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We haven’t yet tested the FP241WZ in our labs, so I can only quote a review published by the respectable BeHardware (“BenQ FP241WZ: 1st LCD with screening”). Vincent Alzieu writes there that the new technology indeed improves the subjective perception of the monitor’s response time, but although only one out of the 16 backlight lamps is off at any given moment, a flickering of the screen can be noticed in some cases, particularly on large solid-color fields.

I guess this is due to an insufficiently high refresh rate. As I wrote above, the switching on and off of the backlight lamps is synchronized with it, so the full cycle takes as long as 16.7 milliseconds (at 60Hz). The human eye’s sensitivity to flickering depends on many conditions (for example, it’s hard to see the 100Hz flicker of an ordinary luminescent lamp with an electromagnetic ballast if you are looking straight at it, but much easier if it is in the area of your peripheral vision), so it may be supposed that the monitor still lacks more vertical refresh frequency, even though the use of 16 backlight lamps brings an overall positive effect. As all of us well know from CRT monitors, if the whole screen was flickering at 60Hz, you wouldn’t need to look for the flickering specially and it would be a torture to work at such a monitor.

An easy way out of this trouble is to transfer LCD monitors to a refresh rate of 75 or even 85Hz. You may argue that many monitors already support 75Hz, but I have to disappoint you: this support is implemented only on paper in most cases. The monitor is receiving 75 frames per second from the PC, but is discarding each fifth frame to display 60 frames per second on the matrix. This can be proved by photographing a rapidly moving object with a high enough exposure value (about 1/5 of a second, so that the camera caught a dozen of the monitor’s frames). On many monitors you will see that the photograph shows a uniform movement of the object on the screen at a refresh rate of 60Hz, but at 75Hz there are gaps in the movement. Subjectively this is perceived as a lack of smoothness of the movement.

Besides this obstacle, which is quite surmountable now that the monitor manufacturers have got to deal with it, there is one more: the higher refresh rate calls for a bigger bandwidth of the monitor interface. To be specific, monitors with native resolutions of 1600x1200 and 1680x1050 pixels will have to use Dual Link DVI to transition to a refresh rate of 75Hz since the operating frequency of Single Link DVI (165MHz) won’t be enough. This is not a crucial problem, yet it may impose certain restrictions on compatibility of monitors with graphics cards, especially older graphics cards.

What’s interesting, the refresh rate increase reduces the fuzziness effect by itself, the panel’s specified response time being the same. This is again related to the persistence of vision effect. Suppose that a picture moves by 1 centimeter on the screen during one frame period at a refresh rate of 60Hz (16.7 milliseconds). The next frame arrives and the retina sees the new picture plus the remaining imprint of the old picture with a shift of 1 centimeter between them. Now if the frame rate is doubled, the eye will fix frames with an interval of about 8.3 milliseconds rather than 16.7 milliseconds. It means the shift of the two images, old and new, relative to each other is twice shorter. From the eye’s point of view, the trail behind a moving object becomes two times shorter, too. Thus, a very high frame rate would give us the same picture as we see in real life, i.e. without any fuzziness.

 
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