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Video Dynamic Range and Color Space

Scott Wilkinson  |  Jul 11, 2012

I recently read HT's review of the Panasonic TC-P55ST50 3D plasma TV. (Good review, by the way.) I noticed that the display was calibrated using a brightness range of 0-255. My understanding is that digital video (Blu-ray, DVD, digital TV) is encoded in YCbCr with a video brightness range of 16-235, and one should set the source component to output YCbCr and set the display to accept the brightness range of 16-235 to preserve the signal and avoid processing and interpolation. Just wanted to get your thoughts and reasons if one way is better than the other.

Stephen Lin

You are correct that video is encoded on Blu-ray, DVD, and DTV in the color space known as YCbCr, and that the industry-defined dynamic range extends from 16 to 235, which are known as video black and video white, respectively. However, because brightness is represented by eight digital bits, it can theoretically have a value from 0 to 255. (Interestingly, the valid range for YCbCr is 1-254; for the color space known as RGB, it's 0-255.) Most digital-video content is limited to a dynamic range of 16-235, but this range is occasionally exceeded, especially at the top end with values above 235, which is called "above-white." Similarly, values below 16 are called "below-black."

In most cases, setting the source device to send 0-255 and the display to accept the same range does not affect the signal's dynamic range by processing or interpolation—it simply lets the entire signal pass through the system. In other words, it opens a wider window for the signal. If the source and/or display are limited to 16-235, any above-white or below-black information in the signal will be clipped out of the image. This is why we always calibrate our displays using the full 8-bit dynamic range. We also test Blu-ray players, AVRs, video processors, and displays to see if they can pass the full dynamic range; this is the Video Clipping test in the Video Test Bench found in all HT reviews of these products.

In addition to allowing above-white and below-black content to be displayed, using the full dynamic range also lets you set a display's Brightness and Contrast controls more accurately. Using a test pattern that includes above-white and below-black elements, you can set these controls to make sure the display is not clipping whites or crushing blacks.

However, while we set Contrast to the highest possible value at which above-white is still visible, we set Brightness to the highest value at which below-black is not visible. Why make below-black invisible? Because virtually no video content is made with below-black information, and increasing the Brightness to make below-black visible would raise the overall black level of the display, which should be as low as possible.

As for whether to send YCbCr from the source to the display or have the source decode YCbCr to RGB, that depends on the specific devices involved—there is no easy answer. Spears and Munsil, developers of the High Definition Benchmark setup and evaluation Blu-ray, have posted a great article about color spaces and choosing the best one for your particular setup.

If you have an A/V question, please send it to askhometheater@gmail.com.

COMMENTS
Kris Deering's picture
Submitted by Kris Deering on July 16, 2012 - 5:03pm
This is a great question and one that doesn't always have the clearest answer depending on who you ask. Scott is correct in every sense of the word with his answer, but where you set the white clipping point is debatable and I find different answers from lots of different people. If you set contrast to its highest point without clipping 255, great. But that does create some limitations. For one you've now limited the contrast potential of the display to allow head room that may or may not be needed. This will also limit you lumen output so the image may not be quite as bright as you would like. Some people set the clip point at about 240 to preserve a bit of headroom but trying to eek the most contrast and brightness possible. You have to be careful though, setting the clip point higher can result in some issues where the display simply runs out of one of the colors and you get some nasty calibration issues. Remember that white on consumer displays is made of red, green, and blue. Depending on the display you're using you may run out of one of those colors when you set contrast high (red is typically the first to go, especially with any bulb based display) and you may not get it back. Setting to 255 is typically the safest bet and is never the wrong answer, but if you want to get that little bit more, make sure you know how those adjustments are effecting the rest of the image. Enjoy!!
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