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That is why our tests of LCD monitors with response time compensation will include one more diagram which looks similar to the more traditional response time graph for black-gray-black transitions. This new diagram, however, shows the RTC error in percent. If during a transition from black (level 0) to gray (level 100) the pixel’s brightness level never exceeds 100, then the RTC error is zero. If the pixel’s brightness reached a maximum of “150” during this transition, the RTC error equals 50% (the maximum is 50% above the desired level of the pixel). The figure below shows you an example of a real-life oscillogram of the switching of a pixel in a monitor with poor RTC setup: the bottom gray line marks the original brightness of the pixel; the top gray line marks the resulting brightness; and the pink line marks the surge that resulted from the too-high overdrive impulse.

Of course, if this error is so gross that the artifacts are easily perceived by the eye, can the monitor be considered really fast? I think that besides the response time proper, such monitors should have another parameter – the time for the pixel’s brightness to get within 10% from the desired level. This time is to be measured by the “sloping” tail of the overdrive impulse and is going to be considerably bigger than the response time proper.

Talking about speed, the monitor manufacturers used to specify the response time as the total time it took to switch a pixel from black to white and to black again. This method has been changed after the introduction of monitors with RTC. If you see a remark “gray-to-gray”, “g-t-g” or “midtone” after the number, it means that the specified number denotes the averaged time of transitions between two gray midtones. In other words, the manufacturer measures the time it takes to switch the pixel between all possible midtones and then calculates the average number. Note also that they used to specify the total “black-white-black” time, while the new standard defines the response time as the time of a transition into one side only.

Of course, this change of the measurement method is mostly due to marketing reasons. The very concept of RTC implies that black-white transitions can’t be accelerated, at least on the existing panels, and RTC wouldn’t affect the response time in the slightest if it was still measured according to ISO 13406-2. But the customer should be given to understand that the new monitors are faster than the older ones, so either the term RTC must be explained in detail (and why this new 8ms monitor with RTC is faster than that older 8ms RTC-less model) or a new method of measuring the response time must be introduced that would ensure advantage to the new monitors.

I must confess I am personally very glad about the manufacturers’ decision to introduce the new measurement method. Although dictated by marketing reasons, it is logical from the technical point of view. The new monitors are really much faster than the older ones at practical use, i.e. when it comes to the midtone response time. And secondly, this way of measuring the response time gives the user a much better picture of the real performance of a monitor.

 
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