How to read a TV review, part 2: Calibration results

To truly find out how well a TV performs, nothing does the job like charts! No, really, it's cooler than it sounds. Learn why!

In my How to read a TV Review article , I detailed all the specifics in a TV review for those new at TV hunting. Well, almost all. I skipped over the really techy sections, like the Geek Box, the Juice Box, and most importantly, the Calibration Results.

Calibration Results tell the informed reader a LOT about a television, and are well worth learning about.

No, really. I swear, it's interesting!

The Geek Box and Juice Box

I love the Geek Box chart. It's a solid compilation about a variety of a TV's performance characteristics. Plus, it's got a subjective "score" for those that don't know what the desired x/y coordinates are.

CNET already has an extensive and well-written description of Geek Box here. If you have any questions, post them in the comments below and I'll fill in where I can.

What we'll concentrate on are the extensive graphs provided at the end of the review. For our discussion, we'll again be using the Sony Bravia XBR-55HX929. You can find the full charts here, though you can click on the images in each of the sections below for an image of where it is on the chart.

RGB Balance

TV's create light with the three additive primary colors: red, green, and blue. Sharp Quattron LCDs have a fourth yellow subpixel, but this is a secondary color, and is added by Sharp's processing. No video signal has yellow as a separate color (it's a mixture of red and green).

In order for a TV to create "white," it needs fairly equal parts of R,G, and B. Too much blue, and the image will appear "cool." Too much red, and the image will appear too "warm." For more on this effect, check out my article on color temperature .

Our example image here is the precalibration measurement showing that across the entire grayscale (from dark images all the way to bright images), there is a consistent lack of green.

After calibration (not shown here), depending on how fine-tunable the TV is, these three lines will overlap on the 100 line.

A "bad" scenario on this graph would be one end radically higher or lower than the other. This would mean that dark images aren't the same color temperature as bright images. On certain scenes this could be noticeable and somewhat odd-looking.

Adjusting the RGB Balance, which adjusts the color temperature, is the main process done when you have your TV professionally calibrated. I discuss this process in my "What is HDTV Calibration" article.

Gamut CIE

Remember those three colors I just mentioned? Turns out, they can be quite different across different TVs. There are standards as to what each color is supposed to be, but for many reasons they can be different. In this chart, the ideal is having the circles within the squares. Too far outside each square indicates that color is inaccurate.

A TV can create any color inside the triangle. Why not make the triangle bigger to create more colors? All TV programming, from Blu-ray to broadcast, have certain specified color points. If the TV's color points are different from these, it isn't accurately creating the colors in the source. Personally I can't stand inaccurate colors, but to others it's less of an issue.

I did an article on color over at, ahem, a different Web site that's worth checking out.

This chart is called the CIE 1931 diagram. It is the generally accepted standard for TV reviews, even though it overaccentuates how much the eye can see changes in green, and underplays how well the eye can distinguish changes in blue. That, however, is a rant for a different article. File the 1931 diagram under "meh, good enough."

The color points in our example here are pretty spot on. A common inaccuracy in many other televisions is an oversaturated green, which would put that color point above the corresponding green box on this cart.

Gamma luminance

Gamma is a weird concept for the non-TV-tech-literate (and to be honest, most people who are TV-tech-literate). As CNET's How we test page describes it: "Gamma is a measure of how much light a display produces when fed a certain level of signal."

The Wikipedia page is actually quite good for this topic (especially the images).

CNET recommends a gamma of 2.2, and judges accordingly how much higher or lower the gamma is from there. Some pundits claim 2.5 is better, though to most eyes this is "punchy" to the extreme. A gamma of 2.0 will appear somewhat flat.

Gamut luminance and numbers

The gamut luminance chart shows the light level (luminance) of each primary and secondary color. The gray bars show the ideal, while the colored bars show the actual measured level. The closer each color is to the reference in level, the better--this especially important for the primary colors of red, green and blue. If there is too much red, for example, the TV is referred to as "pushing red." This can result in people with lighter skin tones having a reddish, sunburny complexion.

The numbers below the gamut luminance chart show the x/y coordinates for the different color points, plus their deviation from standard. The latter is found in the Geek Box. These are the specific numbers used to create the "Gamut CIE" chart above.

Color temperature

This, in kelvin, is what we were discussing in the RBG balance section. Ideally you want all the bars to be roughly the same height, which they are in this case.

For most people, without a reference, the difference of a few hundred kelvin is hard to spot. To this reviewer, it's more important for a TV to be consistent than to be accurate with certain levels of brightness and not with others.

As mentioned in the RGB balance section, my article on color temperature goes further into this.

Related links
What makes a good HDTV?
HDTV settings explained
Other how-to blog posts on HDTV and Blu-ray tech

Numbers, numbers, and a bunch of other stuff (like numbers)

The bottom of the chart lists the x/y coordinates, luminance, color temperature and so on of the TV at certain levels of brightness. Twenty is dark gray, 100 is bright white. The graphs above are representing all of this data visually, so this section is largely a reference.

If you go through the main Calibration Results page, you'll find a chart similar to the one we've looked at, but showing the TV after calibration. In most cases, a TV after calibration will be more accurate than the precal version.

Below the "after" chart are larger versions of the important graphs.

Then there's the Gamma Point chart that shows how the individual colors track at different levels of brightness. This is another way of showing what's going on in the RGB balance chart.

That's largely it.

For most people, CNET's star rating and a skim of the review are enough. For those who want to dig a bit deeper, or really pore over the specifics, these charts are some of the best data on TVs available.

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Tech Culture
About the author

Geoffrey Morrison is a freelance writer/photographer for CNET, Forbes, and TheWirecutter. He also writes for Sound&Vision magazine, HDGuru.com, and several others. He was Editor in Chief of Home Entertainment magazine and before that, Technical Editor of Home Theater magazine. He is NIST and ISF trained, and has a degree in Television/Radio from Ithaca College. His bestselling first novel, Undersea, is available in paperback and as an ebook on Amazon, B&N, and elsewhere.

 

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