Very often, when there is talk about the response time of LCDs, one can hear the term Overdrive. And in fact, this technology has brought some great changes to the millisecond world of LCD monitors. Although named differently by some manufacturers (ADCC, Advanced Amplified Impulse Overdrive technology etc.), there is nevertheless always the same principle working behind it.
In order to depict a new picture on the monitor, the liquid crystals – or short LCs – need to be turned to a certain degree which determines the respective color the pixel is supposed to illuminate in. Of course, the wider the angle a pixel must turn through, the longer it will take to do so. This is also why different color changes result in varying amounts of ghosting.
Until recently, the speed at which liquid crystals turned largely depended on their size, the number of LCs per pixel and their viscosity (i.e. flexibility). In order to make liquid crystals align to their proper position, each pixel is actuated with a certain voltage.
Chart showing responsiveness with and without Overdrive
But, since liquid crystals unfortunately are inert by nature, this voltage is not sufficient to make LCs turn at speeds fast enough that would allow ghosting-free images. This inertia of LCs was mainly visible with displays incorporating VA-type panel technology; but also TN and IPS displays showed some issues with respect to ghosting.
Now, if one calculates in advance the exact alignment each of the liquid crystals is going to have at the next frame (i.e. which color the pixel is supposed to be displaying by then), it is possible to selectively apply larger voltages to them, thereby forcing the LCs to take their respective position very rapidly. And this is what Overdrive comes down to: acceleration of response time by means of well directed "overmodulation".
To avoid voltages being too strong (or too weak), each new Overdrive-featuring LCD must undergo accurate measurements as to how long its liquid crystals take to switch among colors. The reason being, that too high a voltage will cause the liquid crystals to turn too far, which will result in a different form of ghosting, the so called "Corona" effect.
Corona effect on the HP L1955: You can clearly notice the bright shadow contrasting with dark objects.
One can observe this effect on the outlines of objects – for example in games - , as it manifests itself in a form of ghosthing which is brighter compared to the background. Due to its hard-to-describe characteristics, we hosted a video demonstrating the corona effect, which you can download here
But let's get back to the precalculated voltage values: These are programmed into a chip, which, in using the measured values, is able to accurately overmodulate all pixels during operation, ensuring that there will be next to no ghosting visible on screen.
It was particularly the previously slow VA panels that gained the most from the introduction of Overdrive: So now, VA panel do not only provide excellent contrast and color specs, but fast response times as well. However, also TN and IPS displays - having been swift already even without Overdrive - are being equipped with this technology nowadays, thus allowing for these ultra rapid response time of modern gamer LCDs.