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Topic: setup levels - 0 IRE vs. 7.5 IRE
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Mark Ogden
Jedi Master Film Handler
Posts: 943
From: Little Falls, N.J.
Registered: Jun 99
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posted 07-07-2008 11:08 AM
In digital video, the NTSC 0 – 7.5 ire black level corresponds to luminance levels 0 – 16, with 16 at 7.5. There is a great deal of video information between these two levels, but not all DVD player video processors, especially less expensive ones, will pass it, the manufacturer's rational being that 7.5 is “black” and that should be the cut-off point (it is for this reason that the brightness test on the THX Optimizer track on many DVDs goes thru the whole "some DVD players cannot reproduce the shadow around the letters" nonsense). If your player has the option for 0 ire playback, you should select this for maximum dynamic range and shadow detail. Bear in mind however that many DVD authors, in attempting to save space on the disc by lowering the video data, will clip off the blacks at 16 as well using the same rational.
As far as the projector goes, you may as well set it up for 7.5 using the traditional SMPTE bars, perhaps very slightly elevated depending on the device. Keep in mind that the 0 – 16 steps we spoke of above are technically beneath what is considered "black". Your projector may have the oomph to reproduce the shadow detail, or it may not, but you don’t want to drive the image into black clip/wash-out trying to find out. I set my Sony HD-CRT for standard pluge from a Denon DVD player set to 0 ire with a 40 ire window in the image so as not to stress the power supply, and I get fine results in the low-lights (some will recognize this scenario as Title 12 Chapter 11 on Joe Kane’s Digital Video Essentials DVD, which is very much worth having). Additionally, doing things this way saves the NTSC blacks that you may want to input to the projector at some time.
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Robert Minichino
Master Film Handler
Posts: 350
From: Haskell, NJ, USA
Registered: Dec 2005
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posted 07-07-2008 11:43 AM
For video applications (all analog and non-PC digital video), absolute black should be 7.5 IRE and digital 16, with pure white being 100 IRE and digital 235. Signal between 0-7.5 IRE, 0-15 digital, while valid, should show absolute black, and similarly, 100-120 IRE, 236-255 digital should show absolute white.
The player should be set to output an encoded 16 as 7.5 IRE analog and digital 16, and the same for white at 235/100 IRE. You would want the player to be able to output an encoded 0 as 0 IRE, but you would not want the player to output encoded 16 as 0 IRE unless you were using a PC monitor (as opposed to a video monitor).
The monitor, if digital, should be able to differentiate between 16/17 and 234/235, but not between 15/16 and 235/236 for maximum dynamic range. If analog CRT, there should be no visible difference between 15/16 but there should be between 16/17, and the maximum white level should be within the capability of the set's power supply.
This applies to SDI, HD-SDI and HD component. In general, this applies to every real video application that doesn't involve a PC.
For PC VGA and digital (DVI, etc.) purposes, black is 0/0V, white is 255/0.7V. These are so-called PC levels.
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Mark Ogden
Jedi Master Film Handler
Posts: 943
From: Little Falls, N.J.
Registered: Jun 99
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posted 07-07-2008 02:31 PM
Yes, 0 ire for the player, 7.5 for the projector, because really it works out to be the same thing when you look at the final picture. The difference you will notice when using NTSC pluge to set brightness is the ability to see the lower-than-black bar is lost when the playback device is set to only 7.5ire reproduction, as the video processor clips it off.
However, I tend to disagree with what Robert has said above about not wanting to see a difference between 15/16, especially in the analog display device world. First, as I mentioned above, there is actual video information to be found there. Second, remember as your analog CRT or tube based projector is fed a bright video signal; it tends to drive the black levels down somewhat, crushing shadow detail. This is the reason that I set black level using a fairly low 40 ire window signal, it leaves you a little room in the lowlights for just such an occurrence. On the other hand, when looking at SMPTE bars on a consumer based CRT or projector, the pluge becomes almost a worst case scenario, as the rest of the signal is high luminance. It these cases (looking at bars with pluge) I tend to leave the brightness ever-so-slightly above the absolute cut-off point of the center bar for best all around lowlight detail when watching actual video. You will wind up revealing MPEG levels 14 15 16 or so, but, in the long run, to me, it looks better with improved shadow detail throughout the entire range of the picture and still very good black level.
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Robert Minichino
Master Film Handler
Posts: 350
From: Haskell, NJ, USA
Registered: Dec 2005
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posted 07-07-2008 02:52 PM
I agree with the practical necessity of not adjusting a PLUGE BtB for absolute cutoff at 7.5/16 (especially on a CRT), but I didn't want to confuse the issue even more. One compelling reason for erring on the high side of black level even on a set with stable black levels at varying APLs (LCD/DLP in particular) is increased linearity of the 16/17 step; if the set's black level is set to cut off 15 entirely there may be quantization error on the first above-black step (16-17 difference isn't equal to 17-18 difference).
However, even if there is picture data below 7.5 IRE or above 100, it isn't meant to be seen, especially because it doesn't (or at least shouldn't) show up on production studio monitors. I strongly doubt anything unusual shows up there these days, though.
I do disagree a little with setting the display to show 0 IRE as black with a player set to output digital black with the 7.5 IRE setup as you lose a little of the already inadequate dynamic range on digital sets. If you have the black/white levels on your display calibrated to your source, you'll get full dynamic range regardless (assuming the devices aren't doing anything wrong). I try to keep video displays expecting the 7.5 IRE setup as black as that's less confusing to me.
EDIT:
Whoops, analog HD signals shouldn't have the 7.5 IRE black setup, even though digital black remains 64 (out of 1023/10-bit) on HD-SDI and 16 (out of 255/8-bit) on DVI/HDMI.
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