Setting Contrast with AVIA

Discussion in 'Displays' started by Guy Kuo, Aug 20, 2004.

  1. Guy Kuo

    Guy Kuo Supporting Actor

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    Periodically, I hear people struggle with setting contrast. In particular, people get hung up on looking for blooming, avoiding geometry distortion, or thinking that the AVIA disc is only for CRT displays. Here's the five minute summary of the problem of setting contrast (white level) for newbies. It's hard to get it all because there are so many permuations of displays and what to look for. Hence, the on disc instructions could only cover the one - most common case in detail. Of course, a lot of forum members have non CRT based displays or have ones that differ in behavior. In general, here are some goals.

    If you have a glowing phosphor based display like a CRT or plasma, one task is to make sure you don't have contrast set so high that you are rapidly burning the phosphor. You other task is to set contrast such that response to the signal is "linear." In other words, you keep contrast low enough that neither damage to the display or to picture quality result.

    If you have a lamp based display like an LCD or DLP, you don't have to worry about avoiding damage by having too high a contrast, but you still have to avoid huring image quality.

    On older CRT's an unsafely high contrast setting would cause minor defocusing of the electron beam aka blooming. This definitely was a bad thing so we tried to teach people to look for and AVOID ever letting the display run that high. Newer CRT displays don't bloom as much even when contrast is run up high enough to accelerate wear. So, you don't always see blooming and that is a bit confusing if someone new to video adjustments gets caught up in looking for blooming when actually the second part of the advice is the most important - Keep the contrast as low as possible to prolong phosphor life. In some modern CRT RPTV's you won't see obvious blooming but that does NOT mean it is good to keep contrast cranked up all the way.

    Geometry distortion due to power supply problems are less common now. I wouldn't really worry about that on today's CRT's. When it's present and due to things like scan velocity modulation, newbies often can't get rid of it even if contrast is turned down too low.

    On lamp based displays, having contrast doesn't shorten display life but causes clipping of hilites or shifts in the color of white as one primary color or another runs out of dynamic range. Clipping of hilites means that things that are bright but not quite white become indisinguishable from white. Shifts in the color of white means the color of white changes. On DLP's that shift is often blue-green.

    A pro calibrator knows to set contrast to get proper light output, keep within safe limits, and obtain good grayscale tracking. Teaching the consumer to do that is hard because....

    1. The consumer usually has no means of objectively measuring video display light output. Only a few advanced enthusiasts go so far as buying a colorimetry system like the OpticOne. I could have said on the disc to adjust contrast until white measures something like 12 to 16 footlamberts on a direct view or front projection and around 8 to 10 FL for RPTV CRT. Those aren't targets written in stone. Some people intentinally target higher, but they still do so keeping an eye on what is safe for the equipment. Most Avia users don't have a means of measuring how bright is 12 FL. Avia PRO users, yes, but not AVIA users.

    Without a metering system, the best I could say is that the target brightness of white on screen is very roughly how bright a 4 white LED Lightwave 2000 flashlight lights up a white sheet of paper at 28 inches. That is fraught with uncertaintly and isn't that standardizable. People don't have that flashlight and even if they did, the flashlights vary in light output unit to unit and the battery freshness affects things. No matter what I have thought of in common objects as a possible reference, lots of factors ensure that what you see end up with for light from that object won't match what I am getting.

    2. The measured light output level needed to deliver the desired light intensity to the viewer varies with display system. A pro calibrator knows that RPTV's have a high gain screen and need a lower direct measured light output to attain the same visible light level as a direct view. They also know that front projection systems also have screen gain that affects how measured light intensity is seen by the viewer. All those additional situations would take much more teaching than is practical on a consumer disc. If we tried to teach it on disc, almost no one would understand. People get confused with just the simple basic instructions on the Avia disc - and that was about as simplistic as I could get it.


    So this leaves us with the final portion of my advice for setting contrast - keep it as low as consistent with a good image in a fairly dimly lit room in the evening. If you have contrast high enough to compete with sunlight or bright room lights, you are probably too high. Many people are used to really high light levels from a TV. Indeed, my eyes hurt the last time I saw an RPTV at someone's home because the light output was too high. On a home theater display, the light output when set correctly and viewed with DARK ADAPTED eyes will just begin to feel too bright when the scene abruptly switches from an indoor shot to a bright outdoor shot. That's somewhere around white being about 12 to 18 FL when I measure it against my eyeball "too bright tolerance.

    By keeping contrast lower, you are more likely to be in the physically safe and image preserving portion of the display's dynamic range. I wish there were a standard object or light out there that I could say "make white on your screen look as bright as object x." I haven't found anything readily available to consumers that has constant enough of a light output and close enough to D65 in color to recommend as a standard. As I mentioned above, even LED flashlights won't work. They change with battery condition. Fluorescent tubes won't do either because they vary in brightness with age. I once tried to describe the brightness of white in terms of candle flame brightness cast upon a sheet of paper. That wasn't too accurate and I doubt a single soul took it seriously.

    So bottom line - Don't perseverate on looking for blooming or geometry distortions on the test patterns. Instead concentrate on keeping contrast down LOW. Not so low that viewing the picture in a darkened room still results in a dingy image, but as low as still consistent with a bright enough picture in a darkened room. That gives you the best you can do until you have a colorimeter or a pro can come by and get you set up truely right.
     
  2. Gregg Loewen

    Gregg Loewen Video Standards Instructor, THX Ltd.
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    Gregg Loewen
    A bump for another great post on the information hiway.

    thank you Guy!!
     
  3. Dave H

    Dave H Producer

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    Guy,

    What are your thoughts about setting white level this way. Start by having the display's contrast at ZERO on the needle pulses or steps pattern -- keeping your eye on the top most, brightest square. Slowly start increasing contrast until you have just reached a soft white -- with it no longer having a dingy look, but it still being just one ot two clicks away from it being dingy again.
     
  4. Tyson Schettig

    Tyson Schettig Auditioning

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    Just got a new LCD RPTV this week, a Sony 42" Wega kdf42we655 series.

    Having gone through the menu options several times now, I do not seem to have a Contrast setting, just brightness.

    I also have the Avia dvd and after letting the set settle down this past week, I was going to calibrate it sometime early this week, then I saw your post here.

    Does the contrast setting exist in my set?

    Can I successfully calibrate the set without a contrast setting?

    Thanks!
     
  5. ChrisWiggles

    ChrisWiggles Producer

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    Dave, that's essentially what the instructions on Avia say, and is a good basic method for CRT displays. With digital displays you'll be looking for colorshifting and clipping, because you are trying to maximize the contrast in a digital display by getting your white levels and black levels optimized. CRTs are a little different, because you don't have this limited range of contrast that you are trying to maximize.

    Tyson, I'd be surprised it doesn't have some way to adjust the white level. It may be labeled something else, like Contrast, or Picture, or something like that.
     
  6. Tyson Schettig

    Tyson Schettig Auditioning

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    Guess Sony doesn't like using the word contrast. Just tried the picture setting and it indeed changed the perceptable contrast.

    Thanks!
     
  7. Allan Jayne

    Allan Jayne Cinematographer

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    You can do a lot without test equipment.

    One test for contrast / picture / white level (for CRT only) consists of an upright black line that may bend as you increase the contrast. Unfortunately on some TV sets the line may bend a little at a time no matter what your contrast setting is, you will have to pick a compromise point.

    If you see the white square enlarge slightly (blooming) quickly make a note of that point and quickly reduce the contrast.

    The final contrast setting should be somewhat lower than the blooming point and at least a little lower than the line bending point you just made a note of. We suggest aiming for no more than a third up the scale for CRT RPTV and no more than halfway for CRT direct view.

    Further adjustments are now a matter of personal preference.

    Even if you had a light meter, setting the contrast to the foot lamberts suggested can result in faster screen wear than you might like.

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

    We believe that the reason you get eyestrain from watching TV in a totally dark room is that the TV contrast was more appropriate for a relatively bright room and dark room calibration is different.
     

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