How We Test: Displays

Posted 08/26/2008 at 10:54am | by David Murphy

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Testing a display is two parts science, one part art. It's difficult to measure the performance of a display the same way Maximum PC evaluates other products. There is no benchmark that we can just fire up and then report a score from. Nor can we even test displays in their out-of-box condition. A fair amount of tweaking and visual analysis is necessary to ensure fair display benchmarking.  And at the end of the day, determining which monitor reigns supreme is a mix of qualitative testing and the editor's expertise.

That said, benchmarking displays isn't as simple as firing up a DVD and seeing which one looks better. When we pull a display out of its packaging, we hook it up to our standard test bed (Evga 680i motherboard, Evga GeForce 8800GTX, QX6700 processor, 2GB RAM) alongside another display.  Comparing a new monitor against the best-in-class allows us to make judgments about the display's prowess based on the flaws that we observe.  It's much more accurate to look at identical pictures across two screens than to run our suite of display evaluations on two displays sequentially and try to spot the differences from memory.

We hook both displays up to our videocard using identical dual-link DVI cables.  We're running our benchmarks on Windows XP SP3, as we've found much more success cloning the picture for our multimedia tests on this OS versus Windows Vista.  Both displays are each allowed an hour of warm-up prior to our initial round of tweaking.

Synthetic

We start our evaluations by firing up a batch of DisplayMate tests in an attempt to set the monitor's optimum brightness and contrast values.  We look at a number of grayscales on the dark and light ends of the spectrum to discern the point at which adjusting a display's brightness or contrast stops improving the image.  This is a tricky balance, as improving the grayscale range on one end of the spectrum--by adjusting the contrast, for example--can often poison or reduce the range on the other.

Once we've achieved an ideal balance, we begin running through the rest of the DisplayMate tests.  On the color saturation test (shown below), we run through a number of hues to discern the display's ability to produce very light shades of color arrayed against a white background.  A good monitor will be able to produce color all the way down to a two-percent level, whereas weaker displays can look whited-out at four or six-percent.

We then run through a series of gradients that help us discern a number of particulars about a display.  Banding and color-tracking issues appear best in grayscales. The former refers to the dark bars of compression that appear in a display's smooth gradient; the latter refers to unnatural coloration that tints part of a black-to-white gradient. 

We note these when they occur, including the exact number of gray values (128, 256, et cetera) that are on the screen at the first instance of either of these issues.  The grayscale test as a whole also allows us to take a big-picture look at a display's overall range.  For example, displays that are unable to reproduce a wide range of values at the dark end of the spectrum will show a huge black bar (or square) in the middle of our test patterns.

Note that you're looking at the same pattern of grayscales in the above pictures.  The top image uses 256 shades of gray to create the black-to-white fade.  The lower image uses fewer.  It's a serious issue when color-tracking and banding issues appear in gradients with few gray values.

The two above tests are identical in scope: each present the display's grayscale performance using two different gradient patterns.

We also use a DisplayMate pattern to check a monitor's streaking.  We haven't seen this issue pop up in recent displays we've tested.  Nevertheless, streaking is what its name implies: when an image appears to have an aftereffect, or a "tail," as a result of a switch between extreme contrast values.  On the test pattern below, this would show up as a horizontal block--of an opposite color--extending from the white or black bars into the gray right half of the screen.