No More 5 ms Screens in 3D TVs and Monitors Please!

While visiting the Acer stand at this year's MedPi consumer tech show, the head of the firm's monitor division told us that a 'bug' detected by Nvidia had caused the Acer HN247H monitor we tested earlier this year to be temporarily suspended.

The 'bug' in question, Nvidia later explained, was actually the monitor's unpleasantly surprising ghosting time—something we measured in our tests when reviewing the HN247H.

We don't think this can be corrected. Here's why:

3D screens work with an input framerate of 120 frames per second—alternating 60 frames for the right eye and 60 for the left eye.

For a convincing 3D effect with no crosstalk (when the image for the right and left eyes end up overlapping), the liquid crystals in the screen must be able to display and, more importantly, get rid of an image in 1/120 Hz = 8.33 ms.

The time it takes to display and remove an image is exactly what we measure in our 'ghosting time' test. We measured 15 ms for the HN274H, which is almost twice the time required to avoid crosstalk.

It's therefore logical and, we think, inevitable that the HN274H is more affected by this problem than the older GN245HQ, for which we measured a response time of 5 ms. The only solution would be to turn the 5 ms screen into a 2 ms or 1 ms screen, which can be done by activating an overdrive function. This increases the voltage sent to the liquid crystals and in turn speeds up their movement.

Overdrive explained

Basically, the crystals are like little sticks, all lined up and floating in a liquid. When they receive voltage, these 'sticks' start to turn around to let through or to block light as required. In overdrive mode, instead of sending 1 volt to a given cell of liquid crystals, each cell gets 3 volts.

The crystals then become over-excited and turn around much more quickly, which explains the reduction in response time advertised by manufacturers.

Beware of black ghosting: the crystals may turn more quickly in overdrive mode, but the increased voltage must also be stopped which great accuracy. If the crystals, in their over-excited state due to the extra volts, end up overshooting their required positions, a kind of dark blue shadow will be seen behind moving objects on the screen. This is known as black ghosting.

As far as we know, all screens that claim to be 2 ms are actually 5 ms screens with overdrive ... which is handled with variable quality from one model or manufacturer to the next.

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