<Advanced Technical Explanations Of Multi-Image Effect Of Strobing At High Refresh Rates>
Falkentyne wrote:WOW
YES IT IS DOUBLE STROBING AT 240HZ !!!!!!!!!!
It's doing two 120hz strobes, or either a long or short strobe.
Um.... False.
It's single-strobing at 240Hz.
It's simply expected unusually strong strobe crosstalk between refresh cycles, because 240Hz refresh cycles are only 4.1ms long, which means the 1ms LCD GtG starts playing a big factor. Even 1ms GtG to 98-99% still often takes 10ms+ MS to be 99.9%+ complete.
As a rule of thumb, the closer a refresh cycle (4ms) becomes to the time-length of the LCD GtG (1ms), the stronger the double-image effect
DURING SINGLE STROBE PER REFRESH CYCLE occurs. It's simply law of physics.
The LCD response limitations (GtG) is simply bleeding over more refresh cycles simultaneously. In order to eliminate strobe crosstalk you kind of need to force a 2ms blanking interval in. The only way to do that at 240Hz is a 1/480sec scanout, so the scanout takes ~2ms, with a 2ms blanking interval. Meaning, you need a Vertical Total of over >2160 at 240Hz, just to barely match the amount of strobe crosstalk of 120Hz VT1350.
Meaning 240Hz VT2160 would have roughly the same ~2ms blanking interval of 120Hz VT1350. Due to GtG and higher refresh rate, the Vertical Total requirements begins to grow geometrically. If 360Hz LCDs came out, you would now begin to need a ~VT5000 just to maintain a 2ms blanking interval between refresh cycles. At 480Hz, you'd need VT(infinity) -- instant scanout just to cause a 2ms idle time between refresh cycles. This is not possible -- in order to match the strobe crosstalk of 120Hz, so unless LCD GtG goes faster than ~1ms, we begin to finish hitting the limitations of TN LCD panels...
Remember trying to enable strobing at 144Hz on an XL2720Z? The problem becomes worse when enabling strobing at 240Hz because there's no way to get vertical totals (scanout velocity) fast enough to create a long enough pause between refresh cycles to accomodate the 1ms LCD GtG.
(Also, another evidence point that it's single-strobing at 240Hz: the stepping factor in TestUFO is exactly half the horizontal step, at 240Hz, than at 120Hz. It's always 4 pixel step at 960 pixels/sec at 240Hz, which is what the photograph shows.)
Also, at 240Hz, the overdrive is more imperfect than at lower refresh rates, so the technology limitations is also fighting against that too. There will probably be continued innovations to greatly improve strobing at high refresh rates (e.g. NVIDIA's excellent job at LCD overdrive tables -- they are the very best in the industry -- their strobe-optimized overdrive logic even takes into account of the Y dimension, with more aggressive overdrive at the bottom edge than center, to compensate for less time before the strobe). Currently, it's believed that BENQ's 240Hz overdrive is not currently taking into account the Y dimension, to reduce strobe crosstalk the way NVIDIA's ULMB/LightBoost logic does.
TL;DR; BENX XL2540 is not double-strobing at 240Hz. It's simply stronger strobe crosstalk caused by law of physics of LCD GtG limitations. The closer a refresh cycle enroaches into LCD GtG, the stronger the strobe crosstalk becomes, even to the point where it begins to look like to double/triple/quadruple strobe even though it's still strobing once per refresh.
Let's harken back to 33ms LCD days briefly, so let's study
my writing. Look at how the old 33ms 60Hz LCD often displayed 3 refresh cycles simultaneously:
Look at how "6", "7" and "8" are superimposed on each other on this very old 33ms 60Hz LCD. Although most of the transition is complete after 33ms, there is still lots of faint remnants more than 33ms prior, as it takes more than 33ms to get GtG to complete, causing even 2 refreshes-ago to still be faintly visible.
The same thing is sort of happening again at 240Hz --
What we're now encountering with 240Hz LCDs is that the LCD GtG visiblity now spreads much more visibly over more refresh cycles. Even with 1ms GtG, the overdrive isn't perfect enough to get the GtG more than 99% complete before the next refresh cycle, so we're ending up with a situation where GtG remnants bleeds over multiple refresh cycles.
Better overdrive will help greatly -- to a point. It's still the very early days of 240Hz LCDs, and lots of optimizing is possible.
TL;DR #2: People who assume strong double images means double strobing per refresh cycle, are automatically telling you they don't understand how an LCD refreshes. Even in single-strobe situations, it is simply a function of GtG enroachment between refresh cycles.
These are still great low-latency LCD displays for competitive gaming, if you can tolerate the bleeding edge issues of first-timer (less refined) 240Hz operation.
TIP:
1. It helps if you warm up a cold LCD (20-30 minute warm up) as we're familiar with LCDs responding much slower in the cold (like a freezing smartphone or freezing wristwatch in mid winter), the exact same thing happens to monitors. Strobe crosstalk is significantly reduced on a warm LCD than a cold LCD. Temperature is much more important when we're talking about GtG beginning to enroach much more visibly due to the much shorter refresh cycles.
2. It helps if you reduce contrast ratio Slightly bumping black levels and lowering peak white, gives more room for successful overdriven GtG completions, so strobe crosstalk can greatly reduce itself at lower contrast ratios where blacks are slightly brightened and whites are slightly dimmed, to roughly 80-90% of dynamic range.
3. It helps to have as large vertical totals as possible That said, 240Hz monitors are currently as of yet unable to create a 2ms blanking intervals (yet), since that requires a ~1/480sec to ~1/500sec scanout of a refresh cycle. (I.e. 1/240sec = 4.1ms ... but to create a 2ms pause between refresh cycles, the 1/240sec needs to be scanned out in 2ms, which is (2/1000ths) = 1/500th second = just to create a pause of 2ms between 240Hz refresh cycles.
</Advanced Technical Explanations Of Multi-Image Effect Of Strobing At High Refresh Rates>
Now let's get back onto topic.