MAMEWorld Forums - News - Re: Eliminate LCD motion blur -- CRT quality on 120 Hz LCD -- with MAME software black frame insertion

> Even with the lost cost monitors posted, people with mame cabs have
> very specific needs. I had to buy a very specific 16:10 (not 16:9) lcd so it
> would fit into my cab, for example.

Agreed about cabinet requirements. Nothing can be done about that.

On the other hand, if you're looking for a good new desktop monitor for all-purpose gaming -- remember, the 120 Hz monitors, especially LightBoost, are great for all manner of desktop gaming, and some of us want to game with everything. Both sub-$300 LightBoost monitors have a portrait mode, which is excellent for MAME too; they have rotating display stands. You want a wonderful all-around desktop gaming computer monitor that also accidentally happens to be the world's best flat panel MAME monitor -- you've found it!

> Also correct me if I'm wrong, but isn't motion blur caused by running your monitor at
> a refresh rate too high for your pc/videocard/ect to keep up with? So wouldn't the
> solution be to just run a lower refresh rate?

Alas, this is theoretically true if LightBoost strobe backlight worked at 60 Hz.
... Unfortunately, nVidia hardware-locked LightBoost to 100-120 Hz.

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How it benefits 120 Hz LightBoost LCD's

There is 90% less motion blur for 120Hz+LightBoost+BlackFrames(simulate 60Hz) than with plain 60Hz on the SAME monitor as a result! This is because we're getting approx ~2ms millisecond strobe backlight flashes, instead of an LCD frame being continuously shining for a full refresh. The flicker is 1/10th the length of a refresh (1/60sec = 16.7ms), and reduces motion blur proportionally, confirmed in tests by multiple sources and reviewers, including pcmonitors.info (VG248QE review).

At LightBoost 120 Hz, you get two strobes (~2ms) per 60fps MAME frame, which is bad. But if we go back to 60 Hz, we lose the strobe benefit and get 16.7ms sample-and-hold. So we instead stay at 120 Hz to keep the strobe benefit. The workaround is a MAME modification to mask every other strobe by having a black screen between the 60fps frames. This is the "black frame insertion" effect. So you get only one strobe flicker per 60fps frame. You get the full benefit of a strobe length that's about one-tenth the length of a traditional 60 Hz LCD refresh. The motion blur reduction is proportional to that (science & references).

Normally LCD displays don't flicker while CRT displays do. Ever wondered 60fps on LCD is more motion blurred than 60fps on CRT -- now you know why. The stroboscopic effect of CRT flicker is the major player here. Even though we got 2ms TN panels which is nearly the same 2ms as CRT phosphor decay on medium-persistence phosphor. Pixel persistence is not the limiting factor on modern LCD panels. The sample-and-hold (non-strobed) nature is the limiting factor. That's why CRT and plasma looked better until stroboscopic backlight LCD's arrived.

Since LightBoost only works at 120 Hz, we have to mask the unwanted extra strobes by blocking them with a black frame. So we get 60 Hz flicker out of a 120 Hz LighBoost display. One single backlight strobe per MAME refresh. The strobe length varies from 1.4ms to 2.4ms (photodiode oscilloscope measurement, depending on monitor's LightBoost setting). That's an order of magnitude shorter than the length of a 60 Hz refresh (1/60sec = 16.7ms), and the motion blur is also an order of magnitude less on the very same LCD display as a result, when enabling both LightBoost and black frame insertion simultaneously. Everything then suddenly becomes CRT sharp.

Recently, the ASUS VG278H (a 2ms TN LCD panel) was measured to have an effective pixel response time of 1.4ms if you enable the LightBoost strobe backlight; because the human eye only sees the strobes on fully refreshed frames (the pixel persistence is kept in total darkness). The strobes can be shorter than the pixel transitions! The pixel persistence barrier is shattered by strobe backlights.

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How it benefits 120 Hz traditional LCD's

It also benefits other 120 Hz LCD's for a different reason.
Motion blur is proportional to the length of time a refresh is displayed for. 60 Hz LCD's display a frame for a full 16.7 milliseconds, but a 120 Hz LCD displays a frame for only 8.33 milliseconds. If you make every other frame black colored, each 60fps MAME frame is effectively only 8.33 milliseconds long. As a result, the black frame insertion effect reduces motion blur by 50%.

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How it benefits 120 Hz CRTs

Also, display limitations can apply. 31.5Khz arcade CRT's are unable to display 240p at 60 Hz. You must run 240p at 120Hz (fix by black frame insertion), or you must use 480p at 60 Hz (fix by inserting black scan line gaps)