SMPTE Standards And How They Govern What You Watch
Webster’s defines the word calibrate as “to correlate the readings of an instrument with those of a standard in order to check the instrument’s accuracy.” The instrument in our case is your display. The standard is simply the one used to create the content shown on it. For all broadcast and disc-based material, we turn to the Society of Motion Picture and Television Engineers (SMPTE).
Measurements are broken down into a few categories: white point, color gamut, and gamma. These three areas are associated with specific numbers that are derived from the principles of imaging science.
Rec.709
Also known as BT.709, this is the specification governing high-definition content both for broadcast and for Blu-ray disc. And yes, that includes streaming. It starts with a specific color gamut as shown below.
We’ve shown you CIE charts before, so you know that the total area represents the spectrum of visible color, while the triangle is the display’s actual color-rendering capability. And what is Rec.601? It’s a slightly smaller gamut used in standard-def content. It’s only a factor if you watch a lot of DVDs. Very few televisions have a Rec.601 preset these days, so chances are that you’ll be watching all content in the Rec.709 color space.
The actual color points are defined by x and y coordinates. It’s also known as xyY. There are other ways to express the values, but xyY is the most common way for HDTVs. What does Y represent? That is the color’s luminance value. In addition to its position on the CIE chart, it’s also required to have a specific brightness relative to the other colors.
Grayscale & Gamma
But we’re not only talking about color. You see that little triangle at the center of the chart? That’s the white point, otherwise known as D65 or 6500 degrees Kelvin. When you adjust the RGB controls in your HDTV, that is what’s affected. And as we’ve already established, it has an impact on the entire color rendition of a display and is the most important part of a calibration.
The final major parameter is gamma; its standard is currently in transition. Put simply, gamma is the rate at which light output increases as the signal goes from zero to 100 percent. It’s not a linear progression, but a logarithmic one. Here’s an example:
The horizontal axis is the signal level (0-100%) and the vertical represents the output level. You can see that, by the time the signal reaches 50 percent, the actual output is only around 20 percent. To see content as its creators did, your display’s gamma must match up.
For many years, that standard was called 2.2 power function, which is an average of all the gamma values in a given measurement run. In 2011, a new standard called BT.1886 came into use. It’s only a tiny bit different than the above. In fact, it’s so close that you’d have a hard time seeing the difference in a chart. Some of the newest HDTVs (like the Samsung F8500 plasma we just reviewed) conform to BT.1886. Its main goal is to improve shadow detail while maintaining brightness and depth in the mid-tones.
Of course, color, grayscale, and gamma aren’t the only components addressed in a calibration. But they are the three items associated with a SMPTE standard. And that’s what we’re shooting for when we set up any HDTV.
Now we’ll show you how you can adjust your display with a few simple test patterns and some knowledge. You’d be surprised how much can be done without instruments.
