Advanced Calibration Techniques
For many HDTV owners, the Spears & Munsil disc or SpyderHD package will produce excellent results. Picture quality can be greatly improved with a few simple adjustments, and unless you want that final one percent of performance, it’s not necessary to spend thousands of dollars on software and gear.
If, however, you are after the last little bit of potential, we’d like to give you a brief look at some advanced calibration techniques. This will be more of an overview than a block of instruction. Employing these methods requires both instruments and experience. But if you want to take your HDTV calibration to the highest level, it’s a road you have to travel either with an investment of time and cash or the hiring of a professional calibrator.
Two- And Ten-Point Grayscale Adjustments & Gamma
Most HDTVs have a two-point grayscale or white balance feature. That means you can change the RGB values’ high and low ranges separately. We use 30- and 80-percent brightness patterns to accomplish this. Here’s the custom CalMAN screen we like to use. To see all the detail, click on the photo to enlarge it.
On the left are the Delta E errors for brightness levels 20 to 100. The large square is a bulls-eye chart. All we have to do is tweak the RGB sliders until the dot falls within the center square. At the bottom-center is a large RGB Balance chart. It also helps dial in the correct levels when making changes. The upper-right shows the raw data: x & y coordinates plus gamma, brightness, and error values. Below that is another RGB level chart showing all the brightness steps. Finally, there's a gamma tracking chart at the bottom-right.
When we use a two-point white balance control, we adjust the high range (gain) using an 80-percent brightness pattern and the low range (bias or cut) with a 30-percent one. After a little back and forth, most HDTVs return an average error of less than one Delta E.
If some steps show a greater error, and there’s a 10-point white balance control available, we adjust each level starting from 100 percent and working our way downward.
During all of this, we closely monitor the gamma tracking. It’s usually pretty consistent once the right preset is chosen, but it can change on some displays. Once we’re satisfied with grayscale and gamma tracking, we move on the CMS.
Color Management Systems
We’ve talked about these in a few display reviews and in our previous calibration feature articles. They’re not common on computer monitors but some HDTVs and many projectors include them. Our upfront advice is to leave them alone unless you know what you’re doing. You can easily destroy the color balance of a display by misusing it. And in many cases, the controls don’t work properly, leading to even more confusion.
Here’s how we adjust a CMS. Click on the photo to enlarge it.
The first thing to note are the extra saturation levels shown in the CIE chart. We don’t just look at 100 percent (the triangle’s perimeter); we also measure 25-, 50-, and 75-percent saturations to be sure that any adjustments we make don’t negatively impact those color levels. We also watch the luminance chart carefully. It’s usually impossible to get all the luminance values correct, so we have to settle for the best balance as we make changes.
In our observation, luminance accuracy makes the greatest difference in picture quality. And even if a CMS’ saturation controls don’t work (often the case), the luminance sliders usually do. Adjusting them properly can improve your HDTV's picture even when saturation and hue are a little off.
Along the top of the screen are error levels for saturation (Delta C), hue (Delta H), and luminance (Delta L). We monitor them for each color we adjust and try to get all three errors as low as possible.