Display Testing Explained: How We Test Monitors and TVs

Video Processing and Signal Handling

This part of our benchmark suite is unique to HDTV reviews. We use a series of pass/fail tests to determine the ability of a display to process different kinds of video signals. Most of the time, you want your source components handling this because they're more capable. If you own an Oppo Blu-ray player, for example, it will exceed the capabilities of pretty much any TV. Set your player to output 1080p video, and the display does no video processing whatsoever. An example of the reverse would be a cable or satellite receiver, which is usually poor for scaling and deinterlacing.

The first tests consist of a group of video clips from the Spears & Munsil HD Benchmark Blu-ray Edition, which is available to anyone online for about thirty bucks. Here’s a quick rundown of what's covered:

2:2 pulldown: This is the cadence most commonly found in content shot on video cameras (at concerts and sporting events, for example). The original image is interlaced, two fields per frame, and the display must integrate them into a single progressive frame.

3:2 pulldown: The cadence most often used to convert 24p film to 60i video, its order is two fields of the first frame and then three fields of the next, in alternating sequence. If the display doesn’t integrate the extra field properly, there is a very obvious artifact that shows in our test clip and results in a failure.

Accepts 24p: Film content on Blu-ray is encoded at 24 frames per second, and all current players can output the signal at that rate. Most displays can accept this signal and process it to a refresh rate that’s a multiple of 24 by using repeated frames.

Very few displays of any type or price can pass the 2:2 test. Where would you find this in actual content? It’s most common in high-def broadcasts, which are usually 1080i. A notable exception is Fox, which sends its signal out at 720p.

  • Tests performed: 2:2 Pulldown, 3:2 Pulldown, 24p
  • Pass/Fail result

Signal Handling

The second group of tests covers an HDTV’s ability to show signals below black and above white. Unlike PC signals, which range from 0 to 255, a video signal truncates that to 16-235. The areas above and below those values are considered head and toe room, and are not used in correctly-encoded content. It is desirable, however, for a display to at least be able to show the levels between 0-34 and 236-255. It makes calibration easier, and occasionally content does stray outside the limits.

The Chroma Burst pattern shows a series of single-pixel lines, in color, to determine if a display actually achieves its maximum native resolution. Most HDTVs return different results for RGB signals than for component (YPbPr) video. 4:2:2 is the minimum bit depth output from a source; 4:4:4 is more common. Some players can output RGB, which usually eliminates a conversion step in the display. Our test shows which signal mode provides the best resolution performance.

  • Patterns used: Black and White PLUGE, Chroma Burst

Conclusion

We hope this gives you a clear understanding of our testing methods and why the results are important. Which tests have more meaning will depend on your particular application. If you’re a photographer, color accuracy and gamut volume will matter more than input lag or viewing angles. For gamers, contrast and panel speed are likely to be the deciding factors in a purchase decision rather than color accuracy.

As always if you have questions, please let us know in the comments section.

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  • Chetou
    Thank you for this writeup. Though I do find 200 cd/m2 retina scorching on any monitor and viewing conditions, especially with large screens.

    Do you take your measurements in a dimmed/dark room? Even with the meter flush with screen, some light can pass through the glass on the sides.
  • Chetou
    double post
  • cpm1984
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms':
    http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html

    but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'):
    http://www.tftcentral.co.uk/reviews/dell_up3214q.htm

    Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)
  • cpm1984
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms':
    http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html

    but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'):
    http://www.tftcentral.co.uk/reviews/dell_up3214q.htm

    Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)
  • ceberle
    1563406 said:
    Gotta ask about your 'total lag' measurements - they seem to be generally much higher than what TFT Central measures. For example, the Dell UP3214Q has 97ms total lag with Toms': http://www.tomshardware.com/reviews/benq-pg2401pt-24-inch-monitor,3848-10.html but it has only 29ms lag at TFT Central (and only 25ms in 'game mode'): http://www.tftcentral.co.uk/reviews/dell_up3214q.htm Most of the monitors you review seems to have total lag around 80-100ms, which seems really slow. Slow enough that you'd notice the mouse lagging when you move it around. Yet I can't feel any appreciable lag on my Dell 2713HM (but I have no way of measuring....)


    TFT Central uses the SMTT software which measures only the actual display lag. Our test indicates the total time it takes for a user input to translate to the screen. We account for the lag inherent in the input source which in our case is a pattern generator. While this device may be slower than the average mouse or keyboard it is completely consistent and repeatable. If we used a mouse-to-PC test, timing could shift if a driver were updated or we changed video boards. Every monitor we've measured over the past two years has been tested with the same Accupel signal generator.

    Our principal goal is consistency from monitor to monitor. By having the exact same equipment and test parameters, we can ensure that results from last year are comparable to more recent tests.

    -Christian-
  • KevinAr18
    The response time testing method is done wrong and is very misleading. The tesst do not always reflect the real response of the monitors.

    Why?
    Monitors have thousands of different response times, not a single number. Testing just one or two transitions can easily give very innaccurate results; the one transition you tested may be much slower or faster than the other thousand or more transitions that exist. In order to find out if the monitor is any good, you must test a wide range of transitions.

    Want proof?
    See:
    http://www.xbitlabs.com/articles/monitors/display/viewsonic-fuhzion-vx2268wm_4.html#sect1
    In "standard" mode, the black to white transition is low, while almost all the others are high. (Note: the 255 to 0 transiyon is hidden behind the higher ones.) If you had tested this monitor using your current method, you would have concluded it is VERY fast, when, in fact, "standard" mode is actually slow! This is how they used to lie on the box specs: by using only a single number that is fast, while the rest are slow... and your tests only support the lie!


    Examples of how to test response times:
    Xbitlabs tests a range of 72 transitions spread out evenly; the also show the overshoot error rate for all those transitions:
    http://www.xbitlabs.com/articles/monitors/display/samsung-sa850_8.html#sect0
    TftCentral tests 30 transitions & the corresponding overshoot errors:
    http://www.tftcentral.co.uk/reviews/content/benq_bl3200pt.htm#gaming


    If you want, compare your tests of the BenQ BL3200 to tftcentral:
    http://www.tomshardware.com/reviews/benq-bl3200pt-qhd-monitor,3898-9.html
    The tftcentral tests show that the monitor has response time problems of 41ms for the 0-50 transition, but better numbers (6-10ms) otherwise. Your single number tests do not reveal any of these issues.
  • KevinAr18
    Sorry for the aggressive tone in my previous comment. I should have taken more time to write it up nicely.

    This issue with response time testing has always been a problem on tomshardware (even before you ever started writing reviews here), however this is the first time I got a good chance to contact the person that writes the articles. I hope you will be able to look into the response time tests at some point. Sadly, there is literally only two sites on the internet (that I know of) that test response times correctly: xbitlabs & tftcentral. In fact, even tftcentral used to test response times wrong (for many years). xbitlabs was the original site I know of that began testing response times correctly.

    I apologize for not being able to write in more detail right now, but here's two helpful links:
    Response times:
    http://www.xbitlabs.com/articles/monitors/display/lcd-testmethods_5.html
    Overshoot errors:
    http://www.xbitlabs.com/articles/monitors/display/lcd-testmethods_6.html
  • dovah-chan
    You can't really fully benchmark response times accurately as it varies amongst monitors such as the overall overclocking headroom varies by CPUs that are even of the same model.

    Also thanks for this article. It's n-not like I was one of the people who requested it. I'm just glad to see that we are listened to. >__<
  • Drejeck
    I sense a disturbance in the force.
    Is some monitor being reviewed right now? Hope it's a G-sync near 300 euros
  • Blazer1985
    Hi christian!
    I have a request :-)
    Would it be possible for you to compare some cheap chinese tv and monitors to the expensive western branded ones?
    I know that you usually get what you pay for but there might be a sweet spot somewhere in between and would be nice to know :-)
    Especially today that we are beginning the switch to 4k and the prices vary so much (ex. Haier 50" uhd @600€).
    Thanks!
  • KevinAr18
    1572548 said:
    You can't really fully benchmark response times accurately as it varies amongst monitors such as the overall overclocking headroom varies by CPUs that are even of the same model.

    The part I like the most about "benchmarking response times" is that even with minor variations between each monitor, the numbers are very useful in finding out if a particular model is any good. (Personally, I think that the variations between monitors of the same model may be so minor, that testing is still very useful, but I would need lots more data to say with more certainty :)).

    However, instead of me just making comments, I think you may appreciate a good explanation, since I failed to provide one before; sorry about that. :) So... here goes.


    To be clear, the problem that I was referring to with tomshardware's benchmark was basically this: tomshardware tests only one out of thousands of transitions, which may tell you nothing about the monitor's real response times!

    The best way to understand what I am referring to (and why it is so important) would be for me to try and explain the details.

    Consider these details about transitions & response times:

    1. An 8bit monitor uses 256 shades of grey to produce each color. 0 = black; 255 = brightest color (white); 22= very dark grey, etc...
    2. Response time is the measure of how long it takes to change from one shade to another. Say from 0 (black) to 156 (greyish).
    3. A little bit of math shows that if you have 256 shades, there are 65,280 possible transitions. [ (256*256) - 256 = 65,280 ]
    4. Each transition has a different response time; that means an 8bit monitor has over 65,000 different response times!
    5. Consider this example:

    source: http://www.xbitlabs.com/articles/monitors/display/zalman-zm-m240w_4.html#sect3
    Notice how the 255 to 0 transition (white to black) has a response time of ~2ms.
    However, look how many other transitions are maybe 14ms or more!

    Sidenote: overdrive:
    1. Long ago, they dicovered that they could increase response times by applying more voltage to certain transitions (or maybe less in some cases?? not 100% sure).
    2. This "overdrive" method can improve response times a lot (you want it on a monitor). However, if not tuned correctly, too much voltage may be applied and it may "overshoot" the correct color and create bad image problems that are sometimes worse than having slow response times.
    3. This means any response time testing must also test for overdrive errors (commonly called "RTC error").


    The problem with tomshardware (and most sites):
    Currently tomshardware tests only the 255 to 0 (white to black) or the 0 to 255 (black to white) response time and ignores all the other 65,000+. As you can see from the earlier picture, if you test only one transition and not the others, you may end up with a response time that is 100% useless and misleading.
    Also, tomshardware does not test for overdrive errors, which, in some cases, can sometimes be more important than response times

    The correct way to test response times:
    Obviously, you don't want to test all 65,000+ transitions. However, you can test a range of them. Xbitlabs tests 9x9 transitions or 72 transitions scattered evenly around ((9*9)-9 =72).
    This testing reveals some very interesting things. Consider these examples:

    http://www.xbitlabs.com/articles/monitors/display/20inch-6_15.html
    This monitor has a white to black transition of 2ms and a black to white transition of 6ms, but 55+% of the time it is really 16+ms! Tomshardware would have reported false (low) response times if they had tested this monitor using their current method.


    http://www.xbitlabs.com/articles/monitors/display/acer-gd245hq-lg-flatron-w2363d_9.html#sect0
    The response time on this one varys a lot, but you'd never know that without the tests done like on xbitlabs.


    http://www.xbitlabs.com/articles/monitors/display/20inch-4_23.html
    This picture is actually not response times but overdrive errors that I mentioned earlier. What this shows is how MVA panels often have MAJOR image problems when dealing with 0 to 96 and/or similar nearby transitions, but is good most other times. AKA, it means you'd see bad glowing on some dark scenes but a good picture the rest of the time. Note how tomshardware tests reveal none of this important data.