that’s not the case here.

    Sharp on the other hand uses ASV. Unfortunately information on that technology is limited.
     
  11. andrewfee

    andrewfee
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    I couldn't put it down to any one source - I've been wanting to get a new television for over a year now, and have been doing a lot of research into the different technologies, how things work etc, as I've always had an interest in knowing the technical side of anything I buy.

    This is why I suggest that you should really just buy what you think looks good, as any of the current displays from the big manufacturers should make most people happy, and from what I've seen so far, the BRAVIAs are probably the best LCDs out there right now, as they have a wide range of picture controls and a great panel.

    Maybe I shouldn't have made such a sweeping statement, but because I've been focusing on the technical side of things so much, I'm stuck using a 17" Sony LCD (bought as a temporary solution until I could find something bigger) rather than enjoying a great, but not quite perfect, picture on a bigger and better screen.

    I can only comment on what I've used, but what I said is true about the BRAIVA - here's a photo I took of it with the TV on at my normal viewing settings: http://sr-388.net/images/BRAVIA/black.jpg

    (This was actually over-exposed and is brighter than the room actually was at the time, yet it still looks totally black.)

    The picture quality was great too: http://sr-388.net/images/BRAVIA/freeview/CIMG1990.jpg - this was Freeview, but I wasn't completely happy with the VGA input and it had a minor colour reproduction issue that put me off it. (it was perfect 99% of the time)

    The last TV I had was a month or so back when I bought a Panasonic TH37PD60, as I figured I should at least try Plasma, rather than waiting on more LCDs, but it had the problems I mentioned above. (and more)

    No problem, I'm glad I could help. :)
     
  12. jriihi

    jriihi
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    Just adding more information that probably is not useful for 99% ppl but for those 1% that are intrested.

    dvd etc use video not computer levels:
    (0-15) btb(blacker than black)
    16 - video black
    235 - video white
    (236-255) wtw (whiter than white)

    contrast - white level
    brightness - black level
    Use DVE/getgrey to calibrate your DVD player (both contain patterns to set black and white level correctly).

    Andrewfee soon gets me to buy bravia ;)
     

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