HDMI Enhanced Black Levels, xvYCC and RGB

When trying to get the most performance out of your home theater, few things are more important than proper display calibration. Sure, setting up your speakers correctly and room acoustics are important, but if your display is calibrated wrong, even the most clueless of friends are bound to notice. Audioholics has written a wealth of information on tests and calibrations that are more general and that cover a broad range of topics, but this article is more targeted. It has come to our attention that many of you utilizing an HDMI connection might have your black levels incorrectly set. Why the sudden confusion? Options have changed with the introduction of HDMI 1.3 and its tag-along partners: Deep Color and the new xvYCC color space. Read on.

HDMI, as a specification, has been in a near constant state of flux pretty much since its inauguration in December of 2002. As many an audioholic has (at least at times) been confused by the constantly changing spec, Joe Public has little to no chance of keeping it straight. As of this writing, we have arrived at HDMI 1.3b. The problem has been that not only is it confusing for the public, but for manufacturers as well. Plenty of HDMI cables are on the market right now making bitrate and length claims that they just can't pass - not because the companies are nefarious, but because they just don't understand what's being demanded as part of the new spec.

Now, if you are one of those people that eyeballs your settings on your display, you don't need to read any further. Your settings can't be any more wrong than they already are. But for those of us that use a test disc (like Avia or DVE), read on so you can get the correct black levels set on your displays.

YCbCr and RGB

All video discs, including Blu-ray, DVD and Video CDs, are encoded as YCbCr. While many displays accept RGB as well as YCbCr, RGB is generally reserved for output to computer monitors. RGB has a data range of 0-255. The 0-255 essentially refers to the number of colors that are available and R, G, and B, refer to Red, Green, and Blue respectively. With this color setting, each of the colors is given equal weight (which translates into equal bandwidth). In a YCbCr system, the luminance information is transmitted separately and a color difference system is used to derive green. In practical application YCbCr is no different than RGB in terms of quality when done correctly.

Technical Note: YCbCr vs. YPbPr
There is often confusion regarding YCbCr and YPbPr, however it is very easy to understand the difference. YCbCr is simply the digital "component video" format that is used in DVDs, digital TV and Video CDs. Digital camcorders (MiniDV, DV, Digital Betacam, etc.) output YCbCr over interfaces such as FireWire or SDI. YPbPr is simply the analogue version which manifests itself in the form of the three (red, green and blue) RCA connections on most displays (referred to as "component video"). Where analogue YPbPr uses three cables for connectivity, digital YCbCr uses only one.

YCbCr has a native data range of 16-235 (though it technically can do 0-255, but more on that later). The difference? Well, while you would think that RGB and its 0-255 would have a greater amount of color, the discs we all watch are mastered at the YCbCr 16-235 levels. In keeping with our definition of accuracy, the best thing you can do is match the original - this avoids interpolation and possible distortion of the original color and image.

Full (Extended) Range vs. Limited (Normal) Range

What has this got to do with anything - don't worry I'm getting to that. When the HDMI people (Silicon Image) set up the HDMI spec, they put down some ground rules. According to Section 6.6 of the HDMI 1.3 Specification document:

Black and white levels for video components shall be either “Full Range” or “Limited Range.” YCbCr components shall always be Limited Range while RGB components may be either Full Range or Limited Range. While using RGB, Limited Range shall be used for all video formats defined in CEA-861-D, with the exception of VGA (640x480) format, which requires Full Range.

Basically, in YCbCr mode, full range signal (0-255, which it can do) is not allowed and limited range values are specified (16-235 for 8-bit color sources). Initially, analogue sources & displays had something called under- or overshoot which in essence took into account the 1-15 and 236 to 255 values so YCbCr was limited to 16-235.

Closing the Circle

Now that I have you thoroughly confused, let's get back to the issue at hand. ALL video stored on modern discs, be it DVD or Blu-ray, are stored as YCbCr with a range of 16-235. But, the content is mastered such that 0-255 is present in the tape domain (the SDI stream - D5 archive tape, etc.) It is then captured into a computer and then the computer will decide what to do with it - preferably retaining the extended info (all data from 0-255). This entire range is then compressed into the 16-235 range accepted by DVDs, Blu-ray Discs, etc. When you play that back on a system set to 16-235, you get all the information as it was intended with no gaps. If you mismatch and play back with RGB (0-255) settings, you'll clip off the black and white levels and black will look "dark gray". Additionally, you won't be able to view the Blacker-than-Black images found on most video test discs. Let's look at this pictorially to get a better understanding of how this works:

Many of your displays can accept a RGB 0-255 signal. However, if you are using your receiver as an HDMI switcher, there may be a bottleneck. For some reason, many receiver manufacturers have strictly limited their receiver HDMI inputs to a 16-235 range for RGB. This means that if you set your source to output RGB, it will be sending a signal to your receiver that will be truncated before it is sent to the display. If you use this signal to calibrate your black levels, your black levels will be off and it's likely you won't see the Blacker-than-Black pattern from test discs.

Tech Note: SDI and D5 are 10-bit "archive-quality" formats, with valid ranges of 4-1019, with 0-3 and 1020-1023 reserved for Timing Reference Signals (TRS). Similarly in 8-bit transmission over SDI, the legal range is 1-254, with black at 16 and white at 235, and 0 and 255 reserved for TRS.

Blacker-than-Black (Below Black) Test Patterns

In order to see Blacker-than-Black test patterns, you'll need one of the discs below which will give you the ability to calibrate your display or projector to the correct levels. Remember, the proper level is such that the below black information, while present in the system, is NOT visible. TYhis means that you will want to make sure that the Below Black information comes through by cranking up your Brightness (black Level) until you see the below black pattern, and then decreasing Brightness until it is completely hidden, but leaves the next brightest pattern or line barely visible. Here are some of the common places to access a Blacker-than-Black test pattern:

- Black Level + Log Steps, Title 1, Chapter 5

THX Optimizer (see The Incredibles, Ice Age, Star Wars, or any THX DVD)

HDTV Calibration Wizard (ISF/Monster), Title 3, Chapter 1

, Title 7, Chapter 2 and , Title 5, Chapter 3

Note: Avia Guide to Home Theater (regular version) and Sound & Vision Home Theater Tune-up DVDs contain no below black patterns.

xvYCC - a Marketing Gimmick? Mostly.

With xvYCC, the range is again extended to 0-255 as is expected, since digital TVs have no under- or over-shoot as did analogue television signals. This will effectively override (in theory) any RGB or YCbCr settings on your source, AV receiver, or display. The idea is that the xvYCC color gamut is only effective when all components in a chain support it. While several AV receivers and displays support xvYCC color space, currently only the PlayStation3 provides xvYCC as a source. Here's where it gets dicey: The Blu-ray specification for movies (BD-ROM) does not support Deep Color or the new xvYCC color space. Oops.

I'll say it again: Blu-ray and HD DVD movie formats are limited to 8-bit 4:2:0 YCbCr. To our knowledge, there is no move to add xvYCC expanded color capability to the BD-ROM specification. In addition, issues of backwards compatibility would be extremely difficult to overcome, rendering any new 10-bit or higher formats unplayable on legacy BD players. The only solution would be to take advantage of larger BD storage media and issue discs with dual data streams for video (double sided or dual layer if you will).

Currently, Hollywood films are telecined directly to digital, with masters stored on D5 tape in 10-bit 4:2:2 format. While this is better than the 8-bit 4:2:0 present on current media, it's still not 12- or 16-bit Deep Color or even utilizing the xvYCC color space. Without mastering and the ability to store xvYCC on source material (other than games which are generated via PC video cards) it seems that xvYCC is largely a marketing gimmick, save the new lines of camcorders, etc which boast 10-bit recording and xvYCC support. Somehow, eliminating the occasional color banding in home movies isn't exactly the incredible leap in technology for which most of us were hoping.

Manufacturers are not Speaking the Same Language

Unfortunately, manufacturers are coming up with their own ways of dealing with interoperability issues with respect to levels and color range issues. A good example is Epson's new Pro Cinema 1080UB projector. On standard factory settings the Epson does not allow below black (blacker-than-black) pluge calibration. To get the levels right on this model, you need to enable "Expanded" mode on the HDMI Signal Range setting. Epson isn't the only display that requires this odd setting, Marantz' new VP-15S1 exhibited the same behavior.

Simplifying the Process - What to Do

Luckily for us - the net result is that these settings aren't all that hard to get right after all. Here are a few things to keep in mind, however:

• Standard definition DVDs, Blu-ray discs and HD signals are all YCbCr 16-235 range native. Always set your source to YCbCr mode (not RGB) when calibrating your black and white levels on your display.
• Some AV receivers may not pass anything except 16-235 via HDMI. This means that if you are setting your source to RGB (0-255) you may get your black and white levels truncated.
• xvYCC is significant ONLY if all components in the chain support xvYCC. Do not utilize this setting on any component unless everything else can fall into place. Once set, xvYCC should override and RGB or YCbCr settings.

The general rule - the takeaway for this entire article is this:

First, set your source components to output YCbCr if there is an option. Second, get a test disc that has a Blacker-than-Black pattern (AVIA Pro, Digital Video Essentials, any THX DVD with THX Optimizer, etc) and ensure that you can see blacker-than-black on your display. Third, switch your display's HDMI setting from 'Normal' to 'Enhanced' if you do not see the Blacker-than-Black pattern. As long as you can see a below black signal, you are more than likely set up to get the best possible dynamic range from your display. Your final step in resolving this issue should be to set the correct black and white levels.

Again - make sure you can see below black levels in your system, and then calibrate your display to set your black and white levels. This should allow you to enjoy the full benefits of your display and get the most dynamic range out of your DVDs.