How the industry defines the pixel "response time".
- Rise time (tR): Time it takes to go from 0 (black) to 255 (white).
Fall time (tF): Time it takes to go from 255 to 0.
Black-to-Black(tR + tF): This is standard definition of a response time as defined by the ISO 13406-2 method. This is the time it takes to do a complete cycle from black to white and back to black.
Gray-to-gray (G2G or GtG): This is the most common value that's associated with reported response times from the manufacturers, since its usually the quickest and looks better for marketing people to tout. The biggest issue with G2G times is that there is no standardization on test procedures so you never know what you are actually getting. It can mean going from 128 to 255, but it can also be going the opposite way or with much small value changes; or just the complete average. They never say.
Realistically,
G2G is a bit more practical since most of the time, your videos would rarely strobe between 0 and 255 values. If there was a standardized way of displaying the response times to their customers, then this value would have more meaning, but this requires lots of time and money to educate their customers when they really don't need to know too much about this information.
The biggest thing to note is that
Rise time will tend to be very very slow relative to Fall time. Say if you have a tR of 10ms from 0 to 255, 7ms of that time could be spent just by going into an "on" state; from 0 to 32.
Fall time tends to be very fast since its just a matter of increasing voltage. Going from 255 to 0 in most cases can be 2ms or faster. Some panels may have problems when dropping to dark gray values, but typically, they should still be quite a bit faster than similar Rise times.
Overdrive/
Response Time Compensation (RTC) is a technique used in TN panels to apply over-voltage to the Liquid Crystals (LC) to accelerate or motive them into orientation faster. In a TN panel, the LC's are used to block the light coming from the backlight; in this case a black or "off" (0) is actually in an active state where voltage is being applied to the LC. In a fully white state or "on" (255), no voltage is being applied to the LC allowing for full transmission of the backlight.
RTC forces the LC to go into a full white (inactive state) and then to a black (active state) transition first before the LC is allowed to drop to the required grey level. This speeds up the change for G2G values, but does nothing for the actual black to white (tR) response time.
However, when using an RTC mode, a few issues can arise. Color reproduction error/overshoot (aka RTC error %), video noise, and colour trailing resulting in dark halo's on moving objects. RTC error % is the chance of a transition will overshoot the value during the RTC cycle, thus needing to be corrected after the fact. This results in longer response times and can result with artifacts and video noise.
On quality panels, RTC error's are usually very minimal and only occur when going between small values (like 64 to 96); It's occurs more frequently from dark gray to lighter gray transitions. Video noise can become an issue in areas of soft gradation, specially when a panel uses dithering and the RTC error is high for that particular grey value. Colour trailing tends to be a hard issue to see since the black halo's only occur in moving objects, in high contrast scenes and the halo's only last for 2-3 frames; plus, it can only be scene for less than 10-15ms in most modern panels and hardly noticeable.
So now that you just read all of that, lets put that to a practical meaning with an example.
Above is the Samsung SyncMaster SA950 TN panel monitor response time 3d bar graph; It has a stated 2ms response time rating on the box. As you can see, in normal mode, it doesn't even come close. It has an average of 8.2ms G2G with a max of 16ms response time. You can also see that going from any "on" (32-255) state to "off" (0) state is 1ms or less, but requires a little over 9ms to go from 0 to 32.
Above is the same panel, but using an RTC overdrive setting. Clearly it lives up to its expectation for the most part with an average of 2.4ms G2G response time with no higher than 4ms; although it does have some outlier issues with transitions with dark-grays and almost white values. Most panels tend to have similar reproduction problems so its generally acceptable.
Now above is the RTC error % 3d bar chart when running the panel in the RTC mode. This is a bit high for a quality panel but it does highlight some issues with RTC control abilities in trying to hit the right color value the first time. Going from values 32 to 128 will result in an 82% failure rate, requiring the need for a correction for that particular transition. In some monitors, it could take 35ms or longer to correct the color, but generally the rule of thumb is 3 frames to clear and correct.
If you want more information or complete detailed reviews on monitors, visit xbitlabs.com; they have more than enough information to throw a stick at. Other sources is tft central and tom's hardware.
