Falkentyne wrote:Just so we can get clarification, can you please clarify what you mean by "
Because from what I've seen, there may be two forms of crosstalk.
The strobe crosstalk I think most people are familiar with are caused by strobes being *TOO EARLY OR LATE*, because then the strobe completes too soon or too late, than most pixel color transitions can be completed, (e.g. at 6.9ms at 144hz), so then the next frame begins, the strobe is done for the next frame, but most of the current frame's color pixel transitions aren't completed. So then you wind up having the "next frame's" beginning color transition patterns blended in with the previous frame's later color transitions, which creates a very ugly double image of crosstalk and ghosting overlays. At 120hz, the crosstalk is less due to 8.3ms strobes taking more time.
Minor fixes to make this correct.
Strobe length actually has fairly little to do with strobe crosstalk, except as a side effect -- longer strobes create more strobe crosstalk simply because they're longer than the time the previous refresh cycle finishes and before the next refresh cycle begins.
It's important to not confuse "faster" with "higher rate" or "briefer strobe".
A good strobe crosstalk measurement test pattern is TestUFO Moving Photo, Alien Invasion
. Make sure it's animating at full framerate (try different web browsers; not all browsers work properly on all systems).
An early strobe or a late strobe can create strobe crosstalk. A strobe that is too early creates strobe crosstalk (double-image-effect) at the bottom edge of the screen, and a strobe that is too late creates strobe crosstalk (double-image-effect) at the top edge.
Naturally, adjusting strobe timing (phase) moves the strobe crosstalk vertically -- the "double image ghosting band" -- moves vertically. The name of the game is time it in a way to push strobe crosstalk off the bottom edge of the screen but without reappearing again at the top edge. Sometimes it's mathematically impossible (laws of physics).
An LCD is refreshed top-to-bottom (as seen in high speed videos), so the moment the backlight is flashed, you want to time it when the LCD refresh (preferrably done in total darkness) finishes to the bottom edge of screen, but before the next refresh cycle begins refreshing the top edge. So a strobe that's timed a little early will create crosstalking at bottom edge, and a strobe that's timed a little late will create crosstalking at top edge.
The end of the previous refresh -- often ends the last pixel row at bottom of screen. The beginning of next refresh cycle -- often begins on the first pixel row at top of screen. Now you want enough time between refresh cycles. That's the blanking interval (ala VSYNC, or VBLANK, or sync interval, many terminology exist). The blanking interval can be short or long. A large Vertical Total means you've got a longer blanking interval.
How many milliseconds is VSYNC? Depends. If Vertical Total is 1/4th of the visible vertical resolution (say, 1080p visible + 270 pixel blanking interval, for a Vertical Total of 1350), the display is spending 1080/1350th of its refresh cycle actually refreshing the LCD pixels (initiating their beginnings of GtG transition), and the other 270/1350th of the refresh cycle is spent pausing between refresh cycle (which is also useful for waiting for the momentum of any remaining GtG transitions to complete, as well as the strobing).
If the refresh rate is 120Hz (8.33ms per refresh) and using VT of 1350 means 270/1350th of 8.33ms equals ~1.7 millisecond pause between refresh cycles when you're using a Vertical Total of 1350 on a 1080p LCD at 120Hz (1080 visible vertical resolution + 270 of blanking interval). Increasing the Vertical Total while keeping refresh rate the same, speeds up the LCD scan -- but does not speed up pixel transitions. When I say "LCD scan", I mean initiating
the pixel transition of each pixel.
For a Vertical Total of 1350, it simply means the display will take 1/1350th of 1/120th second to finish initiating
the pixel transitions of the first row of pixels of the screen. Then the next 1/1350th of 1/120th of a second is spent initiating
the pixel transitions of the second
row of pixels on screen. And so on. LCD's are refreshed top-to-bottom in terms of initiating the pixel transitions.
(Replace "1350" with whatever Vertical Total is being used)
Obviously, once you're finished electronically
refreshing the bottom row of pixels, that doesn't mean you're visually
finished yet. It still takes another ~1ms to finish pixel transitions (give or take). In high speed videos -- like www.blurbusters.com/lightboost/video
-- LCD pixels simply "fade" from one color to another. Most of the transition (fade) is complete in 1ms.
Now, many modern TN LCD's will finish most pixel transitions in 1ms. This is a good response time acceleration of 1ms for all possible transitions between any shades. Obviously, 1ms is not a hard-and-fast number, as pixels may only be 99% completed in 1ms -- meaning you might have a faint ghost (1% intensity).
Note: Vertical Total is not always controllable by users with all monitors.
Many displays will artificially use internal large vertical totals (accelerated scan for long intervals between refreshes) -- LightBoost does this automatically. The great thing is BENQ XL2720Z's (and compatibles) are one of the few monitors that lets end users manually adjust the speed of the LCD scanning via the Vertical Total trick.
However, strobe backlights are often running at very tight tolerances, so you don't always have
General rule of thumbs (not always hard-and-fast, but usual):Strobe crosstalk factors:
-- The "double-image" horizontal band of strobe crosstalk is taller at higher refresh rates and/or on slower LCDs (longer pixel transitions).
-- If you strobe too early, you get more strobe crosstalk at bottom edge of screen
-- If you strobe too late, you get more strobe crosstalk at top edge of screen
-- If your strobes are too long for blanking interval, you get strobe crosstalk at both top and bottom edge of screen.
-- Reducing strobe length, lowering refresh rate, longer blanking intervals(via large Vertical Totals) or doing 2 or 3 combined -- can reduce the Catch-22 situation of having strobe crosstalk at both top/bottom edges of screen.
Techniques of longer blank intervals
-- Longer blank intervals can be done manually (e.g. via large Vertical Totals) or automatically (e.g. LightBoost firmware in monitor)
-- Automated large blank intervals (e.g. LightBoost) will override & render manual Vertical Total adjustments useless. (frustrating sometimes).
-- Lower refresh rates
The bottom line, is.... Getting good strobing in a monitor, is very fiddly.
At 60hz, the crosstalk is exactly half as much as 120hz (coming from the bottom of the screen as screens are refreshed top to bottom).
Not necessarily exactly half. It's usually half simply because there's twice as long between refresh cycles, but you can compensate by using a large Vertical Total. For example, well-tweaked 120Hz with large Vertical Total can produce better results than 100Hz at default Vertical Total setting.
Also, pixel transition speed can distort the mathematics behind this, to the point where 60Hz might actually result in 3x less strobe crosstalk than 120Hz, especially if pixel transitions are slow (e.g. you're using a 1.6ms blanking interval and pixel transitions are 2-3ms). In this situation you're stuck with strobe crosstalk at both top edge and bottom edge of screen, even with instant strobes (e.g. 0.001ms strobes). This is because if pixel transitions take longer than the blanking interval (e.g. 3ms pixel transitions and a 1ms blanking interval), the monitor is initiating refresh of the top row of pixels long before the bottom row of pixels have finished their pixel transitions.
I hope this is not too confusing to readers, but this is quite advanced stuff for the layperson, to understand the double-image motion ghosting artifacts (strobe crosstalk) of a strobe-backlight monitor.