There are two cases possible, when the compensation impulse is too low or too high. In the former case the “ghosting” effect is more conspicuous than it might have been, but still much weaker than on RTC-less monitors based on the same matrixes.
The second case is illustrated above. The solid line shows the compensated response and the dotted line, the over-compensated response. It’s clear that the brightness of the pixel will not have only reached the desired level, but will have exceeded it by the end of the first frame, i.e. by the moment the compensation impulse will be removed. After the impulse is removed, the brightness will take some time, depending on the matrix’s inertia, to go down to the desired value.
This case is important as it gives birth to a new kind of visual artifacts you cannot see on an RTC-less monitor – they show up as stripes lighter than both the moving object and the background. Below you can see two snapshots of a black text that’s moving from right to left on a gray background. The first snapshot was made on a Samsung SyncMaster 194T monitor, which uses an ordinary RTC-less PVA matrix. We’ve got the typical “ghosting” effect, which is rather strong here due to the peculiarities of PVA technology. The text is almost illegible.
The second snapshot was made under the same conditions, but on a Samsung SyncMaster 930BF monitor which is based on a TN+Film matrix with RTC. You can see a sharp light shadow trailing behind the black text that is moving on the gray background.
On observing this effect, some hardware testers jumped to the conclusion that RTC works by first switching the matrix to a tone lighter than necessary and then into the necessary one. This is not so. Yes, the matrix receives a voltage impulse that, if maintained for long enough, would switch the matrix to a light tone, perhaps to the pure white color even. But this voltage is supposed to be removed before the pixel exceeds the desired brightness, so it is not generally correct to say that the matrix is switched through lighter midtones since it doesn’t have time to reach them if the RTC mechanism is set up correctly. All the “light shadow” effects you may see are only due to the issue of an inaccurate compensation impulse and must be considered as errors in the implementation of RTC. Normally, no such effects should occur.
However, practice shows that all monitors with RTC currently available suffer from this problem in a varying degree. The RTC error may vary from a few to a few tens of percent. That’s why a tester of RTC-supporting monitors must not only measure the response time alone, but also see to how the high speed is achieved. Otherwise, monitors with the strongest RTC artifacts are going to win every test just because sending a too-high accelerating impulse is the easiest way to achieve the lowest possible response time (and the RTC error is not accounted for in the response time measurement method).