BenQ XL2730Z - What is required for smooth 1440p @ 120Hz?

[falkentyne]

280x is a rebadged 7970 ghz edition.
280 is as far as I know, a 7950.

Crosstalk is a very thick double or triple image effect caused by the LCD panel being caught inbetween pixel transitions when strobing the backlight. It's basically caught inbetween frames. Unless you adjust the "Strobe phase" (this has been renamed to Area in the XL2730Z), crosstalk will always be worst at the bottom of the screen. This is because even though the entire backlight is strobed at once, refreshes are done from top to bottom in LCD panels. This is why input lag tests always show the top of the screen having a faster response time (less input lag) than the bottom of the screen. Since the bottom refreshes after the top, when strobing the backlight, the strobe gets completed before the bottom has finished the refresh. So this causes an image of frames caught between one refresh and the next refresh. That is called strobe crosstalk. Strobing turns the backlight on and off once per refresh; the duration that the backlight remains off is "Intensity", aka Strobe duty. It's reversed in the XL2730Z compared to the older blur reduction monitors, but basically, at a high strobe duty (more blur reduction effect=dimmer screen), during a strobe cycle, the backlight remains off for a longer period than it remains on. The on/off total cycle is the same duration in time, but the backlight is off longer than it is on.

When you lower the intensity, the backlight is on longer than it is off= less blur reduction effect, even though the total duration is still the same per refresh.

The problem is, the duration is going to be *faster* than the refresh is. Let's say you are at 120hz. Simple example. 120hz is 8.3 milliseconds of frame time. (1000 divided by refresh rate= milliseconds). 1000 / 120=8.3. Easy enough right?

Now remember the panel pixel response time is different from the refresh time. Let's say the panel has 3.7ms of pixel response time at the top of the screen and 14.3ms at the bottom of the screen.

So we strobe the backlight once per refresh. The refresh is 8.3 milliseconds.

As you can see the top of the screen will complete the pixel response EASILY during the refresh time (3.7ms vs 8.3ms)= NO strobe crosstalk.

the bottom however has a slower response time.
14.3ms response time at 8.3ms FRAME Time.
The strobe finishes, but the bottom hasn't finished the pixel transitions yet. You have an incomplete frame. As you can see. by simple math, you will see that the bottom is caught between the previous frame transition and the current frame transition. So you get two frames superimposed on each other. that's your crosstalk.

At 144hz, the frame time (refresh rate) is even FASTER than 120hz, so logically, you will have even MORE crosstalk (a larger crosstalk zone) at the bottom than at 120hz.

If the XL2730Z were capable of single strobing at 60hz (the XL2720Z is, the XL2730Z is not), you would notice that the crosstalk would be half the size at 60hz than at 120hz. Very self explanatory...60hz is 16.7ms frame time. Since the input lag is also going to rise, the total panel response time also goes up, but the panel can finish pixel transitions of far more of the screen at the slower refresh rate=less crosstalk.


So...what does the AREA setting do?

Simple. It changes *WHEN* the strobe begins and ends on the screen during the refresh, so the crosstalk position is moved up or down. It's basically moving the entire strobe to happen "earlier" or "later" in a set point in time, relative to the refresh itself. While Area is a simple name for "crosstalk area", if you look up the dictionary definition of PHASE, it tells you a lot more about what is going on. By changing the strobe PHASE, you are changing WHEN a strobe begins and ends (when the backlight is turned on or off) during the refresh period. In effect, you are changing the SYNC of the strobe to the refresh itself. I think this makes sense when you think about it.

Using a strobe phase (AREA) of 100 will give you one frame (1000 divided by refresh rate) of LOWER input lag than a strobe phase of 000, but you will have even more crosstalk at the bottom of the screen with a strobe phase of 100 than a strobe phase of 000. If the bottom of the screen is covered by a game HUD, like FPS game, and you can't see the crosstalk, strobe phase 100 (AREA=100) will give you the lowest input lag. You may wonder why this is the case. Well think about it for a minute.

The default strobe phase of 000 has an input lag of 1 frame HIGHER than strobe phase 100. This would only make sense if the strobe sync phase were strobing during the PREVIOUS frame at strobe phase 000 (meaning starting and ending the strobe mostly during the previous refresh) than at strobe phase 100 (the current refresh). And sure enough that's exactly what is going on. So you will have less crosstalk at the bottom of the screen but 1 frame of higher input lag at strobe phase (area) 000 ,and 1 frame faster (lower) input lag at phase (are) 100, but more crosstalk.

To demonstrate this, watch the position of the vertical red bar and the UFO's on this link and change the AREA from 0 to 100.

http://www.testufo.com/#test=photo&photo...&height=-1

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The previous blur reduction monitors (XL2720Z, XL2420Z, XL2430T, etc) allowed increasing the "vertical blanking interval" (Vertical total) via changing the VT from the default, to 1497-1502 (anything in this 6 point range) which lowered strobe crosstalk by 25%, because it gave the panel even more time to complete pixel transitions during each strobe. It basically made the entire strobe 'longer', if that makes sense. The XL2730Z does not respond to VT changes for strobe crosstalk anymore.

When you are not using blur reduction, you don't get any of this "Crosstalk" because the pixels are always on, so you see a completely uniform image top to bottom. It's just the input lag/response time will be slower at the bottom than at the top (although this is not exactly something you are going to notice in a game).

Ghosting is a completely different thing. Ghosting is from overdrive/voltages from pixels not finishing their color transitions, which isn't directly related to strobing itself. You will get ghosting whether or not you are using blur reduction. Just blur reduction makes ghosting more obvious since the blur is removed. There is a way to force an "AMA Low" mode when using blur reduction but I don't know how it's set on the XL2730Z. You're supposed to set AMA To high AFTER enabling blur reduction for this to work, but there may be something else involved that was different than the XL2720Z, so I can't help with this.