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Clarification on PC v Video levels & respective calibration

Discussion in 'Home Cinema Buying & Building' started by ROne, Mar 29, 2005.

  1. ROne

    ROne
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    Having spent several years at this game like most of you - trawling for the most useful bits of information - the one brick wall I have come up against is the aformentioned topic.

    Scanned all the threads over at AVS and TT forums, and even though I thought it was clear at one point I have disappeared into a bundle of illogic.

    The way I have read it is that a PC type display (I am assuming LCD is included in that) can deliver PC levels, 0-254. Media center equipped with the NVIDA decoders supplying VMR 9 can send Video levels across my HDMI connection. THe crunch comes is I don't understand why the calibration is using DVE where you increase WL & decrease BL to fit the video levels of 16 & 235, is any different from using AVIA to calibrate my system.

    It begs the question, how does this influence my chain? Should I make sure the graphics card ouput/tt video controls are default? And only adjust my display device? It's never clear in all the threads how you manipulate your HTPC source to deliver PC levels or Video levels in the first place.

    Secondly when calibrating for video levels, you then use the MCE calibration window and find that the white-level check is showing data at 235 and above as crushed meaning that you have to lower your white level back down to where you previously started.

    On top of this - the Z3 has two HDMI/DVI settings confusingly called standard and enchanced. Enhanced always appears to have better contrast (and my meter readings support this) but seems to crush black a little (unless you adjust your graphics card). Standard shows the full 0-254 range but lacks contrast.

    Can anyone offer any additional clarity on this subject listing whether they adjust the graphics card/TT/MCE settings for video levels over DVI.
     
  2. Gary Lightfoot

    Gary Lightfoot
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    This is how I understand it, but I do agree the finer details can be confusing, so I'll stick to what I know. :)

    Video levels as found on DVDs, are usualy digital bit 16 to 235, and in theory that's all you need see during video playback. When you use Avia which is set for 16 and 235, you will calibrate white and black level accordingly. Your projector is capable of 0 to 255 though, which is why you can achieve a higher contrast when increasing white level up to 255. I've seen this myself and measured higher CR as well. DVE can allow you to calibrate to show blacker than black (BTB) and whiter than white (WTW) information as it has test patterns that allow it. Some THX Optimode disks allow you to set BTB and WTW as well, but the level to which this can be set can vary from THX disk to THX disk as each particular 'Optomode' seems to be designed for the particular DVD it is on.

    The video overlay of a software player if correctly implemented should allow playback at studio levels - sRGB so that the range of 16 to 235 is in use. TTs VMR should do this correctly. If you try using PC levels, the pc will have to try and expand the sRGB range to fit, and this can cause some strange effects such as terrible banding apparently. I'm now wondering if you would compromise the display in some way if you try to fit WTW into sRGB if it cannot go above 235....

    BTB and WTW data can exist on some DVDs, but it can vary from disk to disk. Whilst a CRT can show BTB data (if it's there in the first place) with no ill effect, trying to get a digital display to show BTB can result in image noise and mpg artefacts becoming visible. Showing WTW is less of an issue though, but you're relying on that extra data being on the disks for it to be useful.

    With analogue systems we're sending millivoltages (0mv to 714mv) to the display which are then translated into varying levels of black to white through grey. These voltages are often referred to as IRE levels as you know, but there are 140 ire levels to represent the millivoltages that go from 0mv to 1v, except video only uses 0 to 714mv. Normal DVI will use digital bit 16 to 235, enhanced uses PC levels I believe.

    If the display can accept the higher voltages or the enhanced DVI signal (below 16 and above 235), then you can generate a brighter white and hence achive a higher CR when measuring black and white (you can't get a blacker black). As I see it, you will be expanding the range outside of normal video levels, so you will have to try and expand the 219 levels (235 - 16 = 219) into 255 levels and this is where the problems can occur.

    Not all DVI outputs are implemented correctly judging by some posts I've read over at avs, as some have reported changing the analogue setting for black in an NTSC DVD player (equipped with a DVI port) to 0ire from 7.5ire has also had an effect on the DVI output from that player, which shouldn't be hapening. I found some of the technical info in the exchange a bit confusing IIRC to be honest.

    In theory though, if you use the normal DVI output you should be using 16 to 235, and that's what I would use. Sometimes you can't always see the black bars in Avia using standard settings from the PC (or the drop shadow in THX Optimode), and no matter what you do on the projector you cannot make them visible. That's when you have no choice but to alter the settings on the software player (brightness and contrast and possibly gamma). You may even find that one software player looks quite different from another (ie TT to PowerDVD), so you will have to set them both up seperatley using Avia or DVE.

    One thing I have read but don't know how true it is, or what effects it has, is that some PC graphics cards only output up to 700mv, not 714mv or higher. I can't say I've ever noticed anything to suggest that my radeon cards have ever done this (how would it show up?), so I don't know if it's an issue with the current graphic cards of choice.

    Does that help at all, or have I just mentioned stuff that you already knew so haven't really clarified things?

    Gary.
     
  3. ROne

    ROne
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    That's very useful gary and re-enforces what I thought.

    Just a couple of things.

    "The video overlay of a software player if correctly implemented should allow playback at studio levels - sRGB so that the range of 16 to 235 is in use. TTs VMR should do this correctly"

    I thought video overlay passed an expanded 16/235 to 0-255? Whereas VMR passed 16-235.

    "Normal DVI will use digital bit 16 to 235, enhanced uses PC levels I believe".

    This is where I think my system has contradictions. Enhanced or the z3's L2 HDMI level shows a lower contrast than L1 (or standard). This doesn't make any sense if my enhanced setting (PC level) is supposed to show 255 levels. The enhanced appears to track greyscale better but this could be due to the fact that contrast is not being driven has hard.

    On the Z3, using VMR from within MCE - if calibrated using AVIA and selecting L1 (HDMI standard level) or possibly video levels, it produces a contrast ratio of about 1100:1. If you use the same set up but change HDMI to enhanced the CR drops to about 800:1.

    Another thing - I read MCE passes video levels out the box and it was designed that way - well if that's the case then how come on the white shirt contrast test it passes data above 235?

    Anyway thanks for you clarity on this subject you have certainly help re-enforce what I believe.

    I am going to produce some some low RGB(BTB) charts and put them on DVD to see what I can see myself.
     
  4. Gary Lightfoot

    Gary Lightfoot
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    As far as I know, an overlay can do either, it just depends on how it's implemented. It can be done right or wrong in other words. VMR should be corect though. I use WinDVD and PowerDVD and can't tell if they're not implementing correctly or not. WDVD needs DX9 installed to run, so I wonder if that is using VMR. I can't see anything different in PDVD if it's doing things differently.

    Like you I'm not that definite about the finer technicalities - one possibility is that when you're setting up VMR or the overlay to show btb or wtw, you may be compressing the levels between those two values in order to make those above visible i/e remapping 235 to 230, and wtw becomes 235. Or normal levels work between those levels and btb wtw are allowed to exist beyond them. I think we need clarification on that....

    I wonder if the enhanced dvi is 'enhanced for video' so that's using 16/235. What are you white and black levels like when using Avia or DVE? Do they change from one DVI to the other?

    When you make your charts, try to keep the RGB values at least 4 integers apart. Any less and it's hard to see where the demarkation lines are. I use .bmp before rendering them to .mpg2 because I found using .jpg often spoilt the colour block values - what was all R16G16B16, could end up with a few blocks or pixels of R15G14B16 for example.

    I'll see if I can find some more info. MrD or Gordon might know mopre definitely what's going on here.

    Interesting though. :)

    Gary
     
  5. Gary Lightfoot

    Gary Lightfoot
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    This is from the FAQ over at AVS:

    Not sure if that helps you much, but the midle section referring to mapping studio levels to PC levels is interesting. I wonder how it's being done for VMR and PDVD/WDVD for example.

    Edit: Added this conclusion form AVS but it just reinforces what we already know we should be doing:

    Gary.
     
  6. Gary Lightfoot

    Gary Lightfoot
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    This clarifies the DVI output I think:

    Gary
     
  7. Gary Lightfoot

    Gary Lightfoot
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    And finaly (I think):

    Gary.
     
  8. ROne

    ROne
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    Some stuff there I haven't see on AVS ...

    "When using digital outputs, the major adjustment option you have is the one for digital levels. As is common in consumer labeling, the labels for this can be confusing. The most common labeling will at least hopefully show that you are making an adjustment to the DVI/HDMI digital outputs, and usually the options will read ‘Normal/Enhanced’ or ‘Normal/Expanded’ or ‘Video/PC,’ or some such label."

    That is the bit I'm intrigued by, would you assume from this statement NORMAL would mean VIDEO and Expanded or Enhanced would mean PC?
     
  9. Gary Lightfoot

    Gary Lightfoot
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    Yes, that's the way I read it too.

    When you use PC levels, you can drive white brighter and that's why you're getting a better CR value. I noticed the same thing with both my NEC HT1000 and Optoma H77 as well as some odd changes to desktop colour when adjusting the primaries. I didn't see the same adjustments visibly happening in the overlay, so I assume the overlay is working correctly by using a diferent range which I presuem is sRGB.

    I wouldn't worry that enhanced mode can give a higher CR, as it's going beyond sRGB and into PC levels, which isn't what you should do for accurate video reproduction. I think it's easy to attain another 200+ CR by overdriving the gains but at the detriment of white level detail.

    Most of the info above I just copied from Chris Wiggles over on AVS btw, and it's a conglomoration of info from al the bigwigs like Guy Kuo and Stacy Spiers, so we can assume it's correct. :)

    Here's the link:

    http://www.avsforum.com/avs-vb/showthread.php?s=&threadid=491711

    I have it bookmarked now as it's a usefull reference.

    Gary.
     
  10. ROne

    ROne
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    I've had another quick look at this and still seem to be going round in circles.

    The Z3 has two HDMI settings, L1 and L2.

    L1 clips below 16, no matter how you increase the brightness you can't see any RGB/VMR data below this point. The white is much brighter but doesn't clip.

    L2 Allows me to see below 16 (BTB) and WTW, yet appears to be dimmer.

    Now assuming 16 is the floor of Black video - why would I need to be worried about using L1? It's only clipping below black (and I know there is a raging debate about this) but I would imagine 90% of useful picture data starts there - and I get much better contrast to boot.

    If I didn't use this option - what would be it's actual use on the projector itself?
     
  11. Gary Lightfoot

    Gary Lightfoot
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    I wonder if L1 somehow sets D16 to 0 (shifts it down), ignoring and clipping everything below, and allows white to run up to 255? Sounds like you may have to speak to Sanyo's tech people for clarification.

    For a digital display, I don't see there being much of an issue using L1 as seeing BTB can display image noise etc, so it's not necessarily a bad thing.

    L2 looks like the input of choice in this case, especialy with VMR as it's maintaining video levels correctly within PC levels. Why it looks dimmer I don't know. Does it measure dimmer? Did you reset the brightness control on both TT and the pj, and did you try to reveal BTB using TT?

    It does sound confusing as to what's really happening..

    Gary.
     
  12. ROne

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    Yes L2 measure's notably dimmer.

    I think instead of going all out for accuracy(with L1) I think any clipping trade off in the BTB may be less of an issue than the loss of contrast.

    I may try Sanyo, but it will probably be one of those things they just can't answer.
     
  13. ROne

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    Yes, you can reveal BTB by lifting the brightness of the TT levels, would this not be a bad idea though ...
     
  14. Gary Lightfoot

    Gary Lightfoot
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    It depends, it might be necessary so that you can get full range of video levels on the pj. Sometimes you have to do that to show the black bars in Avia for instance, and adjust both TT and pj to get the settings correct. I had to do that with my H77 for instance. I'm not sure what's actualy happening in the digital domain there though. Would be interesting to know.

    I know what you mean about Sanyo not knowing - most of the comapny people you can speak to in the UK aren't that technical usualy.

    Gary.
     
  15. jriihi

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    Rone. Any word from sanyo on this issue? Should i use L1 instead of L2 to get better CR(?) or something even if it clips btb data and leaves wtw intact? Assuming it does it correctly.
     
  16. ROne

    ROne
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    You won't get a word from Sanyo on this, or at least sanyo UK.

    I use L1, it's just too dull on L2. (about 300:1 difference on CR)
     
  17. jriihi

    jriihi
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    I wonder if L1 crushes blacks and shadow detail or clips right at 16? I tried DVE and there didnt see anything odd so its probably ok.
     

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