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REVIEW: BenQ XL2420T 120Hz Part 7


Responsiveness

We carried out measurements on the XL2420T in its native resolution at 60 and 120 Hz via the DVI connector. The monitor was restored to its factory settings for the measurements.

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Image construction time and acceleration behaviour

We measure the image construction time for both black-white changes and the best grey-to-grey change. In addition, we also find the average value for our 15 measuring points.

The datasheet indicates the response time as 2 milliseconds (GtG). In fact, we measure a speedy 2.2 milliseconds for the fastest grey change and 3.0 milliseconds for black-white. The overall image construction time (rise and fall) measured for our 15 measurements is extremely fast, at just 3.1 milliseconds.

Factory setting 60 Hz: Fast switching times, pronounced acceleration with large overshoots.

The AMA overdrive option is active in the OSD as standard. The luminance gradients measured show a very powerful acceleration. We find large overshoots at all measuring points and these continue to increase as the luminance decreases.

AMA off: Very long rise times, no acceleration.

As soon as the overdrive solution is switched off, a completely different image emerges. No acceleration can now be seen and the rise times increase hugely to values in and around 18 milliseconds. Typically for a TN monitor, the fall times are only slightly affected. Thus, all overshoots disappear.

With active overdrive, the panel is equally fast at 60 Hz and 120 Hz.

Such a fast FullHD panel is, of course, very interesting for 120Hz usage, an area where hardly any of this monitor’s rivals can hold its own. Evidently, BenQ has a very good grasp of this technology, since the switching time characteristics are retained completely even at t the high image frequency.

Without overdrive, the pace becomes leisurely once more.

When overdrive is active, the 120Hz times are identical to those at 60Hz. Without overdrive, the values are reduced by about 60 percent due to the faster image sequence. The effects on the frame (which only lasts about half as long) are identical.

Lag

We measure the lag as the sum of the signal delay time and half of the average frame change time. When the monitor is in use at 60 Hz, we measure an unnoticeable (but not particularly short) signal delay of 6.4 milliseconds on the XL2420T. On average, only another 1.5 milliseconds pass until the target luminance is achieved, so the average total lag is rather short at 7.9 milliseconds.

In 120Hz usage, the lag time adjusts to the faster frame change and drops to just 3.4 milliseconds. The average image construction time remains the same and thus, the average total lag in this case is a very short 4.9 milliseconds.

Backlight

The LED backlight in the XL2420T is PWM controlled, which means that short periods of darkness are set for luminance settings of less than 100 percent. At 140 cd/m² at the workspace, we measured a rather low switching frequency of just 180 Hz. Since the maximum luminance is not very high, it is not necessary to reduce much and therefore, the duty cycle is fortunately very high at 71 percent. Nonetheless, we cannot exclude the possibility that sensitive users may notice an irritating backlight flicker in this model.

The backlight is PWM controlled.

Subjective evaluation

The good measurements can be confirmed in the subjective test; there were no streaks or motion blurring in gaming or in video playback. Therefore, the monitor can also be recommended for hardcore gamers.

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