Black Level, 7.5 IRE, 0 IRE,

Discussion in 'Displays' started by Guy Kuo, Mar 7, 2003.

  1. Guy Kuo

    Guy Kuo Supporting Actor

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    Note: Because this type of discussion sometimes gets archived. I have edited this first posting to be more complete even when read out of the context of this and another discussion thread. I have incorporated some information from Jim Doolittle and Stacey Spears so we can have a better stand alone piece. Consequently, some comments and clarifications appearing later in this thread may seem to have been already covered in this first posting. No prescience really took place. GK

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    I've seen people confused or posting confusing errata about 0 and 7.5 IRE black level settings in DVD players and displays. The concepts are actually quite simple but laborious to explain. After seeing one too many confused answer and explanation about the difference between American NTSC (black at 7.5 IRE) and Japanese NTSC (black at 0 IRE) and how that affects the picture, I couldn't take it any more. Here is my attempt at clearing some of the mess up......

    Unless a display or processing system has insufficient ability to adjust its black level (brightness), one can use either a 0 or 7.5 IRE setup as black. The concepts of IRE, voltage, and black seem to create an undying amount of confusion and misinformed posting on the boards.

    One could describe the video signal in volts, but IRE units are more convenient to talk about than 0.7 volts from porch to white. By definition there are 100 IRE units from zero (the porch) to the top of a white signal. The sync pulses are at -40 IRE.

    In both the North American and Japanese NTSC systems, these levels stay the same. What changes is the voltage or IRE level at which black is represented. In North American NTSC production practice, black is represented in the video signal at 7.5 IRE. In Japanese production and equipment, black is represented as 0 IRE. In any case, max white is at 100 IRE.

    As Jim Doolittle points out....

    ".... The black level switch on a DVD player will add set up (bring the black level up to 7.5 IRE) if the end user wishes to use "S" video or composite video outputs on the player, or if the display device only has 1 memory for brightness level, the end user could choose the 7.5 output level for component video so that it matches all his/her other sources.

    To simplify, if using component the player should be set to output 0 IRE. If using "S" or composite the player should be set to 7.5 IRE. This regardless of what the picture looks like after making a change. After making a change the display must be recalibrated.

    Now, not all players offer the choice and if your not sure which one to use, you will have to have a waveform monitor or a lot of time and knowledge to figure it out. The players that do offer a choice have chosen marketing terms to describe the difference, such as "dark", or "enhanced".

    end quote.



    Not every DVD player or device implements black at the IRE or voltage level expected. However, black in the picture is intended to be just as black, and white just as white no matter the levels implemented. The key is your display must represent the incoming signals correctly through proper calibration of black and white levels.

    Again, if your display is adjusted correctly for the type of incoming video signal, black is no darker or lighter than black with one setup level than another. The output dynamic range should stay the same.

    I can hear the guffaws now, "But, my the picture looks different in brightness and shadow detail when I switch my DVD player between 0 and 7.5 IRE ("enhance black" or "darker" on/off) for black!"

    And I would patiently ask, "Did you recalibrate the display's brightness and contrast controls each time you switched between sources to compensate for the changes in the video signal?"

    This is where the disconnect seems to happen. In order to properly compare the effects of having a 0 vs 7.5 IRE setup black level you must adjust the display so it creates black at 0 IRE for an 0 IRE setup signal. You also have to readjust it to create black at 7.5 IRE for a 7.5 IRE signal. You also have to adjust the contrast so the max white is at the same light output level for each one. In other words, the picture ends up looking the same.

    People sometimes calibrate their display for black at 7.5 IRE and then switch their DVD player to "enhanced black." Naturally, that will darken the image. Indeed anything which is black and up to about 7.5 IRE above black will turn black. That clips off shadow details. You can do that, but you should realize you have now set your system to intentionally alter the image and make everything darker than intended and hidden shadow details. Set the display so it displays 0 IRE as black, matches the incoming type of video signal, and you find yourself back at the same type of picture.

    In short, if the video signal has a 0 IRE black (aka "enhanced black" or Japanese NTSC, 0 IRE setup), the display's brightness control will need to be set higher than if the signal has a 7.5 IRE black.

    There are caveats. It is possible that a display or video processor is unable to pass or display signals below a certain threshold. If a projector or scaler simply is unable to accept any signals below 7.5 IRE and you feed it a video signal which uses 0 IRE for black, no matter how far up you turn the brightness control, shadow details will be clipped off. For such a device, it is imperative that the video signal have a 7.5 IRE black level to ensure that the entire signal gets through.

    One might run into a display device which has a very limited range in brightness adjustment ability and also expects all its signals to have 0 IRE as black. Feed such a device a 7.5 IRE black leveled signal and no matter how much you turn down brightness you simply cannot correctly adjust the projector to make black in the video signal be the darkest black possible on projector. Even black in the signal would cause the display to glow. Images would appear foggy. If one has such a limited projector, then it would be important to always feed it a 0 IRE black leveled signal. Fortunately, such a severely crippled range of black level (brightness) control should be very rare. A properly designed display should always be able to properly calibrate its brightness to render 7.5 IRE as black.

    So it comes down to the proverbial newbie question – Should I set my DVD player to "enhanced black" or not?

    For display systems which can handle either 0 or 7.5 IRE as black, the answer is that it really doesn't matter. You can select either standard for black. You just have to calibrate your display's brightness control to behave correctly with the type signal you are feeding it. The picture ends up the same either way because proper calibration removes the difference. You can of course decide to improperly calibrate the display and allow your picture to look different, but that is up to you. I personally, tend to recommend people stay with 7.5 IRE as black from the DVD player because that matches all their other video sources. That allows the same settings to be used with other video sources. Otherwise, one must remember that the brightness setting for everything else needs to be a bit lower than for DVD player.

    Now as I have said, there is the possibility that you have the oddball display system which cannot differing signal levels well. Just make sure that black is black, and there is no clipping of shadow details while setting brightness. If you can't see the two moving near black bars in AVIA or Home Theater Tune-up, you ARE clipping shadows details.



    This leads me to another misconception which keeps circulating on the forums --- the idea that black is encoded onto a DVD as being at a specific IRE level independent of the DVD player. That is not true. Not for Videl Essentials, Not for AVIA, Not for Home Theater Tune-up, Not for any theatrical release DVD. What is encoded onto the disc is true black in the video signal is at a specific digital level which I'll refer to as "digital black". All properly mastered material should have black in the image encoded at this same "digital black." The DVD player or HTPC takes the "digital black" and outputs it as a voltage. If the player is set to output black at 0 IRE, then it comes out of the player as 0 IRE. If the player is set to output black at 7.5 IRE, then it comes out at 7.5 IRE. It is a small distinction, but is very important.

    You understand the difference if you understand why the follow statement is false, "VE has black encoded as 0 IRE but AVIA has black at 7.5 IRE."

    No, both discs encode black as the same digital 16 (digital black). There is no way for a disc to independently specify that a signal will come out of a DVD player at a specific voltage regardless of what the DVD player is set to do. The setting of the DVD player is what determines the actual output IRE or voltage which comes out when something "black" on the disc is played. So again, if the player is one which outputs black as 7.5 IRE then black would be 7.5 IRE with either disc. If the player outputs black as 0 IRE then black comes out as 0 IRE on either disc.

    Confusion happens when you take a disc like AVIA which labels the patterns as being a certain IRE. The labels are true if and only if the DVD player is itself NTSC accurate. In the cases, AVIA, VE, and HTT, the labeling is correct only if the player outputs digital black as 7.5 IRE and digital white as 100 IRE. If the player is set otherwise, the actual voltage or IRE level won't be exactly that said by the labeling. This is true for all three calibration discs. Fortunately, most DVD players you find in the store will come pretty close, so those labels are useful.

    That means that if the DVD player is set to output digital black at 0 IRE, all the labels will be a bit higher than the actual measured IRE. It would be possible to create another set of patterns which were correctly labeled or signal leveled for a 0 IRE black player, but that would be of little real utility.

    There is another more insidious problem. As Stacey Spears of Secrets of Home Theater points out, there is still variability in how well DVD discs encode black in the picture correctly at digital 16. Even on THX discs measured black is at, "… 20, 25, 14, etc."

    He goes on, " The THX master plan was to feed test patterns in at the start so they would go through the same pipes as the video. Then they wanted to have THX DVD players look at special markers and would alter the levels on-the-fly on a disc-by-disc basis. They failed to get this second portion into any THX certified players. My problem is if they know the delta between their disc and what is correct, they should have fixed the disc or the end-to-end production chain."

    Variance in black and white levels is a mastering problem, not a sign that ones display is awry. The existing variability shows that even more work needs to be done to improve production standardization. That way the entire chain from production to home playback can have predictable results. Everybody gets to see the picture as intended. We take care to calibrate accurately. The production house should as well. We all see the same picture. If it looks good on their displays, it should look the same on ours.

    We could simply give up and let everybody to follow their own "standards" and ignore SMPTE's for black and white levels as well as saturation and hue for their production, but we'd be stepping back into the dark days of unpredictable results. The fight for everyone meeting standards needs to be an ongoing one. Otherwise, we would have some houses violate reserved headroom space in the digital levels to extend picture information. The result was severe clipping on displays which are correctly calibrated. That house could then claim their discs have a more dynamic image. Our whites are whiter, our blacks are blacker, colors more vivid, and sound effects louder! Do we really want a world in which discs vary greatly in brightness and loudness? That would make it impossible to accurately recreate the appearance and sound as originally intended without recalibrating for each production house's own "standard."
     
  2. John Royster

    John Royster Screenwriter

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    Thank you Guy!

    Your explanation of "digital black - the one on the disc" and how that relates to the actual IRE/voltage output makes everything clear now.

    For me, my TV has seprate white leve/black level/color/tint/sharp per input so I set my DVD player to "darker" and calibrate accordingly. The white level/black levels stay with this input which is DVD only.
     
  3. John Stone

    John Stone Supporting Actor

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    Excellent post, Guy. This should be a "sticky" post.
     
  4. Jim Doolittle

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    Hi Guy and all,

    Yes, a great subject of confusion for many. I would like to clarify a few points about the different black levels, 0 IRE and 7.5 IRE.

    I've never heard it called NTSC verses the Japanese system, but rather NTSC verses component video. Quite simply, the dynamic range of all NTSC video is 7.5 IRE to 100IRE and the dynamic range of all component video, including HDTV is, 0mv (0 IRE) to 100mv (100 IRE). This indeed does result in a little bit more dynamic range for component video sources, granted, just a little bit.

    Both VE and AVIA have component video encoded on them, therefore they were encoded at 0 IRE. The black level switch on a DVD player will add set up (bring the black level up to 7.5 IRE) if the end user wishes to use "S" video or composite video outputs on the player, or if the display device only has 1 memory for brightness level, the end user could choose the 7.5 output level for component video so that it matches all his/her other sources.

    To simplify, if using component the player should be set to output 0 IRE. If using "S" or composite the player should be set to 7.5 IRE. This regardless of what the picture looks like after making a change. After making a change the display must be recalibrated.

    Now, not all players offer the choice and if your not sure which one to use, you will have to have a waveform monitor or a lot of time and knowledge to figure it out. The players that do offer a choice have chosen marketing terms to describe the difference, such as "dark", or "enhanced".

    I will share a problem I'm having as a result of a manufacturer not understanding this documented engineering choice in SMPTE 274M (1080i & P) and 296M (720P)(NTSC is documented in SMPTE RP170A). I have a few different HDTV set top boxes that are all routed through a switcher and then through an Extron CVC200 component to RGB transcoder and then to my display device. Since HDTV is component video the black level should be 0 IRE. Well on my Dish 6000 the black level at the component video outputs is set to 7.5 IRE. This is apparent when I'm watching any of the other 5 HDTV sources, and when I switch to the Dish 6000, the blacks are all washed out and I must recalibrate the displays black level if I want to continue to watch the program coming from the Dish 6000 box. As far as I'm concerned, this is unacceptable as I'm losing dynamic range and I'd have to do a lot of programming in order to automate this switch. Instead, I want the manufacturer to read the standards before they design the gear, am I asking too much!

    At CES I approached Dish Network's booth and located the top engineer (that's top engineer they had AT THE SHOW) and politely, in private, slowly tried to explain the mistake they had made. He replied that the Dish Network 6000 had been designed to the "Dish Network" standards, of course of which there are none. The SMPTE write the standards, not Dish Network! I returned the next day and spoke with him again (all of our conversations were very polite and he seemed interested) and handed him the documentation in hopes that he would get that to the proper people so that when the Dish 921 comes out it will hopefully be properly designed. Let's hope it makes it!

    Jim Doolittle
     
  5. Guy Kuo

    Guy Kuo Supporting Actor

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    Thanks, all. Jim, I applaud your trying to get a manufacturer to mend their ways. I refer to the difference in setup level as North American and Japaneses NTSC because Japanese equipment and production for NTSC has black at 0 IRE. Whereas we use 7.5 IRE here in North America. I wasn't thinking about the setup level in component. I think you'd agree that component doesn't always have the same setup from player to player model. In other words, just as you described regarding the Dish receiver, not all DVD players implement things as expected. That means one really must calibrate for the source. The problem on DVD doesn't end there. Despite the years of existing standardization, production isn't always striving to get things exactly right before encoding to disc. Thus, not all discs are coming out with black encoded at the proper digital 16.


    I would like to clarify, "Both VE and AVIA have component video encoded on them, therefore they were encoded at 0 IRE." because that might again confuse people into thinking that the discs themselves can specify an IRE level. As Jim, states the IRE level which comes out for black is determined by the player. Both discs properly encode black as digital 16 (the recommended level for black in the digital video realm). A properly working player outputs black from the discs at 7.5 IRE (if setup is 7.5) or at 0 IRE (if setup is zero).

    For those who are truely confused now, the video world, luminance doesn't run the full range from 0 to 255. On a computer digital graphics card, 0 is black and 255 is white. For digital video, not all bit levels are used for picture information.

    Digital levels 0 and 255 are reserved as special flags. Black is to be encoded at digital 16. The levels from 1 to 15 are reserved as slop for undershoot, but not actual visible material. That's how one can put "blacker than black" type signals into a disc. One really isn't supposed to use those levels, and some equipment simply won't pass anything in that range. On the upper end, there is also a range of values reserved for overshoot headroom. If the mastering or telecine isn't accurately done, black and white might end up a bit off from the standard levels recommmended. Discs are still showing up with variances. It's not a problem with display equipment, but a problem in production which needs to be corrected before encoding to disc.
     
  6. Jim Doolittle

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    Absolutely Guy,

    And to further confuse, here are the specs for component video levels in both the digital and analog domains.

    In the digital domain, component video black is supposed to be 16, and white is supposed to be 235. This is a particularly overlooked aspect of digital component design. As Guy points out, in the computer digital world, black is 0 and white is 255. When designing components that are destined to display pictures in an analog world, that headroom must be in place or it will adversly effect the picture.

    In the analog domain, we have black at 0mv and white at 700mv. We have headroom to 714mv and since the synchronization signals run all the way down to -300mv, we have headroom below the 0mv black level and this is how we can still show "blacker than black" in component video products that are properly designed.

    I hope this helps somebody!

    Jim Doolittle
     
  7. Brent Hutto

    Brent Hutto Supporting Actor

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    OK, this thread has been very educational and I think I've got it. Let me pose a question about a compromise for the sake of convenience...

    Since I use a component connection from my DVD player to the TV I've always set the DVD player to output black at 0 IRE. That means no "set-up" from the player in other words, right? I now have an HDTV cable box (Scientific Atlanta 3100HD) and when I use my DVD player-specific TV calibration settings the black level is obviously too high. I have no control over fact that that cable box is set to 7.5 IRE black so I'm tempted to change my DVD player output to match and re-calibrate. That way I don't have to change between calibration memories on the 36XBR800 every time I switch sources.

    For some reason, I've never been able to duplicate my best calibration, stored in the "Pro" memory, in any of the other three memories. I use the "Movie" one for normal TV viewing but when I've tried setting the DVD player to black at 7.5 IRE and running an Avia calibration in "Movie" it never ends up looking like what I got in "Pro". There's some hidden difference between the "Pro" and "Movie" modes on that Sony.

    So what would I be giving up by optimizing my "Pro" settings with Avia using 7.5 IRE black on my RP82? It sounds like once I get the proper calibration in that circumstance the result will be indistinguishable from the excellent calibration I have now with 0 IRE black. Or did I miss something?
     
  8. Doug_L

    Doug_L Stunt Coordinator

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    Thanks to Guy and Jim for the excellent explainations. I've got a follow-up question:

    Where does the Pluge pattern fit into this issue? I've always thought that on the Video Essentials disc the blacker-than-black bar was lower than 0 IRE - but now I'm not sure that this is even possible. So here's a few follow-up questions:

    1) What does the Pluge pattern really test?
    2) What are the relative values of the black background vs. the blacker-than-black bar?
    3) Will a pluge pattern work with enhanced black level (or the like) turned-on?
    4) How can I see the pluge pattern if I'm using S-Video cables?

    Thanks - I'm pretty confused right now.
     
  9. Guy Kuo

    Guy Kuo Supporting Actor

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    ... I've always thought that on the Video Essentials disc the blacker-than-black bar was lower than 0 IRE .....

    That goes with the incorrect assumption that the signals are hard coded so they always come out of the player at the same IRE level. That simply isn't so, but people get stuck right there. If you work back through the previous posts and understand the relationship between digital black and how that is "translated" into voltage and IRE by the player, then you will realize by now that the blacker-than-black bar isn't at a specific IRE level unless the state of the DVD player is also known. The "PLUGE" pattern encodes as bars which are at 4 IRE below black, black, and 4 IRE above black. but is about 4 IRE below black (whatever IRE black happens to be set on the player.) If the player is set to have black at 7.5 IRE then the blacker than black bar ends up at about 3.5 IRE. If the player is set to have black at 0 IRE the blacker than black bar ends up at -4 IRE.

    As for questions 3 and 4. It depends. The player must pass the blacker than black signal and also is able to produce video signals below 0 IRE for you to see it with enhanced black turned on (0 IRE as black). Some won't pass blacker than black. Some won't generate signals below 0 IRE. You have to experiment a bit to find out if your particular system allows you to use the blacker than black bar of a PLUGE signal. If you turn the display's brightness control intentionally too far up and make the black background glow and don't see a bar which is darker than the now glowing "black" background, then your system is not passing or reproducing the blacker than black bar.

    On the other hand AVIA uses two moving bars which are respectively 1 and 2 IRE above black so they should always pass through equipment unless the equipment is clipping shadow details. This was done to avoid reliance on blacker than black signals which may or may not be passed properly. You can use PLUGE signals instead, but your equipment should pass blacker than black signals if you want to use PLUGE signals instead of AVIA "Black Bars."
     
  10. JohnnyG

    JohnnyG Screenwriter

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    Brent, most displays maintain seperate internal 'offsets' for 480p and 1080i input, so using your 480p settings as determined by your DVD player might be very wrong for 1080i. It all depends on how accurately the offsets were set-up at the factory. Without 1080i test patterns, the best you can do is eyeball it.
     
  11. Rich H

    Rich H Second Unit

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    I had to revive this thread because, although I've read the whole thing I don't quite see the distinct answer I'm looking for. I hope an expert can help out.

    I own a Panasonic plasma and the Panasonic RP-91 DVD player (which gives me the "lighter" "darker" black level setting option).

    My situation is that I often watch movies on my S-Video input, and sometimes on my component input.
    I do this based upon which input will offer the preferable picture quality per DVD (my S-Video input is a bit cleaner looking, my component input more color rich etc.)

    Obviously, then, I'd like picture optimized for both settings. My questions:

    1. Is there ultimately any discernable picture quality advantage to using the darker black level setting (0 IRE) for my component input, vs the lighter (7.5 IRE) setting?

    2. What are the picture quality consequences for using the "darker" setting when viewing on my S-Video input?

    It would be most convienient for me to leave the player in the "lighter" setting that suits my S-Video viewing, and simply calibrate my S-Video/Component inputs to match. However, I'm wondering what I give up PQ-wise on my component input if I do that (that last bit of dynamic range?).

    You know us videophiles, we want the best of everything. [​IMG]

    Thanks for any info.
     
  12. JohnnyG

    JohnnyG Screenwriter

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  13. Rich H

    Rich H Second Unit

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    "Ultimately? Yes. In practice? No."

    In otherwords, the sharp eyed videophile is likely able to see the difference or not? Do I assume correctly: Not?
     
  14. Rich H

    Rich H Second Unit

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    I just spent quite a while comparing the lighter vs darker settings. Hear is what I saw:

    When my RP-91 DVD player is set to it's Darker mode (black output 0 IRE...which is how I normally have the player set), my COMPONENT signal looks rich and smooth, with deep blacks yet quite good shadow detail. However, my S-Video image looses a *bit* of detail in dark areas that I see in the COMPONENT signal, and I can't seem to quite recover the detail even when upping the brightness.

    Now, when I switch my DVD player to the Lighter mode (outputing black as 7.5 IRE), I seem to recover that shadow detail on the S-Video signal, giving me the full range of blacks that I get from the component signal.

    HOWEVER, on the Lighter setting (S-VIDEO) there is a distinct loss of richness, dimensionality and punch to the image. Now, I'm quite familar with the effects of turning brightness lower and how it can seemingly make for richer colors and a higher-contrast look. But that doesn't seem to be the case here. When I calibrate the brightness levels on the Lighter setting (downward) to match the look of the Darker setting, I can't get the same image quality. It looks flatter, lower contrast and less colorful. Even pushing up the contrast fails to recreate the image I get on the Darker settings, in terms of color dimensionality and three-dimensional punch.

    Can anyone offer explanations as to what I'm seeing, and why the Darker setting would seemingly produce better image quality on my S-VIDEO input?

    Thanks...
     
  15. JohnnyG

    JohnnyG Screenwriter

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    Rich, just setting the player to the 'Lighter' setting is not enough. You have to match the TV to the black level of the DVD player. So, in each case above, you have to run a PLUGE or other similar pattern and set the TVs Brightness control appropriately. Only then is it a 'fair fight'. You are likely to conclude that there is no visible difference.
     
  16. Rich H

    Rich H Second Unit

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    John,

    I suppose I still haven't been clear enough. I know that simply setting the player to "lighter" is not enough. I have made adjustments to re-calibrate my display's brightness (downward). That has been my whole point.
    Even when I do so, the picture does not match the quality of the darker setting.

    I've not used the pluge, but have used freezed reference scenes from a DVD, in which there is a range of shadow detail all the way to black. When I switch my DVD player to Lighter setting, I redial the display's brightness down watching particular shadow details until they are just visible - exactly matching their visibility on the darker setting. Yet even when the black levels seem to exactly match on the darker/lighter settings, the rest of the color spectrum and mid-tones on the lighter setting seem to have a flatness and lack of punch compared to the darker setting.

    It's almost like a change of gamma when comparing the darker/lighter settings. Could that be a clue?

    Thanks...anyone? ....anyone?...Bueller?
     

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