Black level and Avia

Discussion in 'Archived Threads 2001-2004' started by Dave H, Aug 20, 2001.

  1. Dave H

    Dave H Producer

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    If I follow the black level pattern on Avia and eliminate the one black bar that I'm supposed to (leaving only one moving black bar visible) I find my picture a bit too dark on my TV where I'm losing delicate shadows, etc.
    If I increase the brightness a few notches, my problem is settled. According to Avia, this would be incorrect because both bars will appear on the pattern.
    The bottom line is preference and common sense. I guess Avia points you in the right direction and gives you a guideline to follow, but that doesn't mean you should, necessarily, follow it to the tee.
    Anyone else notice tweeks which are necessary?
     
  2. Dave Gogebic

    Dave Gogebic Extra

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    I also find that Avia gives you a good starting point - I then visually tweak it up or down a little depending on the input that I have it on. I find that it is pretty much "Right on" on my Progressive DVD input - but I end up turning up the contrast and brightness a little when watching my SVideo Direct TV input. I find setting the contrast the most difficult. You are supposed to adjust it so that there is no bluming - but mine never does blume. I then heard that you should adjust it so the graduated boxes on the top are each 2x brighter than the one below it and that the top white box is a true shade of white. This seems to make about the most sense. Interestingly when you go from the combined contrast/brightness test pattern to the next one which is only the brightness adjustment - You seem to have to adjust the brightness back down....once again I think this is only a starting point that gets you in the ballpark.
    Just my two cents worth from playing around with it for a while. Anyone else have any other experiences?
     
  3. Dave H

    Dave H Producer

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    While I feel my contrast is "dead-on" now, I found it more difficult than any of the others because my set barely blooms and the lines in the right and left corners barely move. Looking to see each block is twice as bright as the other helps, but it's not exact either. But I used this and wathcing reference quality DVDs to see that the white level was correct.
     
  4. ThomasL

    ThomasL Supporting Actor

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    If your set doesn't hold black level well across varying picture intensities then most likely using the half gray/half black with moving bars pattern will result in a compromise setting. Depending on what annoys one more, one'll either find that this makes some dark scenes look too washed or that shadow detail is lost especially in dark areas on brighter overall images. There's really nothing one can do about it other than to tweak the brightness setting for the particular source you're watching.
    hope this helps,
    --tom
     
  5. Luis Gabriel Gerena

    Luis Gabriel Gerena Second Unit

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    Thomas beat me to it...Remember two things: Avias ia NOT perfect but is a great base line just don't think of it as "The Law" so feel free to tweak it to your preference. Second, the black level problem is indeed caused by your tv. It happens with my Toshiba also but not with my projector. If I am not confusing terms it is called DC restoration.
     
  6. Guy Kuo

    Guy Kuo Supporting Actor

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    Here's the deal. The darker of the two moving bars is acually visible above black if one examines the actual signal. Black bars appear in multiple patterns in AVIA. They always have the same signal level relative to black, no matter which pattern, but they will appear different due to real world display limitations. On a perfect display, one would adjust black level to make the black background perfectly black and the two moving "black bars" visible. We don't have perfect displays.
    Most displays have imperfect DC restoration and do not hold black exactly at the same level as overall picture intensity varies. We deal with this by adding the gray half of the screen to load the display's video amplifiers with an average signal level. If you adjust to this, then you get a compromise setting that often works well. Mind you there IS compromise if the display doesn't hold black level steady. You can either get black blacks with shadow details obscured as black or adjust to get shado details visible but make black glow slightly. On higher end displays, the two extremes of choice are more close together. You can use the plain black bars (no gray half) to set things to generate perfect black level for low APL scenes. You can use the black bars with half white or the black bars in the lower right hand corner of a color bars pattern to set black perfectly for a high APL scene. For the instructions that most people are going to follow, I chose to lead them to a compromise. I don't think most people are interested enough to understand DC restoration, APL, and the compromises their display forces them to make.
    Even if the display has perfect DC restoration, there is still reason that the darkest moving bar won't be visible when black level is correctly adjusted. This is most true on projection displays like RPTV's and FPTV's. Light scatter from the gray half of the screen spills over to the black half and hides near black details. If this is happening and I have people adjust black to make the darkest bar visible, they'll actually be set too high. Again, I simplify for the average AVIA user.
    If the display is one which is known to have very good black level stability (good DC restoration) then my preference is actually to use a lower APL pattern to set black level. With a low APL pattern like Black Bars + Log Scale, there is less light scatter and one can adjust to see the darker black bar.
    On CRT front projectors with good DC restoration, I go one step further and totally remove the effects of room wall light scatter by adjusting black level while looking INTO the green CRT projection lens and setting things exactly right on the phosphor. I use the green gun because the other two will most likely be at a different cutoff point and the green gun produces the most luminance in the final picture.
    Even worse is trying to use settings which are dead on correct for your DVD player with other sources. The VCR's, cable boxes, and other video sources in your system most likely don't produce signals with signal intensities and characteristics which exactly match the DVD player. AVIA is a very accurate calibration tool for DVD playback, but if you use something which is not sourced through that same playback chain, your results may not be very palatable. A more NTSC accurate source will be closer to the DVD, but I doubt anyone will make that claim for a cable feed.
    Hope that makes what is going on clearer, but the more you know, the more nuances with which you find yourself dealing.
    ------------------
    Guy Kuo
    www.ovationsw.com
    Ovation Software, the Home of AVIA DVD
     
  7. Dave H

    Dave H Producer

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    Guy,
    The compromise you speak of is what I've been doing the last couple of days. At this point, I prefer to see delicate shadow detail and allow a tad bit more "glow" than have my darks too dark where I lose detail. However, on some films (such as Closencounters that I watched tonight) I tended to see a little bit more grain by having the brightness up a notch. Again, I had to weigh the compromise because I saw more detail at this level.
     

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