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Better to have more GAIN or CONTRAST?

Discussion in 'Projectors, Screens & Video Processors' started by jrwood, Aug 14, 2002.

  1. jrwood

    jrwood
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    I was having a play the other day in the service menu where you can change the GAIN/BIAS settings. As a test I looked at one of the icons on my desktop (a CDROM icon with various shades of grey!) which looked very good.

    I increased the GAIN for r/g/b by 10 in the service menu which made the colours more vivid on the screen but obviously this CDROM icon on the desktop had now lost its detail because the whites were being crushed. I turned the projector contrast down by about 10 and the detail returned because the whites were no longer 'crushed'.

    I was wondering if the gain is the same as the contrast setting?, if not is it better to have a high gain for each r/g/b in the service menu and a much lower contrast setting on the projector?.

    For example if the settings were
    -- setup #1
    gain
    red=64
    green=60
    blue=74
    contrast 70
    -- setup #2
    gain
    red=104
    green=100
    blue=114
    contrast 30
    --
    As long as a greyscale test pattern made sure that all the greys were being displayed which would be theoretically better?
     
  2. ROne

    ROne
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    I've tackled this one with my set-up couldn't come to a conclusion.

    Isn't shifting all the RGB at the same time equivelant to shifting contrast anyway? Because RGB together will move white level to a different pont.

    I would have thought moving RGB would be better as you can move the individual deficient elements of RGB to find the optimum output of your panel. IE Red my clip before green so you can compensate whereas with contrast you will push them all.

    I've not come to a conclusion on this, my personal angst is gamma - where does this fit in. According to my tests you can have different black levels set correctly with different gammas (from the overlay control panel) and produce types of picture (low gamma crushed blacks and less shadow detail but better upper intensisty contrast) or high gamma (better shadow detail and poor upper intensity). Anybody any tips at what to arrive at with GAMMA.

    Does anybody find AVIA still a bit subjective? Like when it says to set white level when the white portion turns from grey to bright white? Or even on the black level if you check the different black level tests some say to make the right bar visible and other black level tests say to make the right bar barely visible - this is about 5 steps on my projector!
     
  3. Mr.D

    Mr.D
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    OK.

    RGB bias and gain control RGB levels at the top end of the intensity range (gain) and the bottom end of the intensity range (bias). Basically you've got two control points : one in the darks one in the whites.

    You get your black and white points set correctly . Then you look at a low IRE patch say 20IRE and make it neutral ( I'd say by dropping values until it looks neutral then up them all equally to get the original luminence ( brightness level) back to where it was before you started adjusting. Then you do the whites by adjusting gain in the same way but looking at a higher patch say 80IRE. Then you check the lows again and recalibrate ( as the two will be interactive) then back to the whites ( changing the relevant IRE patch each time) and ping ponging back and forth until both highs and lows are as good as you can get ( helps if you have a colour analyser handy).

    The idea is that this will sort out your gamma at the same time ( as analogue ecvices rarely have kinky curves unless they are bust)

    However on a digital device the curve on the display is normally linear so when you watch material on it that is designed to be displayed on a device with a nonlinear intensity distribution ( video on a CRT for example) you need to perform a software gamma correction before values of the image get sent to the linear display) The downside of this in digital devices is that the intensity response of the panel is rarely all that large and when you start correcting your material if you are not careful it will exhibit posterisation on display on your linear display.

    Most decent digital display device take into account the fact that you are likely to be feeding them an input ( say RGB from a PC) that is expecting a CRT type response on the other end. (you may additionally have to make some gamma tweak to the overlay though as you are looking at a situation where you have video material being displayed on a system that has a linear colourspace expecting to drive a display that has an inherent gamma of 2.2.

    To be honest its unusual to have both RGB bias and gain on a digital display: could be its simulated in software so effectively its giving you control over the incoming RGB gamma correction on the input source. My panny ae100 only has RGB drive controls but I assume it does the whole linear display leavign me to tweak the highs and lows before they go into the projector using hte overlay gamma in my HTPC.

    its all just curves trying to counteract other curves before you run out of intensity range.
     
  4. ROne

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    Usefull stuff ...

    Okay what happens when you have different Gamma settings on your projector (my projector has just RGB gain) but has a standard gamma setting, film and sport.

    However you would think the film mode would look best but it remaps the intensity so there is a boost in shadow detail at the expense of top end intensity and overcompensates in the dark areas and looks weird.

    this is obviously product of an LCD projector trying to do better shadow details but it looks quirky even though it does boost them.

    So do I use standard or film gamma to set-up with?
     
  5. Mr.D

    Mr.D
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    They would get on much better if they just gave you options which were the actual real gamma of the display based on sending it material with a gamma of 1 : ie 1 , 2.2 .

    The film modes and other sundry nonsense tend to be ways of trying to make the image look punchy: think AI and black stretch modes on a TV.

    A good start is to view a non-dithered gamma patch via a PC and this should tell you the rough display gamma of the display.

    Avia has a gamma pattern but I'm a little unsure how to read it: I think it gives the display gamma with videospace taken into account so its the 2.2 gamma you want to look correct for video. ie video material is about 0.6 giving an idealised end to end gamma of about 1.2 on a correct sytem with 2.2 display gamma.

    Basically i use the metered display criteria for my graphics card ( available from the theatertek forum. Assume this is relevant with the intention of driving a CRT display ie 2.2 gamma for video material. So the 2.2 patch should be aimed for to be correct on the linear display ( my projector) and adjustments made to make this so are correcting for the inherent linearity of the LCD display and any weird additional gamma correction it may or may not be performing on the incoming image. ( I kinda suspect that the reason the ae100 looks as good as it does is an additional correction the unit is doing for video material: most LCD projectors probably don't)
     
  6. jrwood

    jrwood
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    Should of made a new topic for gamma!...

    Does anyone know which is best?; higher gain values with low contrast setting or high contrast with lower gain values?. Maybe they are just the same although in my factory menu I can change the contrast for each r/g/b value individually...

    I also have a gamma on/off which makes the picture go very dark if its turned off...

    James

    p.s Im surprised more projectors dont have individual BIAS/GAIN settings for each of the r/g/b values?. Its rather handy looking at a greyscale calibration test pattern and then adjusting the GAIN so that the mid grey to white looks perfect, likewise with the BIAS setting you can adjust it so that the midgrey to black looks perfect.
     
  7. jrwood

    jrwood
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    I thought that too, but GAIN just changes the mid greys to white level, obviously too high a gain will make very light grey look white, unless you decrease the contrast..

    The contrast setting changes the whole range, whereas the gain/bias only affects the high/low rgb values..

    hmm oh well!
     
  8. zcaps57

    zcaps57
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    That's because your panels are overdriven.

    Assuming that they both produce the same colour temperature, correctly set white, black levels for NTSC/PAL standard.....raising RGB gains with lowering overall contrast, and low RGB gains with high contrast - They're the same thing.

    Actually, I've experimented this while back with my 11HT, and photocell detector as well. Boosting RGB gains upto 255 max value, and lowering contrast. And photocell detector was used for maintaining correct colour temp., gamma curve, white & black level. My conclusion was they are the same.

    If you see the difference, its either you're seeing different colour temp, overdriven white level, or black level.
     
  9. jrwood

    jrwood
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    Thats the thing, Im not seeing any difference just wondered if changing the contrast instead of the gain was basically the same thing which you have pretty much confirmed aswell. Im now at a point where all scenes look really good, especially dark scenes and Ive finally achieved full grey scale being shown on the projector.

    gain and bias are your friend :)

    btw its interesting that the 10/11HT allows you to ramp up your gains to 255 and 226 for green/blue to get a much better picture. On my cx1 projector the defaults were like 64,60,60 or something like that and Ive just increased the gain by another 10-15 ..... going up to even like 128 for each colour out of a possible 255 makes it over saturated..strange!
     
  10. zcaps57

    zcaps57
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    Jrwood, what kind of method did you take to ensure the linear grey scale & colour balance ? (ISF calibrated, or something like a photocell detector with SMART software ?)
     

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