One of the Holy Grails of Home Theater (an avocation clearly overstocked with the darn things) is achieving the proper display of near-Black details in your video. This of course starts with the proper display of Black itself!
It should be EASY, right? As I detailed in my post on Digital Video, every format for Digital Video DEFINES a particular pixel value as representing "Black". All the TV has to do is make that pixel, well, Black! No light output. And uh, brighter pixels should be brighter than that. Of course your particular TV might not be able to turn a pixel TRULY Black. But that's a detail. You get it as black as you can.
Then with a wave of my hand (something oft accompanied in the teaching game with a sotto voce, "Step 2: A Miracle Occurs!") I mentioned there is ALSO a portion of the video data range reserved for describing pixels as, "Blacker Than Black". Um, say WHAT?
Well, Bunky, it's time to get dark. I mean REALLY dark. So slip into your most Goth outfit, put on that sombre music, and lower the lights. For we are about to encounter Blacker Than Black pixels, and learn what to do with them. And what NOT to do with them!
It begins with Signal Processing. Not even the Digital Signal Processing I've mentioned in a few posts already. No, that came later. This is ANALOG Signal Processing. And one of the things discovered early on when man first sought to manipulate Analog recordings of real world events is that "boundaries" cause problems.
Things in the real world are continuous and, for the most part, unconstrained by boundaries. You've recorded a low volume sound? Well there's a volume even lower than that. And there's a volume in between those two, and so on. This continuous and unconstrained nature actually SIMPLIFIES the math involved in manipulating that recording. But what if there's a wall blocking the sound? Now things start to get complicated; reflections and such. And the same SORT of complexity arises in the processing math if you constrain the recording you are processing in time (it starts abruptly HERE and ends abruptly THERE) or range (it gets no SOFTER than this nor any LOUDER than that).
And indeed a whole body of Practical Art was developed around massaging signals BEFORE they were processed; to ensure sufficiently simple math could be applied in the processing without producing unwanted artifacts. For example, raise the volume slowly at the start and lower it slowly before the end. This masks the fact the signal was actually recorded with an abrupt cutoff at the beginning and at the end.
In the case of range, the trick was to allow for "headroom" above the high end of the recording level and "footroom" below the low end. That is you recorded -- and PRESERVED -- signal levels both above and below the range of levels you were really interested in. And you did this just so you could process those INTERESTING levels (in between) without problems. I.e., without requiring fancier math, implemented in more expensive electronics!
As Analog Signal Processing evolved into Digital Signal Processing, so did the science of Information Theory: Showing the "real world" problems, and their "practical" solutions all truly came out of the math itself. It wasn't a matter of the quality of the equipment or the skill of the recording engineers. Certain rules and techniques were NECESSARY to avoid problems cropping up in Signal Processing.
The concept of Blacker Than Black video dates back to the dawn of TV. The old Cathode Ray Tube (CRT) TVs steered a beam of electrons across the inside surface of the glass Picture Tube. The electrons would cause phosphors coating the inside surface of the glass to light up according the level of the beam.
But it took a certain strength of beam to initiate that, and you didn't want it to be willy-nilly with some phosphors beginning to light up while others stayed dark.
So the TVs were designed to expect a "Setup" level of voltage, above which the phosphors would light reliably. The Setup level, of course, defined Black. Voltages above the Setup level produced increasingly brighter grays leading to white.
Meanwhile the electron beam also had to be repositioned from time to time, such as from the end of one line to the beginning of the next, or from the end of the last line at the bottom of the image to the start of the first line at the top of the next image. You needed to be sure the beam was not lighting up the phosphors while it was retracing its position like that! So a lower level signal, called Blanking would be applied during these period of repositioning -- called the Blanking Intervals. The idea was to make sure the Blanking signal was low enough you could be guaranteed no phosphors would light up.
In the NTSC, Analog TV standard used in the US, if you divided the signal from Blanking up to Peak White into 100 steps, the Setup level -- meaning Black -- would be at 7.5 steps. (Relative values are used to describe this stuff because the actual electrical signal could be different voltages depending on how it was amplified at any give stage in the video signal path.)
The term IRE is used to refer to these steps -- for the Institute of Radio Engineers. So Blanking is 0 IRE (often, but not always, zero volts), Peak White is 100 IRE, and Black is 7.5 IRE.
TECHNICAL NOTE: There are also control parts of the video signal, such as the "Sync Pulses". These are placed 40 IRE *BELOW* Blanking to make them easy to distinguish. So if Blanking is zero volts, the sync pulse would be a negative voltage. Thus the full range of the signal from Sync to Peak White is 140 IRE. Again, relative values because the actual voltages depend on the amplification of the electrical signal.
Now if you are guessing ahead, you may have already figured out the portion of the video signal BETWEEN 0 IRE and 7.5 IRE is the Blacker Than Black pixels! And that's precisely right. It is the "footroom" of the Analog video signal.
This allows for some variation -- tolerances -- in the video cameras and the TVs. That is the video signal also preserves light levels captured by the camera in the range from 0 to 7.5 IRE. Of course IDEALLY the camera is capturing "Black" -- i.e., the level the video engineer wants the audience to see as Black -- at 7.5 IRE, and the TV is also rendering 7.5 IRE as true Black. But if there's a little bit of variation between cameras and between TVs the image doesn't fall apart, because there's imagery capture "below black" (as well as "above black", of course) to accommodate such variations.
This also means the video can be PROCESSED more cleanly. Suppose you want to overlay text on top of your video broadcast -- perhaps advertising "film at 11:00!" The character generator will produce letters with black borders to make them easier to read. Merging that with the real TV program is a form of processing, and "getting the edges right" for that text is substantially easier if the processing is allowed to float around a bit either side of black. Which means the underlying video ALSO needs that footroom present.
Getting all this set up correctly -- in cameras, and in TVs, and in intervening video transmission and processing stages -- required test equipment. Something that could generate standard level, Analog video signals you could rely upon for adjustment.
In the case of TVs, this was called Picture Line Up Generation Equipment -- or PLUGE, for short. The manufacture would use a PLUGE device at the adjustment stage in its assembly line. A repair technician would use a PLUGE device when fixing a broken TV or correcting a miss-adjusted TV. The PLUGE signal would produce a known pattern of black and near-black on the TV screen, and the electronics in the TV could then be adjusted until that displayed correctly.
Repair technicians and professional display calibrators still use signal generators today: Much more sophisticated than the original PLUGE devices, and now producing DIGITAL Video output instead of Analog.
But most Home Theater enthusiasts will instead encounter PLUGE patterns as test charts found on a calibration disc, such as the "Spears & Munsil, 2nd Edition", Blu-ray, disc I linked in my post on Calibration Discs.
Different calibration discs will each have their own take on the best way to present such a test chart, but they all allow you to verify your settings are rendering Black pixels as Black (or as close to Black as your TV can produce), AND that near-Black pixels (brighter than Black) are indeed visible as they are supposed to be, AND that Blacker Than Black pixels are . . . . what?
That's the crux of it! What are you SUPPOSED to do with Blacker Than Black pixels? The answer is, two things:
- First, make sure they are PRESENT. I.e., that the Blacker Than Black content in the test chart is actually making it through to your TV, and
- Second, make sure they are uniformly BLACK. That is, all the Blacker Than Black pixel values should be indistinguishable from "Black" itself. They should all merge into a single, uniform, "Black".
Well if you are not supposed to SEE them (Rule 2), how are you supposed to tell they are THERE in the first place (Rule 1)?
The answer is, you temporarily MISS-adjust your TV to make them visible. Then you correct the adjustment to make them invisible again.
Before we get into the details of that, let's get clear why these two Rules are important. For the 1st Rule: There are many ways to misconfigure the devices in your video chain which will delete the Blacker Than Black pixel values. Technically, they are "clipped" to a single value -- usually Black, but maybe not. The failure to preserve the Blacker Than Black portion of the content you are playing negates the value of recording that stuff in the first place. You are back to having a "hard cutoff", at Black, in the video data. Depending on where this is happening, and what video processing occurs after that point, this can have a more or less negative impact on the near-Black portions of the video you are SUPPOSED to see.
SO, as part of setting up the Black Levels in your TV, you should ALWAYS take a moment to miss-adjust things -- i.e., to raise Black Levels too high -- so that the Blacker Than Black portions of your calibration PLUGE chart become visible. Thus proving those pixel values are indeed making it all the way to your screen.
The 2nd Rule is equally important. Having demonstrated the Blacker Than Black values are PRESENT, you must now make them invisible! Why?
This is a serious question, and one that trips up many Home Theater enthusiasts! Remember, the Blacker Than Black pixels represent REAL content, which was carefully filmed, edited, and delivered to your TV. Why NOT raise levels so you can see them? Wouldn't that reveal even more near-Black detail?
Indeed if you try it on a few sample, dark scenes, you may even convince yourself allowing at least some of the Blacker Than Black pixels to show through is "a good thing"!
But it's not.
First there's Artistic Intent at risk. The filmmakers and the engineers who produced the home media version of the film have determined the correct level of near-Black details to best tell their story. You really should want to see the film the way it was intended to be seen.
But even more important, the QUALITY of the Blacker Than Black content is -- usually -- significantly lower than the intended, visible content. There's no mystery to this. The studio monitor TVs used to produce both the film and its transfer to home media are calibrated CORRECTLY -- producing true Black and nothing less. All of the filmmakers efforts are focused on making sure Black-and-above looks correct. They USE the Blacker Than Black content to help make sure that happens, but they do NOT check the consequences of misconfiguring the TV to make that now-processed Blacker Than Black content visible!
So let's discuss checking this stuff for SDR video. (The process for HDR video is a little different, but I'll defer that.)
If you refer back to my post on Digital Video again, you'll recall that each pixel is defined by three component values specifying its brightness and color. Those component values might be 8, 10, or 12 bits each, but for purposes of this discussion will focus on the 8-bit version as that's typically the way the calibration charts also describe it.
As described in that post, the different formats of Digital Video use the components differently to represent brightness and color, but you can simplify things by referring to the "Luma" of a pixel -- i.e., it's gray scale brightness somewhere between Black and White.
Representing Luma in 8-bits gives you 256 values, between 0 and 255.
For Home Theater content, Black is defined as Luma 16. Reference White is defined as Luma 235. The values from 1-15 are the Blacker Than Black values. The values from 236 to 254 are the Peak White values. (0 and 255 are reserved.) I won't be discussing Reference White or the Peak Whites in this post.
(The 10-bit and 12-bit representations have the same division between Black and Reference White with Blacker Than Black footroom below and Peak White headroom above, but the numeric values are, of course, different.)
Don't Fall for the Extended / Enhanced Video Range Trap!
At this point I need to digress, and steer you away from another common trap foisted on us by the crack Marketing teams at video game player companies.
These game devices are essentially specialized computer graphics computers. That is, they generate their video on-the-fly, and pass it to the TV with the expectation that little or no video processing will be done on it in between. And so --much like actual computers -- they have adopted video formats which forego both the Blacker Than Black and Peak White ranges of Home Theater video.
That is -- in an 8-bit format -- they define Black as Luma 0 and Reference White as Luma 255. Since there are no values available lower than 0 or higher than 255 there's no place to put any Blacker Than Black content or Peak White content.
The idea is this lets them use more steps for rendering the stuff you are supposed to see in games. But using such a format for Home Theater content will NOT produce the best results. The gray scale in the Home Theater content (16-235, you'll recall) has to be "stretched" into the 0-255 range. This results in rounding errors -- which show as "banding" on your screen. And, as just stated, the Blacker Than Black and Peak White values simply get discarded.
So don't use that format for movies or TV shows. Sounds simple right? Except the Marketing people, in an effort to tout the nifty new video from these game devices, decided to call this alternate video format "Extended" or "Enhanced" video!
And TV makers, wanting to simplify things for folks attaching games machines, adopted the same terminology: Extended or Enhanced. And, umm, what do they call the format you are SUPPOSED to use for Home Theater content?
They call it "Limited" video.
Guess how many people incorrectly rush to change their settings from Limited to Enhanced?
Deep sigh.... The Power of Marketing Compels You!
In the case of Sony's PS3 they compounded the problem by clipping the Blacker Than Black and Peak White values even if you WERE smart enough to select "Limited" video. To get THOSE back you needed to select yet ANOTHER setting -- which the Marketing guys decided to name "Super White".
It's stuff like this which shows why it is so important to actually CHECK that Blacker Than Black (and also Peak White) pixel values are truly getting to your TV.
There are two, basic video level controls in TVs for setting the end-points of gray scale, SDR video. I.e., the Black Level and the White Level.
The control for adjusting the Black Level is pretty uniformly called Brightness. You can remember Brightness is for Black Level because they both begin with the letter "B".
The other control, for adjusting the White Level is usually called Contrast, but may be called Picture. In any event, it is the control that's NOT called Brightness and is not related to Colors.
In older TVs, these two controls interacted quite a bit. That is, changing Contrast (to adjust the White Level) also altered the Black Level to a lesser degree. Conversely, changing Brightness (to adjust the Black Level) also altered the White Level to a lesser degree. So you had to go back and forth between those two controls until you honed in on the sweet spot pairing of the two that combined to produce the best Black Level and White Level together.
In modern Digital TVs this interaction has been largely eliminated. But it is still a good idea to go back and forth a few times to confirm adjusting one of these controls has not altered the best setting for the other control.
Raising Brightness produces a brighter Black Level. That is "Black" starts looking like dark gray and the Blacker Than Black values start to become visible.
So checking Rule 1, above is easy. You view the PLUGE chart on your choice of calibration disc, and temporarily raise the Brightness control a bunch to confirm the Blacker than Black values are becoming visible.
Let's take an example. The "Spears & Munsil, 2nd Edition", Blu-ray, video calibration disc includes a "PLUGE 0%" chart which can be used for this adjustment.
At first glance it looks like a black screen with a few, vertical, dark gray bars. In detail what you have is two bars to the right of center which are 2% above Black on the one nearest center and 4% above Black on the one further to the right.
Meanwhile, left of center are two more bars. The one nearest center is 2% BELOW black, and the one further left is 4% BELOW black.
So if the Brightness adjustment for Black Level is set correctly, both of the bars on the right SHOULD be visible and both of the bars on the left SHOULD NOT be visible.
The background of the chart is Black -- pixels of Luma=16. And that background has overlaid on top of it a checkerboard of large squares one step ABOVE black -- pixels of Luma=17.
So if you raise Brightness for the Rule 1 check the background and checkerboard will become readily visible -- dark gray. The two bars on the right will be even brighter than that, and the two bars on the left will look like darker holes in that dark gray background. And that -- the visibility of those two bars on the left as darker holes in the now-raised, dark gray background of the chart -- is your Rule 1 proof. If you see those bars then the Blacker Than Black data is getting through to your TV screen. If not -- if you only see the checkerboard and the two bars on the right, then your Blacker Than Black data has been clipped. Time to go check the video output format settings in the devices feeding that signal to the TV -- as well as the TV's own settings -- to see what's causing this. START by making sure you have not fallen into the Extended / Enhanced Video Trap!
The next step is to lower Brightness again until the chart background returns to Black and the most prominent thing you can see is the two bars on the right (2% and 4% above Black).
Now at THIS point you want to make sure your room is lit as you intend it be used for your most critical viewing of your most cherished content. For most videophiles this will mean the room is darkened but not completely black. Just dimly lit. Think how lights are lowered in a commercial movie theater. Even that is more room light than you'll likely want in your Home Theater.
If the room is brightly lit, your eyes will lose the ability to distinguish near-Black details. You can of course choose settings for your TV that work better for bright room viewing. But for best quality viewing you should be in a dimly lit (but not blacked out) room. For now, I'll assume that's how you are doing things.
After you have dimmed the room, and given your eyes time to adapt, adjust Brightness so that the 2% bar on the right is visible but not easily visible. The 4% bar on the right should be easily visible. The two bars on the left should be completely INVISIBLE.
Now, for fine tuning, get up close to the screen and adjust brightness so you can JUST begin to see the large checkerboard of Luma=17 against the chart background of Black -- Luma=16.
THAT'S your correct Black Level setting!
At this point the checkerboard will be just barely visible -- perhaps only with your nose right up to the screen. The 2% bar on the right should be visible with some difficulty, and the 4% bar should be easily visible.
These are pretty vague statements, and you may wonder just HOW visible these should be? Should you raise or lower Brightness a notch to change them?
This gets into the province of the "response curve" for Gray Scale between Black and White. Focussing on the near-Black end of that, just how much MORE visible should each step be above Luma=16.
This is controlled by the Gamma Correction setting in your TV -- another complicated topic and yet another of those Holy Grails of great Home Theater setup! I'll just give you a taste of it in this post.
The ideal response curve between Black and White for SDR video is NOT linear. It's more like an exponential curve sweeping upwards. From a practical point of view, this means that SDR video should "Come Out of Black SLOWLY". That is each step above Luma=16 should ideally produce only a small increase in the level of dark gray you see.
Thus when set up correctly, the Black Level which lets you just barely distinguish that Luma=17 checkerboard on this chart should still leave the 2% above Black bar kind of hard to see from your normal seating position.
But I'll leave it at that for now. In the meantime, say a few kind words for the hard working Blacker Than Black pixels in your content, which are helping to make your near-Black details looks so good. Think Black Thoughts. You'll be glad you did!