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G-Sync and PWM [Good news: It's PWM-free]

Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.

G-Sync and PWM [Good news: It's PWM-free]

Postby Stryker7314 » 28 Jan 2014, 05:16

Can someone confirm that the G-Sync module on a VG248qe makes it PWM free, want to verify before ordering. I saw that the Chief Blur Buster said so in one of his comments before release, but did not back it up with a test once the module was reviewed. I need your help Chief. :|
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Re: G-Sync and PWM

Postby Stryker7314 » 28 Jan 2014, 12:23

Looks like I found were Chief Blur Buster mentioned this ( http://www.blurbusters.com/nvidia-confi ... sync-kits/ ). "The G-SYNC board appears to make VG248QE become PWM-free or high-PWM (5-digit KHz); as I no longer can detect PWM, so that part seems solved." Chief can you please expand on the PWM of the VG248QE if there is anything else to add? Maybe there isn't much more to say other than it runs at PWM-free spec.
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Chief Blur Buster » 28 Jan 2014, 16:05

Yes, the G-SYNC upgrade takes over all backlight driving and makes the VG248QE backlight PWM-free (or hi-PWM beyond my measuring equipment's limitations).

I did a 10KHz basic photodiode oscilloscope test, and it was flat DC. And I tested using the fast-camera-wave-and-take-photo (TFTCentral PWM test. I couldn't see any pulses.
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby nimbulan » 28 Jan 2014, 19:47

Chief Blur Buster wrote:Yes, the G-SYNC upgrade takes over all backlight driving and makes the VG248QE backlight PWM-free (or hi-PWM beyond my measuring equipment's limitations).

I did a 10KHz basic photodiode oscilloscope test, and it was flat DC. And I tested using the fast-camera-wave-and-take-photo (TFTCentral PWM test). I couldn't see any pulses.

I'm curious, does that mean that the G-sync upgrade hardware is technically capable of performing simultaneous G-sync and strobe operation as outlined in your article since it seems to be capable of both constant brightness and strobing? Obviously the firmware is not capable of this yet but it would be a very nice thing to have as an update.
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Chief Blur Buster » 29 Jan 2014, 00:05

nimbulan wrote:I'm curious, does that mean that the G-sync upgrade hardware is technically capable of performing simultaneous G-sync and strobe operation

Theoretically. However, achieving this reliably without artifacts may not be feasible. To do so, you would want a strobe range of about ~85Hz-120Hz. Below 85Hz it needs to migrate to flat DC or it flickers at low strobe rates; above 120Hz(on 144Hz panels) there's not large enough blanking intervals to permit strobing.
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby michaelius » 29 Jan 2014, 06:24

What would happen if framerate falls below 85 fps then ?
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Chief Blur Buster » 29 Jan 2014, 12:23

michaelius wrote:What would happen if framerate falls below 85 fps then ?

And there liein the engineering challenge in combining ULMB and GSYNC simultaneously.
-- You could automatically disable strobing (but could cause a sudden screen brightness/color change)
-- You could continue strobing (but that causes flicker to become visible at low frame rates, especially 60Hz and less)
The engineering challenge is to make it all seamless.
However, I did describe some potential algorithms to allow combining variable refresh and strobing, such as slowly blending strobing gradually into PWM-free mode (or high-PWM).
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Stryker7314 » 29 Jan 2014, 14:12

Chief Blur Buster wrote:Yes, the G-SYNC upgrade takes over all backlight driving and makes the VG248QE backlight PWM-free (or hi-PWM beyond my measuring equipment's limitations).

I did a 10KHz basic photodiode oscilloscope test, and it was flat DC. And I tested using the fast-camera-wave-and-take-photo (TFTCentral PWM test. I couldn't see any pulses.


Awesome! Thanks for the info Chief! :D
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Chief Blur Buster » 29 Jan 2014, 18:52

Camera proof of ASUS VG248QE monitor being PWM-free, via TFTCentral moving camera method.

Photography Info: Casio EX-ZR200, 1/125sec exposure, non-strobed mode, camera sliding very fast horizontally on a Drylin W-1080 Camera Slider Rail (which I use for pursuit camera tests) at approximately 1 meter per second slide speed, from a focus distance of about 4" away from the monitor plane. The horizontal lines are the unblurred screendoor effect (gaps between pixels), due to the high precision of the camera rail (and confirmed not a rolling-shutter effect on HiPWM).

Without sliding:
Image

Camera sliding very fast at 1 meter per second:
Image

Zero PWM. At 1 meter per second, a 1/125sec exposure (one refresh cycle) blurs the one-pixel thick line by more than 20x width. I've been able to detect 10KHz PWM using a 1 meter/second camera sliding speed, but I see zero PWM in this photo.
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Re: G-Sync and PWM [Good news: It's PWM-free]

Postby Chief Blur Buster » 31 Jan 2014, 13:10

On OCN, wondered how the LED can become PWM-free during the GSYNC upgrade. The GSYNC motherboard replacement appears to include a new LED voltage driver. No LEDs do PWM by itself, the PWM is caused by the LED driver. PWM (Pulse Width Modulation) is simply a power supply whose voltage swings from 0 volts to full voltage, back and fourth, rapidly. Voltage pulses from a power supply! That's it. The LED doesn't know how to do PWM, that's done by the LED driver circuit (aka LED power supply). And guess where the LED driver is? Yup! Not on the panel, but on the monitor motherboard! Which is replaced when you upgrade to GSYNC.

It likely needs to do that to do the more powerful abilities, and because PWM has some nasty interference-effect artifact with variable refresh rate. For those not familiar with the PWM artifact problem, please see LCD Motion Artifacts. Viewing http://www.testufo.com/ghosting while Brightness=0% shows the PWM artifact on older VG248QE monitors. The problem is when doing variable refresh rate, this would happen to receive ugly temporal variances in the PWM artifact, due to the beat-frequency interplay between strobe rate versus refresh rate. Strobe/CRT familiar users will be familiar with the CRT 30fps@60Hz double image effect, or the LightBoost 60fps@120Hz double image effect. Or on old PWM-dimmable monitors, 60fps@180Hz triple-image effect (low-frequency 180Hz PWM), where the number of copies of images is a mathematical function of (framerate)/(flashrate). Look at http://www.testufo.com with LightBoost enabled, lower framerates have more copies of images. Naturally, I can understand why NVIDIA has intentionally engineered all GSYNC monitors to be PWM-free, to prevent the ugly variable-image-multiplying-effect of low-fixed-frequency PWM during variable-refresh-rate situations.
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