by Oleg Artamonov
06/11/2003 | 10:42 AM
One of the most important characteristics of liquid-crystal displays (LCDs) is pixel response time. It is the time interval between the change of the signal in a particular cell and the change of the state of this cell. This parameter is also specified for classic cathode-ray tube displays and gas panels, but it is less crucial for them, as their response time is measured in microseconds. As for LCDs, the response time in them can reach tens of milliseconds, which may be noticeable for the human eye.
Despite some evident visual effects, such as the fuzziness of moving objects, response time cannot be more or less accurately estimated by the eye. One of the reasons is the inertia of the human eye itself (thanks to that, we don’t see the flicker of ordinary CRT displays, although every pixel of the screen glows only about 10% of the time).
In fact the problem of measuring response time looks simple at first sight: you could measure the time between the dispatch of the signal for cell state change and the actual change, tracking the pixel glow with the help of a photodiode. But it’s not that simple. The point is that you can’t pinpoint the time when signal is issued without meddling with the electronics of your display.
Another way: we could follow the vertical synchronization impulse and calculate at what time after it the data for our pixel are transferred, and adjust this time by the line synchronization impulse for the line containing that pixel… But such arithmetic only makes sense when you know exactly all time parameters of the scanning and assume that the display’s electronics doesn’t bring about any delays. The phenomena that can only be hypothesized upon, but not measured, don’t increase the reliability of the results. That’s why we put this method off, too.
After some brainstorming we chose another way of measuring pixel response time: we only need to draw the graph of pixel luminosity with a photodiode and oscilloscope and measure the time between the beginning and end of a luminosity change. There appears one question – how exactly should we measure response time for a display? Unfortunately, most manufacturers of displays and matrices don’t specify the measurement conditions and tell just the final result. Only technical documentation on certain TFT-LCD panels (not the off-the-shelf displays based on them) contains a description of the methods used to measure their response time parameter. That’s why we chose to stick to the ISO 13406-2 standard, which tells that LCD pixel response time is the total time it takes to turn a pixel on and off. Moreover, not the full time is taken, but the time from 10% to 90% of the total pixel luminance and then back to 10% (the graph is taken from the specs of Samsung Electronics’ LTM170E4 panel):
You don’t even have to watch a single pixel: it’s quite possible to light up the whole line to increase the input signal for the photosensor. In theory, as pixels light up one by one, but not simultaneously, this will blur the signal front, but such a measurement error is too tiny and can be neglected in our case. At 1024x768 resolution and 60Hz frame scan, pixels are output onto the screen at about 50MHz frequency. For example, when ten pixels come into the sensor’s window, the delay between the first and the last will be only 0.2 microseconds. It is too small considering we deal with time intervals of several milliseconds and higher.
To make the signal stronger, we can register not just the color of one sub-pixel (the red, green or blue dot), but of the whole triad, that is, draw a white line on a black background under the photosensor’s window. The three different colors of the triad are created by means of external color filters, while the three sub-pixels are actually identical, so we won’t get less accurate results measuring light-up time of the three sub-pixels rather than one. Meanwhile, the total signal intensity will be doubled in comparison with the signal from red sub-pixels only (doubled, not tripled, because the sensitivity maximum of the photodiode lies on an infrared range of 950 micrometers, and the sensitivity to green or blue colors is lower than to red).
The photosensor was made from a Siemens BPX90 photodiode and an Analog Devices AD8604AR precision amplifier. The photodiode’s shunt resistance was set to 10 megaohm to achieve required sensitivity. The amplifier was powered directly by the computer. In order to reduce the noise from the switching mode PSU, power was sent via an LC-filter and compensation regulator based on the 7805 chip.
Amplifier with the photodiode
The whole contraption was put into an aluminum case to avoid noise and external light. So, without any special measures (like a separate PSU, precision regulators and the like) we reached the noise level of about 10mV. It allows performing measurement tests without any problems, considering the maximum signal level is about 4.5V (the operational amplifier limits it as it has unipolar +5V supply).
Assembled sensor, top view
The bottom of the case was covered with black mat film to reduce noise from side lighting. An aperture was drilled against the photodiode. Its diameter was about 3mm.
Assembled sensor, bottom view
During the measurements the sensor was installed onto a horizontal panel and a small program was drawing a horizontal white line, one pixel wide, on the screen just against its light-sensitive window. The sensor’s output was connected to the oscilloscope. The trigger of the oscilloscope was set to switch on along with the fall edge of the signal (when measuring pixel fadeout time) or its rise edge (when measuring pixel light-up time). Thus, the oscilloscope’s scanning is switched on when the white line appears and disappears, fixing the signal from the photodiode.
We tested the system on an ordinary CRT-display (Samsung SyncMaster 750s and CTX PR705F). That’s how the phosphor lights up and fades out at 85Hz screen refresh rate (the oscillogram was taken against a simple white background, but the phosphor under the photosensor was highlighted by the electron beam at a refresh frequency, so there were periodical “flares”):
First of all, we checked the reaction time of the photodiode. The rise edge is about 400 microseconds long, which complies with the calculated reaction time (the maximum operational frequency is 5…10kHz, considering the resistance of the shunt and capacities of the photodiode, wiring and the input of the operational amplifier). Evidently, this is enough to register processes taking several milliseconds.
After testing the system on CRT-displays, we put tried it on an LCD display from NEC - LCD 1525V. The display worked in 1024x768@60Hz mode; its brightness and contrast were set to maximum.
Pixel light-up time
Pixel fadeout time
In the oscillograms above the levels of minimum and maximum brightness are marked with horizontal lines, and the time of transition across 10% and 90% brightness – with verticals. Thus, the light-up time for NEC LCD 1525V was 48ms, the fadeout time – 10ms, making the total of 58ms.
Now, that we have discussed in detail our methodology, let’s go over to the tests. All the tested displays worked in 1024x768@60Hz mode with the maximum brightness and contrast. Ten oscillograms were taken for each display (five – with pixel light-up and five – with pixel fadeout). The results were then averaged.
It is a mainstream model from Compaq, which is now merged with HP. It has rather average specifications according to today’s standards: 120/100 degrees viewing angles (along the horizontal and vertical, respectively); 35ms response time; 250nit brightness (1 nit = 1 candela per square meter) and 350:1 contrast ratio. Nevertheless, we didn’t reveal any inconveniences at work: the effective viewing angle allows reading text on the screen even looking in parallel to the matrix, although colors become yellowish at a more than 45 degree deviation from the direct view. After brightness and contrast were set to 40-50% of the maximum, it was possible to work in a brightly lit room. Controls are quite handy, too: large and easy-to-press buttons, a clear menu system… The display has two outputs – VGA (D-Sub 15) and DVI, which is a rare feature among 15” models.
The display base allows turning it to portrait, which is quite useful when working with documents (historically, the orientation of pages is portrait) or in the Internet (many sites are intended for 800x600 resolution and at higher resolutions there is a broad blank margin on the right). On the other hand, the height of the base cannot be adjusted, and the power supply unit is external, which is not always handy.
During the tests, this display showed 32ms pixel light-up time and 7ms pixel fadeout time – overall, we have 39ms response time. It is a little above the specified 35 milliseconds.
This display doesn’t boast any extraordinary features, as well. Its specifications are just a little better than those of the Compaq model: 120/100 degrees viewing angles, 30ms response time, 320nit brightness, 300:1 contrast ratio. The viewing angles turned to be rather strange: the manual said you could look at the screen at an angle of 40 degrees from above and 60 degrees – from below. In practice, however, when you looked at the display just a little from below, its upper part got noticeably darker. At the same time, it’s all right when you look at it from a side or from above, and the picture is quite discernable even when your line of view is in parallel to the screen. Subjectively, the eyes were least of all tired when brightness and contrast were set to 40%.
The design of the model is a bit clumsy and bulky, especially that broad bezel. The base has no height-adjustment options; the portrait mode and wall mounting (a useful thing sometimes) are not available, either. Although this model has rather large dimensions, its power unit is external.
The menu is far from perfection. It’s too ascetic, although there are all necessary functions present. The most common settings – brightness, contrast and self-tuning – are accessed with one button, though.
This display also proved a little worse than its specs said: 35ms (28ms pixel light-up time and 7ms pixel fadeout time).
Pixel light-up time
Pixel fadeout time
There is one more interesting thing about this display: the backlight lamp is modulated. To control brightness, the manufacturer used a width-pulse modulation of the lamp power supply (as you know, LCD displays use fluorescent lamps with high-frequency power supply, about 40…60kHz) with a frequency of about 260Hz. This was quite clear in the oscillograms, taken at a low brightness level (when brightness is high, the impulse width gets larger and independent impulses practically converge into one):
Of course, the eye cannot discern such frequencies, so all the things mentioned above are nothing more but just a curious fact.
If someone asked me to describe this display in one word, I would say “nice” about L1511SE. That’s the impression left by the display in general as well as by every single component: from the case design to image quality… The specifications don’t stand out too far from the previous two models: 25ms response time, 130/100 degrees viewing angles, 250nit brightness and 350:1 contrast ratio. We would like to add a few words about brightness. Most people who buy an LCD display for home use don’t like too much brightness: such a display is hard to look at in the evening, in twilight, but if you reduce brightness and contrast to 20-30%, the number of displayed colors is perceptibly lower. L1511SE differs (for the better) from other models by its low brightness level, which doesn’t strain the eyes even in the dark. Meanwhile, its brightness is enough to work in a brightly lit room. Thus, we ran a word-processing program with 70% brightness and 90% contrast.
The viewing angles are somewhat worse by this model than by the previous two. It’s all right when you look at it from a side, but when you look from above or below at an angle of 50 degrees or more the screen gets considerably darker. Still, the manufacturer didn’t promise anything better than that, although the above-described displays had the same specified angles, but much better effective ones.
The design of the case is good enough. This display is smaller in dimensions than Iiyama’s model, notwithstanding its internal power unit. However, the height of the base cannot be adjusted, the portrait mode is not available. The menu is not rich in settings, but quite user-friendly. The three basic functions – brightness, contrast and self-tuning are accessed with one button.
During the tests, this display showed 19ms pixel light-up time, and 6ms pixel fadeout time – 25ms in total, just like the manufacturer said. As with Iiyama BX3814UT, we can see in the oscilograms wide-impulse modulation of power supply of the backlight lamp when brightness is reduced. The modulation frequency is lower here – 200Hz.
Pixel light-up time
Pixel fadeout time
Another mainstream model, at least according to its specifications, belongs to the older LCD display series from Samsung, which is now replaced by the 152 series. The manufacturer specified 25ms response time, 120/100 degrees viewing angles, 250nit brightness and 330:1 contrast ratio. The display is really not too bright. When running a word-processing program, comfortable settings were 60% brightness and 70% contrast. The viewing angles are good with the same exception: when looking from below, the upper part of the screen appears dark. This wouldn’t be a problem at all, if the display didn’t have a pivoted base: when you work in the portrait mode and look just a little from the left, the right part of the screen (the former upper part) is dark. For your information: Samsung labels pivoted-base displays with SA-index (SAN, SAS, SAB) and ordinary ones – with SS-index (SSS, SSN, SSB). The third letter in the marking tells about the case color: N is for white, S is for silver and B is for black.
The case design is rather bulky: a massive base, broad bezel… However, thanks to the large case, the power unit is internal. Control buttons are on the right side of the screen. Brightness and self-tuning are accessed with the same button.
This display showed 24ms pixel light-up time and 7ms pixel fadeout time in our tests. In total – 31ms response time.
By the way, this display was the only one, which was assembled without any latches: they used simple screws. So, it was an easy task to take it apart. The examination uncovered a curious fact: the manufacturer of the TFT LCD module is not Samsung Electronics, but Tottori SANYO Electric Co., Ltd. The module is Torisan TM150XG-26L10.
To all appearance, this model is going to replace the out-dated 151S and 151B models in the value sector, while the 152 series will take the more expensive part of the market. The specifications repeat those of the 151S model, but the design is slightly different: 151N inherited a massive base from the 151 series and the front panel – from the new 152 series. The plastic bezel is narrower, and the control buttons moved down. However, the buttons are small and hard to press. Moreover, this model, just like 151S, has only two quick-access buttons – for brightness and image self-tuning. The power unit is internal.
The comments about the viewing angles are the same as we made for the previous model: the upper part of the screen is darker when you look at it from below and this causes some discomfort in the portrait mode. There is a big reserve of brightness and contrast. Subjectively, most comfortable settings for work were 70% brightness and 45% contrast.
This display performed worse in our response time tests than the 151S model: 40ms in total, a very long rise (36ms) was followed by a very short fall (4ms).
Pixel light-up time
Pixel fadeout time
One more difference from the previous model: at low brightness we saw the modulation of the power supply for the backlight lamp with a frequency of 540Hz.
This display represents a new series from Samsung and features new design. The power unit is taken outside, which made the case slim and light. The elegant-looking base allows adjusting both the height and skew (from vertical to horizontal position). When the screen is positioned horizontally, the base is in fact folded up and you can hang the display on the wall (mounting screws and other necessary items are included). The cables are now attached to the base, not to the display, which is more flexible. Overall, this display seems to have the best case among all the considered models: a stylish and handy design. Unfortunately, they had to sacrifice the portrait mode for that: the new base cannot be rotated to the portrait mode at all. So, the second letter in the model marking no longer indicates the base type, it now stands for the availability of built-in speakers (Z) or their absence (D).
The specified characteristics are astonishing: 25ms response time, 160/150 viewing angles, 350nit brightness and 450:1 contrast. In practice, the image is perfectly seen even when your line of sight is nearly parallel to the screen. But when you look at it from below, there is the same problem: the upper part of the screen looks dark. Still, as the display cannot be used in the portrait mode, it is no big inconvenience. Working in a brightly lit room, we set brightness and contrast to 40%.
The buttons on this model can be easily pressed, but the quick-access button for contrast is not here.
This display showed the same response time as SyncMaster 151S – 31ms (25ms – light-up and 6ms - fadeout).
Pixel light-up time
Pixel fadeout time
At low brightness, we could see the modulation of the backlight lamp with a frequency of 1kHz.
This model is a variation of the previous display, but it features built-in speakers (the letter “Z” in the marking tells that) and a DVI input. Otherwise, this display resembles the 152B. Note that the speakers are in the base, not in the display’s bezel. So, the base became thicker, it now has a line input, headphones output and volume control. Of course, as is the case with nearly all multimedia displays, the quality of the speakers is only enough to give sound to Windows and ICQ.
Brightness and contrast of this model were higher than in those of the previous display: we had to set them to 30% and 20%, respectively, for comfortable work.
During the tests, SM 152T quite expectedly showed 32ms response time: 23ms light-up time and 9ms fadeout time.
Pixel light-up time
Pixel fadeout time
As the main goal of this review was to measure pixel response time of 7 LCD displays, we list their results in a table for your convenience:
Light-up time, ms
Fadeout time, ms
Total response time, ms
Claimed response time, ms
Samsung SM 151S
Samsung SM 151N
Samsung SM 152B
Samsung SM 152T
Interestingly enough, all displays (except LG L1511SE) showed higher response time than their manufacturers claimed. However, nearly all the displays exceed their specs just slightly: by 5ms on average. The exception is Samsungs’ SyncMaster 151N: notwithstanding its excellent specifications, this display is now not the best choice.