[A5hun] How LCD Response Times are Measured, and Why 10% to 90% GtG Measurements are Moderately Deceptive

[axaro1]

I'm a big fan of A5hun and I wanted to share a very informative and technical analysis of how G2G is calculated and what the G2G table charts from your average monitor reviewers compare to a normalized gamma response curve (that is a more accurate and panel specific form of Rise/Fall interpretation).

phpBB [video]

[Chief Blur Buster]

Just added [video] tags to embed YoutTube video.

His stuff is indeed great too. The red phosphor article was a collaboration between multiple reviewers that strived to find out the problem of the red-phosphor ghosting issue, and understand the weird nuances thereof.

Some related topics to this universe:

1. Understanding GtG versus MPRT: Two Pixel Response Benchmarks
2. What Causes GtG Increment When Lowering Refresh Rate?
3. Microsecond LCD GtG Times Possible in Blue Phase LCDs
4. The Complexity of Testing GtG
5. We Need 100-Level Overdrive Adjustments
6. The Complexity of VRR Overdrive
7. We Need User-Defineable Overdrive Lookup Tables

Even things like a 1-degree room temperature difference changes GtG response dramatically. Even a 1-week break in period after receiving a new monitor changes GtG response dramatically. For example, a 3.2ms boucneless becomes a 2.4ms bounceless. Even the pressure spot from a mis-shipping (takes a few days of breakin to fade: shipping pressures can be like a finger pressed on the LCD for several days; and sometimes the pressure mark takes a long time to fade). Even panel lottery and panel aging can change GtG slightly. So many subtle factors that complicate GtG benchmarking.

I also want manufacturers to use overdrive sliders (100 settings) rather than 2 or 3 or 4 Overdrive settings.

• Some users prefer no overdrive at all (vision gets distracted by coronas, slowing them down)
• Some users want super-excess blatant overdrive (BenQ AMA Premium) because it's like a tracer-bullet assist feature
• Freezing homes (artic) will slow pixel response, requiring slightly higher overdrive
• Hot homes (tropics) will speed pixel response, requiring slightly less overdrive
• A monitor that ages can have slightly different GtG response in two years than when initially received
• Some users have a preference to faster pixel response with slight coronas

A compromise is that the existing series of Overdrive settings can have a "User Defined" setting added to them, which then unlocks the Overdrive Gain slider that directly accesses the scaler's overdrive gain register (usually a range of 0 to 128 or thereabouts). That way, it won't complicate users since only advanced users will bother to access "User Defined".

(I would love to see A5hun begin participating again in the Blur Busters Forums!)

[Digika]

I also want manufacturers to use overdrive sliders (100 settings) rather than 2 or 3 or 4 Overdrive settings.

I think the reason they dont do that is to reduce the amount of testing, if you have set up of 20/60/80/100 you just have main 4 passes, but with step of 1, 1-100scale that becomes 100 main passes. Takes crapload of time.

[Chief Blur Buster]

I also want manufacturers to use overdrive sliders (100 settings) rather than 2 or 3 or 4 Overdrive settings.

I think the reason they dont do that is to reduce the amount of testing, if you have set up of 20/60/80/100 you just have main 4 passes, but with step of 1, 1-100scale that becomes 100 main passes. Takes crapload of time.

Actually, surprising to many people, is that the scaler/TCON already has this register, and the testing/work is already done at the scaler vendor. All panels have this slider built into them.

But this is not exposed at the menu level. How it works at the manufacturer level is that they choose their favourite OD Gain settings (e.g. 27, 45, 93) and give them names, like "Wimpy", "Normal", "Hypnotizer" or whatnot.

Imagine being only able to select brightness number #20, #40, #60, #80 and #100 and not letting you access in-between values! Yet monitors let you have this, and even provides you RGB Gain settings (an advanced user setting)

The good news is that a few monitors DO have this overdrive slider available in main menus. For example Nixeus monitors and the upcoming Eve monitors do have the overdrive slider. While others (ASUS, BenQ) have it accessible in Factory Menu (of which setting is sometimes lost/forgotten after power cycling the monitor). This needs to be in the user accessible region, something like Settings -> Advanced area, or perhaps Overdrive -> [Light, Normal, Maximum, User Define] -> (Select User Defined) -> Overdrive Slider Appears 0%[||||||---------]100%

[Chief Blur Buster]

I also want manufacturers to use overdrive sliders (100 settings) rather than 2 or 3 or 4 Overdrive settings.

I think the reason they dont do that is to reduce the amount of testing, if you have set up of 20/60/80/100 you just have main 4 passes, but with step of 1, 1-100scale that becomes 100 main passes. Takes crapload of time.

<Programmer POV>
Since you're a software developer I think....

The Overdrive Algorithm
How overdrive is generated is that they run photodiode oscilloscope on many passes to find correct overshoot colors. For example using grey value 220 to speed up a transition from grey 50 to grey 200. That's an voltage-level overshoot of +20 for a specific color combo without showing a vision-level overshoot ripple on an oscilloscope. That's the "strongest visually overshootless overdrive" you tune for -- the biggest voltage overshoot you aim for that doesn't produce a visible/measurable photon-level overshoot. There's 3 separate monochrome pixels per color pixel, so overdrive is processed 3 times independently for each color channel, since overdrive only cares about the monochrome transparency of the pixel in the LCD layer, without regards to the color filter layer. So you have one LUT that can be used for all color channels.

The Overdrive Lookup Table
You measure this for all grey combos to create an Overdrive Lookup Table of A(B) = C where A is source grey, B is destination grey, and C is the grey to use to accelerate transition from A to B. There are 65536 values in a 256x256 OD LUT on an 8-bit panel, though most manufacturers use a subset such as 17x17 OD LUT and use bilinear interpolation to generate the rest of the LUT (256x256) to save scaler memory. Ideal OD LUTs is generated on a per-Hz basis, calibrated at 20C room temperature. Also, reduced-resolution OD LUT is a big problem in the 240Hz+ and 360Hz+ refreh rate race because of stronger localized hotspots in the GtG heatmapping, which would benefit from finer-resolution original OD LUTs. VA panels, in particular, benefits significantly from larger-sized OD LUTs, combined with an OD Gain, to help better mitigate the dim-colors VA ghosting.

The Overdrive Gain
Overdrive Gain is a simple multiplier to the delta (difference of B and C in the A(B)=C formula) in the OD LUT. The middle setting is usually a 1.0 multiplier. So the overshoot is a +20 there. Now if you slide the slider lower, the multiplier may become 0.6. So your overshoot becomes only +12 in that case. Now if you slide the slider higher, the multiplier may become 2.0. In this case the overshoot becomes +40.

Deviating from neutral (visually overshootless) overdrive, will produce more ghosting or more coronas. And if temperature changes, the calibration may require a slight overdrive gain shift to compensate for temperature or panel lottery effects. Also, values outside the gamut is clipped (e.g. clipped to 0-255). Also, Overdrive LUTs can be multiple refresh depth if need be (e.g. needing to clamp a pixel to color value 0 for two or three refresh cycles to speed a transition from greyscale 50 to greyscale 5, which is unusually slow on a VA panel during cold temperatures, so multiple-refresh-cycle overdrive can be a big benefit for such panels. But it's more expensive to engineer for. However, Overdrive Gain is a simple engineering move, and it is frankly, it is surprising that manufacturers have been resistant to letting users adjust Overdrive Gain.

Overdrive Gain is absurdly simple mathematics, and that's why an overdrive slider is simple Math 101 stuff.
</Programmer POV>