REVIEW: NEC PA301W-BK Part 23
In accordance with the older ISO 13406-2 standard, the monitor was switched from black to white and back again. The switching times measured between 10 and 90 percent brightness and back (rise + fall) are added together and the sum forms the image construction time. The current ISO 9241-305 standard describes the image construction time as the average transition time both ways between five different grey values; here, the manufacturer may seek out the most favourable individual values from the field of measurements. The image construction time is also referred to as the reaction time or response time.
Explanation of diagram: The first series of measurements includes the times for switches from black to grey values between 30 % (RGB 77) and 100 % (RGB 255 = White, corresponds to ISO 13406-2). This series describes the monitor’s behaviour for strong contrasts. The second series of measurements includes the times for a switch between two grey levels whose RGB values are 30 percent apart respectively. This series describes the monitor’s behaviour for low contrasts.
Switching times with RI off, left: black-to-grey; right: grey-to-grey 30%.
Switching times with RI on, left: black-to-grey; right: grey-to-grey 30%.
The data sheet of the PA301W indicates detailed response times: black to white is listed as 6 + 6 = 12 milliseconds, grey to grey as 6 milliseconds. Our measurements confirmed these values, but the complete grey changes forth and back take considerably longer than the values indicated. The Response Improve (RI) option has a very clear effect: when RI is deactivated, we measure an image construction time (forth and back) of 11.4 milliseconds, whilst with active RI, this value drops to just 8.4 milliseconds.
The investigation shows that the IPS panel used in this model switches much more quickly than earlier IPS models and also most current IPS panels. These times would be more like what can be expected of a TN panel.
Every brightness level of a pixel is allocated to a particular control voltage, with which the target brightness is achieved precisely. The switching speed can be increased if a larger difference in control voltage is initially applied than the difference in brightness would actually correspond to. Then, the voltage is corrected in order to bring the brightness to the target value. This process is referred to as Overdrive.
The Overdrive process is determined by the RI option on the PA301W. The two pictures show a series of three brighter and three darker images with grey values of 80 and 50 percent respectively.
Overdrive for grey change 80-50, left: moderate for RI off; right: overshoots for RI on.
Without RI, the Overdrive is moderate and the panel switches as quickly as possible without brightness overshoots. When RI is active, on the other hand, clear overshoots arise for all changes in brightness. After two frames, the panel has then levelled off at the target brightness.
As well as short switching times, which are necessary for fast image construction, the lag time (latency) of a monitor is significant for the perceived responsiveness. This applies especially for games with fast movements such as racing simulations or shooting games. If too much time passes between input and image output, the controls become too indirect and the gamer’s enjoyment is affected. This effect is referred to as input lag.
There is no standard target for the determination of the lag time in monitors (here, see also our special: "Investigation of the input lag test process"). First, we measure the lag time until the clear start of illumination (10 percent of the final luminance). We then add half of the average image construction time (there and back) to this value.
Explanation of diagram: The lag time of the LCD arises as the difference in time between the control signal (red) and the illumination of the pixel (10%-threshold, green curve).
The PA301W delays the image for a very long time.
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