Eizo 240Hz VA monitor uses strobe backlight. No interpolation.

Posted Sep 5, 2013 by

Upon studying the Eizo FDF-2405W manual for Eizo’s upcoming monitor, there is good news on page 15:

Reducing motion blur “Blur Reduction”
Motion blur occurs when the eye recognizes liquid crystal transitions which comes from changing screens (frames). When “Blur Reduction” is set to “On”, the backlight flickers in sync with liquid crystal transition*1 so the change cannot be seen, thereby achieving clear images with less blur. (Default setting: On)

*1 This monitor converts 120 Hz input signals into 240 Hz within the panel, and doubles the refresh rate to draw two images per frame. By applying a voltage higher than the input signal to speed up response (overdrive) for the first image, and then drawing the second image with the original input signal, the liquid crystals are stabilized. The “Blur Reduction” function turns on the backlight only for the stable duration of the second image, and off for other durations.

This means no interpolation is used, so no input lag from interpolation! The Eizo “240Hz” monitor achieves 240Hz via a two-pass refresh. One overdriven refresh in the dark, unseen by the human eye, followed by a single backlight strobe flash on a very clean 120Hz refresh.  This should produce excellent LightBoost-style quality, reasonable input lag, and excellent VA colors. Although this model is targeted at GIS/mapping, this could potentially become an excellent casual-gaming 120Hz monitor with great color! An interesting question is the strobe flash length, as shorter strobe flahes results in less motion blur.

UPDATE: Eizo has released the gaming version of this monitor, the Foris FG2421.
It uses exactly the same two-pass refresh followed by strobe, at 120 strobes per second.

About Mark Rejhon

Also known as Chief Blur Buster. Founder of Blur Busters. Inventor of TestUFO. Read more about him on the About Mark page.

3 Comments For “Eizo 240Hz VA monitor uses strobe backlight. No interpolation.”

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quaixor
Member
quaixor

Sounds impressive. Though I’d like to know if there is a perceptible difference between a regular 120hz IPS and this one to justify the (5-fold? Who knows!) increase in price.

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11 years 12 days ago
Mark Rejhon
Author
Mark Rejhon

That depends on the strobe length, one backlight flash per unique refresh/frame. When compared to 120fps@120Hz (8.333ms frames):
50%:50% duty cycle = 4.167ms flash = 50% less motion blur than 120Hz
25%:75% duty cycle = 2.075ms flash = 75% less motion blur than 120Hz
10%:90% duty cycle = 0.833ms flash = 90% less motion blur than 120Hz

When viewing http://www.testufo.com/#test=photo&pps=1440 in a 120Hz-friendly browser such as Chrome, one can easily tell apart:
LightBoost=100% (2.4ms flash, 1/400sec, 400fps@400Hz equivalence)
LightBoost=10% (1.4ms flash, 1/700sec, 700fps@700Hz equivalence)

At 1440 pixels/second motion during LightBoost=100%, the castle windows at the top becomes slightly blurrier (about ~1.5 pixels of extra motion blur). Although there are points of diminishing returns, they do not stop for a long time — there is definitely a benefit to “1000fps@1000Hz”, and it also becomes even more important for virtual reality (see Michael Abrash’s writings, of Valve Software), but quite noticeable for many motion blur sensitive people during FPS gaming, especially if your eyes are used to CRT-quality motion.

Mathematically, a 60Hz display flickering at 1/120sec flash per refresh, has the same amount of motion blur as a 120Hz sample-and-hold display, for motion of framerate matching Hz. Flicker at any Hz with strobe of (1/X)sec, has equivalent motion blur to an “X” Hz display running at “X” frames per second. “X” is the persistence value, for the length of the off-on-off cycle (not transition time, e.g. on-off only or off-on only). We’re excluding stroboscopic/wagonwheel effects which is worse at lower Hz; and comparing sample-and-hold motion blur — the type you see at http://www.testufo.com/#test=eyetracking — it actually becomes rather simple once a person understands how sample-and-hold motion blur works.

The one short flicker per refresh, for CRT, is why CRT 60fps@60Hz has motion blur than LCD 120fps@120Hz (non-LightBoost). This is even despite the fact that many medium-persistence CRT phosphors (e.g. FW900) have slower decay times than modern LCD pixel transition times in today’s 1ms panels. The sample-and-hold effect is enforcing motion blur; the shorter the persistence (flicker cycle, off-on-off), the less motion blur.

I will be somewhat disappointed if the strobe is 1/240sec. That will make it more motion blur than LightBoost (which has motion as clear as a 700fps@700Hz display — visualize it like a 700Hz display with lots of black frame insertion, and only 120 visible frames lasting 1/700sec long eacah). But the manual suggests that the strobe only occurs at the clean period of the second refresh, which suggests the strobe is shorter than 1/240sec.

I will be quite interested to put an oscilloscope against the surface of an Eizo 240Hz VA monitor, to determine its strobe cycle, and compare it to CRT and LightBoost. I would optimistically hope for an approximately 2ms (1/500sec); a strobe length that make a lot of sense. (That strobe length is currently economically doable at their quoted average ~350cd with a brief boost voltage to an overengineered LED edgelight. This requires approximately a ~1500cd surge of a flash from the edgelight which is already possible with newer LED’s, and by being on 25% of the time, would average down to about 350cd. CRT phosphors can ‘flicker’ at ~5000cd to compensate for the dark period between refreshes, so we’re getting “there”). This would make motion blur roughly equivalent to a LightBoost monitor running LightBoost=60%

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11 years 12 days ago
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