Articles: Monitors
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As usual, there’s some tricky issue with the new method, though. I mean the word “average”. Easy to notice, this measurement method will make a monitor that does all the transitions in exactly 15 milliseconds and another monitor that does half the transitions in 27.5ms and the other half in 2.5ms (the example is purely theoretical, of course), look exactly alike – they will both have absolutely the same response time! In other words, the averaging doesn’t tell you how the response time varies between different midtone transitions which may be important information in some cases (one of them will be presented below). Having some even sketchy response time graph for all the possible transitions in the monitor’s specification would help greatly, but I think we won’t see such a thing in near future.

In LCD monitor reviews on our site I have published only 2D response time graphs which showed transitions from black to different levels of gray and back again. Transitions between two tones of gray were not shown. Such a diagram doesn’t give you the full information about the tested matrix, but it does give more than enough info to evaluate the real-life performance of a monitor. However, we have improved the software part of our testbed to measure the speed of gray-to-gray transitions, too, and to present the results as 3D histograms.

Below is an example of such a histogram, built for the ViewSonic VG712s monitor (a 17” TN+Film matrix without RTC). The vertical axis shows time in milliseconds; the left horizontal axis shows the level of gray from which the transition occurs (from 0 to 255 stepping 32); and the right horizontal axis shows the final level of gray. So, for example, the transition from pure black (0) to light-gray (192) corresponds to the farthest light-blue column. There are all zeroes along the histogram’s diagonal due to obvious reasons, and there are also zeroes for the transitions between the lightest tones (224-255) because the monitor is tested at the maximum brightness and contrast settings when some of the lightest tones are in fact indistinguishable from each other or there’s such a small difference between them that it doesn’t allow to make any measurements with an acceptable accuracy.

And yes, the response time of this TN+Film matrix is far from perfect. The “fast” transitions into the pure white color are obscured completely by the tall columns of slow (over 20, and sometimes even over 30 milliseconds!) transitions between mid-level tones. The low but narrow line of gray-to-black transitions along the left side of the histogram can’t save the day for this matrix. Thus, TN+Film matrixes have a rather low real speed despite the beautiful numbers in their specifications.

But as soon as we take a ViewSonic VX724, a monitor on practically the same TN+Film matrix but featuring a RTC block, the picture changes dramatically: the columns are all much shorter (I’ve built both the diagrams on the same scale for better comparison). The maximum response time is 13.8 milliseconds; the minimal is a mere 2.0 milliseconds!

Unfortunately, not all RTC-supporting monitors can produce such pretty-looking diagrams as the ViewSonic’s. Below is the response time diagram of the Samsung SyncMaster 760BF which uses a TN+Film matrix, too. As you see, the diagram differs a lot from the one of the RTC-less monitor, yet some transitions still take quite a lot of time, up to 30 milliseconds. However, if you calculate the average response time for the 760BF, it will of course be much lower than that of any ordinary RTC-less TN+Film matrix.

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