Brilliant!
One of the biggest changes in a long time in Home Theater Video has been the recent introduction of High Dynamic Range (HDR) content, and equipment which can display it properly. HDR allows elements in scenes to be much MUCH brighter than with prior home video technologies (in comparison to other portions of the same image) -- excellent for sparks, glints, flashes, direct views of light sources, and details in bright objects such as clouds in bright sunlight. However, there is plenty of Standard Dynamic Range (SDR) content out there which you will still want to look its best. And the foundation for understanding what HDR brings to the table begins with an understanding of the proper rendering of SDR Video.
In my prior post on Blacker Than Black Video, I introduced the concept of the Headroom and Foot Room portions within the video encoding. The "Peak White" pixels found in SDR Video simply reflect the Headroom authored into that video content. Whereas HDR and SDR Video are quite similar in their treatment of Blacker Than Black pixels, they differ dramatically in how they handle these brightest pixels.
In this post we'll focus on setting up your TV to render Peak Whites properly whenever you are viewing SDR Video content: I.e., what you SHOULD see and what you SHOULDN'T see. So break out the sunscreen and get ready for a few Bright ideas! COOLNESS NOTE: Shades are optional.
To begin, let's try to clear up some confusion surrounding the naming of the controls used to adjust this stuff. In the post on Blacker Than Black Video mentioned above, I said the "Brightness" control was used to set Black levels -- memorable because both Brightness and Black begin with the letter "B" -- and the "Contrast" control (sometimes called "Picture" in newer TVs) was used to set White levels.
So, umm, why the HECK did they use the name BRIGHTNESS for a control used to adjust the DARKEST parts of the image? And why use Contrast (a term which means the comparison of brightest vs darkest) instead of, oh, White Level?
This all goes back to the Analog video circuitry used in the first, picture tube TVs. What that circuitry allowed was control over the average level of light output from the picture tube (the amplification of the Analog video signal) and the range between the darkest and brightest portions of the image.
The Brightness control set that average level of light output -- thus the name "Brightness". However, the main thing you wanted to achieve with it was assurance that Black content was rendered as Black (no light output) while content just above Black remained visible. That is, if you set the average light output of the picture tube too high, you would not be able to achieve a nice Black when called for. And if you set it too low you would not be able to distinguish the near-blacks above Black.
Meanwhile the Contrast control set the range of light levels the tube would render -- from darkest to lightest -- thus the name "Contrast". The Contrast setting allowed you to make sure there was enough visible gradation of light levels so you could see all the available gray scale details and also get a pleasing "white" appearance to the brightest portions of scenes. But if you pushed Contrast too far, you would get "Blooming": In the brightest portions of the image, the beam of electrons lighting up the phosphors on the inside of the picture tube glass would light up too big an area of phosphors -- too large a spot size. And so your ability to see details would drop dramatically in those bright portions of the image.
Thus, from a practical point of view, Brightness established the starting point of the gray scale and Contrast established a range of brighter imagery visible above that.
Now if you think about it, that means these two controls will INTERACT whenever you are trying to set correct levels for both Black and White. First of all, if you raise the "black" starting point (by setting Brightness higher) that ALSO raises the "white" ending point -- because the Contrast setting is still trying to maintain the same range above that initial starting point. And if you lower the "white" ending point (by setting Contrast lower) that ALSO alters the "black" starting point because BOTH ends of the light output change when you compress or expand the contrast range.
But the Brightness setting still had MORE of the desired impact on setting Black levels than it had on messing up White levels. And similarly, the Contrast setting had MORE of the desired impact on setting White levels than it had on messing up Black levels. Why? Because the natural "response curve" of picture tube TV is "non-linear". It is more like an exponential curve starting off slowly at the black end and rising fastest at the white end -- the so-called "Gamma curve response", which is an inherent feature of picture tube TVs.
TECHNICAL NOTE: This "Gamma" response was not something particularly desirable in picture tube TVs. It was just a fact of life. The manufacturers could have made picture tube TVs with linear response (or more nearly linear), but the extra electronics involved would have significantly raised the price of the TVs -- slowing down the adoption of this newfangled technology. So the Marketing guys put their foot down and said leave the dang TVs alone. INSTEAD, spend money, as necessary, to create content which ASSUMES it will be viewed on home TVs with this expected Gamma curve response. E.g., design your TV cameras to produce this expected response curve, even if that made the cameras more expensive.
The result was, folks setting up these TVs simply got used to the fact Brightness was the control to reach for when you needed to fix Black levels and Contrast was the control to reach for when you needed to fix White levels. Then you ALSO had to deal with the interaction BETWEEN these two controls, by going back and forth between them a few times looking for the sweet spot, combo of the two which yielded the best result for both Blacks and Whites.
When Digital TVs came along, the game changed. The Digital video processing in the TVs could now offer independent -- non-interacting -- controls for establishing proper Black and White levels. (In reality, there was still some lesser degree of interaction between such controls due to how the Digital Video processing was implemented, but to much less a degree than with the old, Analog controls).
However by this point, the idea Black levels should be set with a Brightness control and White levels should be set with a Contrast control had become too ingrained, and so the "Brightness" and "Contrast" NAMES for these controls were retained, even though they no longer served the original function which FIT those names in Analog TVs. (There was a halfhearted attempt at addressing this confusion with the use of "Picture" instead of "Contrast" in some TVs.)
Meanwhile, the natural response curve for fixed pixel, digital display technologies (e.g., Plasma, LCD-backlit, and OLED panels, for example), no longer bore any relationship to the Gamma curve of traditional, picture tube TVs. Indeed, the natural response curves were all over the place! However, there was so much CONTENT out there already produced for traditional TVs (along with infrastructure such as cameras and editing equipment built to produce content for those TVs), the TV makers simply decided they would use some of the Digital Video processing power in their new TVs to EMULATE those older, picture tube TVs. That is, the new TVs would apply "Gamma Correction" to insure a response curve which matched the prior generation of TVs.
So now you had "Brightness" for setting the Black levels, "Contrast" for setting the White levels, and "Gamma" for adjusting the shape of the response curve between Black and White. (I won't be discussing Gamma further in this post.)
For some technologies, the Contrast control could even be divorced (to a degree) from the setting of the maximum light output of the display. That is, Contrast would control the number of distinguishable "steps" between Black and White, but another control could alter how much light output was produced by White. For example, the Backlight control in LCD-backlit panels, or the OLED Light control in OLED panels, or the Iris setting or bulb brightness choice in projectors.
Getting the light output correct for White is important because the brain will interpret too little light as a dingy grayish, instead of White.
The unofficial term "Nit" has come into vogue as a shorthand for the official unit of Luminance (i.e., light output), which rejoices in the name Candelas per Square Meter (cd/m^2). So 1 Nit = 1 cd/m^2.
TECHNICAL NOTE: A Candela is roughly the light output of a single, typical, wax candle, viewed from one direction. Compare the older name, Candlepower. There is, of course, a more precise, technical definition of the Candela, but this will suffice. Smear that light output over a surface 1 meter square and you get a Candela/Square Meter, or 1 Nit of light output. That is, the entire square meter combines to produce this 1 Candela of Luminance. Each portion of the square meter produces a proportional fraction of 1 Candela.
If you were setting up your TV for SDR video using an optical light sensor, a typical setup for viewing in a dimly lit room would be to target 100 Nits light output for Reference White. In a completely blackened room, that might even be too much light. Something closer to 50 Nits might be more comfortable to the eye. In a room with more ambient light, you might target 150 Nits. A typical, SDR, LCD-backlit TV might have a max light output capability around 200 Nits.
TECHNICAL NOTE: HDR TVs are designed with a much higher, max light output. For example 600-700 Nits for OLED panels, or several THOUSAND Nits for LCD-backlit panels. However, for some of these technologies, the max light output diminishes dramatically with the amount of the screen you are trying to light up like that. The max light output for a full screen White may be significantly lower than the max light output for a smaller patch of White. This is not as big a problem as you might expect. Lighting up a whole screen to such high levels would be painful to the eye. So better quality HDR content is typically authored to limit the percentage of the screen calling for such high light output at any given moment. Think a jagged bolt of lighting across a portion of the screen as opposed to the whole screen flashing at that same brightness.
If you are NOT using an optical light sensor to set up your TV, the goal is to adjust the light output for Reference White so you see it as a pleasing, "white", with no hint of dingy gray -- but at a level that's not uncomfortably bright when viewing normal content. As noted above, the adjustment here will vary depending on the amount of ambient light in your room during normal viewing. (The ability of the eye to distinguish contrast -- the range between darks and lights -- diminishes as the room ambient light increases. So a brighter "White" will be less jarring in a room that's already lit.)
The standard recommendation for Best Quality Viewing is your room should be dimly lit -- NOT blacked out. This applies to critical viewing of content you really care about. For casual viewing -- the TV running in the background while you do other stuff for example -- there's no problem having the room more brightly lit, so long as you have set up an alternate set of video settings in your TV for watching that way: Settings which are chosen to work well with that brighter room lighting. You might not be able to see quite as much detail or refinement in the image as with your settings for dimly lit room viewing, but this is only casual viewing, so no problem!
I've referred several times above to "Reference White". This goes back again to my previous post on Blacker Than Black Video where I laid out the the range of pixel "Luma" values for SDR video.
TECHNICAL NOTE: As in that post, I'm going to use 8-bit values for describing this stuff here. If you are thinking in terms of 10-bit or 12-bit SDR video, the concepts are exactly the same, although of course the actual numbers are different.
To summarize, SDR Digital Video for home theater uses a value between 0 and 255 to denote the gray scale brightness (Luma) of a pixel. (Two additional values describe how to color that pixel.) In this encoding, Luma 16 is defined as Black. Luma 235 is defined as Reference White. The range from Luma 1-15 denotes the Blacker Than Black pixels. And the range from Luma 236-254 denotes the Peak White pixels. (0 and 255 are reserved.)
Pixels authored as Blacker Than Black are NOT intended to be seen. They are retained to improve the quality of processing of the video (as discussed in that prior post).
Pixels authored in the Peak Whites ALSO improve the quality of the video processing, but unlike the Blacker Than Black pixels, the Peak White pixels are OPTIONALLY available to be seen!
That is, when the video is captured and edited, the filmmakers strive to insure that all CRITICAL, bright details are visible as Reference White or below. However, additional glints and highlights can be allowed in the content as Peak Whites. Setting up a TV to show these can add nuance in the bright details, but it is also OK if they are NOT distinguishable from Reference White on a given TV.
Why do this? Because any TV, regardless of its technology, will have a maximum light output it can produce. If you adjust the TV so that the Peak Whites are distinguishable, that means you have, necessarily, adjusted Reference White to be DARKER than that. And THAT might be too dark to produce a pleasing White from Reference White content!
This would be particularly relevant to projector owners, since projectors are typically light-challenged in their output (made up for by the fact they can produce that light output over a very big projection screen, which still keeps the overall image looking great).
Some lower quality TVs may also have cut corners in their video processing such that they can not render the full range of the video encoding -- perhaps for lack of processing power. I.e., to keep the costs down, they may clip the Peak Whites. (And typically, also the Blacker Than Black values!) It is acceptable for them to do this (albeit not ideal) so long as they can still do a proper job of rendering the video all the way up to Reference White.
The Factory Default settings for most TVs have Contrast set WAY too high. This is one of the Torch Mode settings I discussed in my prior post on Extinguishing Torch Mode Settings in Your New TV. If the TV has a separate setting for controlling overall, max light output, that, too, is likely set too high.
What do I mean by too high? The typical Rule of Thumb when setting up a new TV -- i.e., when working to get rid of its Torch Mode Settings -- is to expect you'll have to start by dropping Contrast to HALF of the Factory Default setting!
So what's the problem? That is, why NOT just use that Factory setting? Well first of all, Contrast set that high will likely make the screen painfully bright to watch in a dimly lit room. Very tiring, too.
But even more important, Contrast too high will likely clip your Peak White pixels!
Back in the old days, with a picture tube TV, you would have seen this as Blooming -- inability to see fine details in brighter portions of the image because the electron beam is too strong (producing too big of a dot size in the screen's phosphors).
You won't see Blooming in modern, Digital TVs, but you WILL see the clipping of brighter pixels -- the inability to distinguish them from nearby pixels which SHOULD be noticeably darker. Indeed the Factory Default Contrast setting may even be clipping to a degree that you can't even see up to Reference White!
Think of a bright, sunlight cloud. Will you see the swirls and other details in the cloud, or does the cloud look flat and uninteresting, like looking at a whiteboard?
What's even worse is that TVs will often clip sooner in just some colors. So for example, if Blues and Greens clip at a lower level than Reds, your brightest whites will develop a distinct, reddish tinge. The increased Red component in the brightest pixels is showing through, but the Blue and Green components have run out of gas -- just can't get any brighter.
Even more so than when setting Black levels, it is CRITICAL when setting White levels that you've first taken steps to extinguish the Torch Mode settings in your TV. In particular, that you've chosen an appropriate "Picture Mode" for quality viewing, and turned off the various video "enhancement" features manufacturers love to lard into their TVs.
In addition, setting Black and White levels (with the Brightness and Contrast controls) should be tackled at the same time. Even though modern Digital TVs don't have nearly the amount of interaction between these controls as with older, picture tube TVs, there will still be some minor amount of interaction. So plan on going back and forth a few times in your quest to find the best COMBO of Brightness and Contrast settings for your TV, in your viewing conditions.
So suppose you've prepped things properly -- i.e., have your room set up the way you want to use it for critical viewing (dimly lit!), and have already taken steps to extinguish the Torch Mode settings in your TV which might get in the way (in particular, finding the correct choice of Picture Mode). How do you now go about adjusting White levels with the Contrast control?
First, you will want to use test charts from a calibration disc. If you try to set this stuff up using real movie or TV show content you will likely end up just frustrating yourself. There's no consistency between such programs, and some of them may even have authoring flaws.
In my post on Blacker Than Black Video I discussed using calibration charts from, "Spears & Munsil HD Benchmark, 2nd Edition", to set Black levels using the Brightness control. There are other calibration discs you might use, but lets stick with that one for this post, too. (The disc is linked in my prior post on Calibration Discs.)
Launch that disc and, in the column on the left, select Advanced Video. Then in the tabs across the top select Setup. Then in the list of charts that appears below, select Contrast Y.
What you will see is two strips of bright, white vertical bars across the top and bottom of the chart, with a gray scale "ramp" in between, which is brightest in the center and dark to either side.
Concentrating on the two strips of bright, white vertical bars, note that they have numbers in the range 231 to 253. Each of those bars represent Luma of that value. They are displayed against a background of Luma 254. The strip on the top and the strip on the bottom are simply reversed as to which end of the strip has the brighter bars.
Now the FIRST thing you need to do is verify that the Peak White values are actually making it through to your display screen AT ALL. The way to do that is to lower the Contrast control (on your TV) -- a bunch. At some lower value ALL of the bars up through 253 should become visible against the background of 254.
If you can find no low setting of Contrast which makes all those bars appear then your Whites are being clipped. Typically this would mean that the highest number bar you can distinguish -- even when looking closely -- will be the one numbered 234 -- i.e., one step BELOW Reference White, which you'll recall is 235. That's because all of the HIGHER Luma values -- from 236 to 254 -- are being clipped to Reference White -- 235. You can't distinguish the 235 bar itself because the background it is displayed against is now also showing as if it was that same Luma value of 235.
This clipping may be happening in your TV, in your AVR, or in your source device (i.e., the disc player you are using to play the Spears & Munsil disc). Typically what's going on here is one or more of your devices has fallen into the Extended/Enhanced Video Range Trap. See the details in my post on Blacker Than Black Video.
Finding the culprit device(s) and correcting their settings can be a nuisance, largely because manufacturers have not settled on how to name this stuff. If you see settings related to Extended/Enhanced Video or SuperWhite you are likely on the right track. In the OPPO disc players, the only way you can get this wrong is to set their video output to RGB PC Level, so don't use that choice.
In TVs, AVRs, and some source devices the critical setting may be labeled as something to do with "black levels" and it will offer only TWO choices. the problem is, those two choices could be named just about anything. For example Low vs. High. And what's worse, the MEANING of Low and High may be REVERSED in a TV compared to an AVR or source device.
This has long been, and remains, a MAJOR point of confusion in setting up Home Theater gear, and it may be you'll have to seek help from other owners of your gear if you don't stumble upon the answer.
For reference, in the OPPO disc players, setting video output to any YCbCr choice, with all their Picture Adjustment controls left at factory defaults, insures that the full range of Blacker Than Black and Peak White values will be sent out from the player. That's then a comparison point for finding the right settings in your other gear.
In the Sony PS3 you can achieve this be setting video output to YCbCr and ALSO setting SuperWhite to ON.
A variant on this problem is if you can only get SOME of the Peak White bright bars to become visible no matter how much you lower Contrast in your TV. For example you may be able to distinguish bars up to 245 against the background of brighter white, but none of the bars from 246-253.
This is almost always due to the TV cutting corners. It has reduced the demands on its video processor by clipping PART of the Peak Whites. In such cases, there may be no setting in your TV which will cure this fault -- it is built into the design of the TV. Check with other owners of your TV to see if they have any help to offer. A TV designed like this is definitely better than a TV that clips ALL of the Peak Whites, but there are plenty of TVs out there which don't cut this corner.
TECHNICAL NOTE: When testing whether Peak Whites are getting through to your screen you MUST use the Contrast control IN THE TV. If your source device (for example) offers its own Contrast control, lowering that one may make the bars visible, but that's a bogus result! All you've done is LOWER the Luma values for those bars as sent out on the HDMI cable to your TV. I.e., they are no longer Peak White values, because you've told your source to make them darker. In general when doing setup like this you want to set your source (and your AVR if it offers video adjustments) to do NOTHING to the video: To send out the calibration chart content just as it comes off the disc. Do all adjustments using the controls in your TV.
So now let's suppose you've managed to confirm your gear really is passing Peak White values -- all the way up to Luma 253 -- to your display screen. You may have had to fix some video format settings in your gear to get here, but let's assume you've accomplished that.
What's next?
Well now you want to start RAISING Contrast in your TV until the higher numbered Peak White bars (near Luma 253) start to VANISH. This indicates you have found the Contrast setting which starts to clip those highest, Peak Whites.
Now adjust Contrast until you find the highest setting which still leaves the Luma 253 bar visible. Note that it will likely be JUST BARELY visible against the background of 254.
TECHNICAL NOTE: Your eye is more sensitive to gray scale (and less sensitive to colors) towards the edges of your vision, so try turning your head to check for that 253 bar out of the side of your eye. Go up and down with the Contrast control a few times to verify the correct setting.
OK! Are we done? Nope!
The next step is to check that you haven't clipped any of the COLORS -- Red, Green, or Blue. You see if any one or two of those is still increasing up to 253, the white bar for 253 will reflect that, but you still may have clipping of the color itself below that.
To check for this, move on in this set of Spears & Munsil Charts. NOTE: On this disc, Right Arrow moves you to the next chart without having to first return to the list of charts. Left Arrow moves you back a chart. Enter button returns you to the list of charts.
The next 5 charts in order are Contrast Cb, Contrast Cr, Contrast Red, Contrast Green, and Contrast Blue. Check each of them to see if you can distinguish up to Luma 253 on those charts as well. If NOT, reduce Contrast a bit more to get there.
Now you have your candidate setting for Contrast. There are still a few more checks to do before declaring victory!
First in that same page of charts (Advanced Video > Setup), find the chart labeled Crossed Steps White. This chart shows large vertical bars stepping from Black to Reference White. (There are two strips, reversed in direction of the steps). On this chart look for two things. First are the bars a "neutral gray"? If you see coloration of the bars -- perhaps a blue tint, or perhaps a red tint -- odds are your Color Temperature is not set correctly in the TV.
TECHNICAL NOTE: The Color Temperature setting adjusts the color bias of "white" towards blue or red in accordance with the "temperature" of a light source producing that white. For example a very hot light source would have a bluish tint to white. A cooler light source would have a reddish tint to white. The standard Color Temperature for Home Theater content is 6,500 degrees Kelvin -- or more precisely, a technical value denoted "D65". But it is easy to fall into the trap of setting the TV to the wrong Color Temperature. For example, the Torch Mode, Factory Default settings in TVs typically include a Picture Mode choice which has the Color Temperature too blueish, because that tricks the eye into thinking the picture is brighter. Odds are, if you've selected the correct Picture Mode to begin with as part of extinguishing the Torch Mode settings, it will come with the proper Color Temperature pre-selected. but if you see coloration of the gray scale step bars (in the calibration chart just described), that's an indication you need to go hunt down and correct your Color Temperature setting.
The other thing you want to check on this chart is whether the brightest bar (Reference White) is, in fact, a pleasing "white" to your eye and not dingy gray. Remember, this is something you want to check while your room is lit as you will use it for critical viewing (and after giving your eyes a bit to adapt).
If you are not sure about this result, try raising Contrast a few steps to see if the "grayness" of that bar changes dramatically towards "white". If so, your display may be light-challenged in its output. Consider a compromise setting -- with higher Contrast -- which clips SOME of the Peak White values but also gets you a more pleasing Reference White. If your display is REALLY light challenged -- as with some projectors -- you may have to forego the Peak Whites altogether. Just make sure you don't raise Contrast so much that you can no longer distinguish Reference White (Luma 235) against the background of Luma 254.
Next go back to the list of charts, scroll all the way up to Setup in the tabs across the top, and then move right to select Evaluation. A new set of charts will now be shown.
In those charts, look at both Dynamic Range RGB and Dynamic Range YCbCr. The RGB chart shows bars of Luma 231-253 in strips of Red, Green, and Blue. The YCbCr chart shows the same for strips of gray scale (Y) and the two color difference components (Cb an Cr) which you will see as Blue and Red.
What you are looking for in these two charts is whether the difficulty of seeing the higher Luma values appears to increase at about the same rate for all 3 strips. If one strip appears to become harder to see in the high Luma bars before the others, that too may indicate clipping. Try tweaking Contrast a step either way to adjust.
The white bars strip in the Dynamic Range YCbCr chart is also a good check for coloration of higher Luma white values. The high Luma bars in that white strip should remain a neutral white -- not tinting towards Red or Blue or Green.
Now you are really close to done. Just a few more double-check steps.
Back on the Setup page of charts, return to the 6 Contrast charts, this time focussing on the ramp shown in the middle between the two strips of bars. What you are looking for here is whether there's any sign of stepping or banding in that ramp. This can happen if the implementation of the Contrast control in the TV lacks some precision. If you see stepping or banding, try tweaking Contrast up or down a step or two to see if you can clear that up. You many need to pick a compromise setting which minimizes clipping while also minimizing banding.
Finally, go back to the Evaluation set of charts and look at the Field 100% chart. This is a full screen of Reference White. It should look a pleasing "white", not dingy gray. But is it TOO bright, or perhaps not bright enough? If you need to adjust the light output without screwing up the good results of the Contrast setting, you may be able to use an overall light output setting in your display -- such as Backlight in an LCD-backlit display, or OLED Light in an OLED display, or the iris or lamp low/high setting in a projector.
With an optical sensor, you could target a specific Nits level when doing this, but the REAL test is how you see it by eye. Is it "white" enough for your taste? Is it not so bright it is hard to look at without squinting?
If you do have an overall light output setting to use, you may want to revisit it after viewing some real, SDR content.
TECHNICAL NOTE: When adjusting settings for SDR content viewing on an HDR display, odds are you will need to lower the "overall light output" setting significantly below its maximum setting. That maximum setting is meant to service HDR content. Typically, these displays will remember settings separately for each type of content. So the changes you make for great viewing of SDR content won't screw up your HDR viewing, and vice versa.
It's taken a fair number of paragraphs to describe all this, but if you've followed along, by this point it should be pretty obvious there are only a couple things to check in each of these charts. So now I recommend you cycle through the set of charts, described above, and re-confirm everything is still as desired.
While you are at it, now also go recheck your Black Levels (Brightness setting). If you've made big changes to Contrast, you may need to tweak Brightness again. Go back and forth a couple times to find the sweet spot combo of settings which works best for BOTH Black Levels and White Levels.
Then go watch some SDR content! If your display was clipping Peak Whites beforehand, I think you'll be DELIGHTED at the new stuff you can now see in brighter portions of the imagery.
Proper Peak Whites AND Proper Near Blacks are two of the most important of the numerous Holy Grails of Home Theater Video. Proper Gamma ties them together -- but that's a subject for another post.
For now, step out into the Bright lights, and enjoy!
--Bob