FAQ: Understanding HDMI Quick Frame Transport (lower lag)

[Chief Blur Buster]

So, I assert that VRR alone is not as good as VRR plus QFT. For my 1080p example, VRR+QFT would technically be equivalent to VRR+864Hz refresh rate.

Terminologically, all VRR already always include QFT for framerates below max Hz.

What you mean extra QFT above and beyond -- so that max Hz is still QFT.

This is already possible on some monitors if you create it in a Custom Resolution Utility and then add FreeSync range to it.

One problem is that this will only work to reduce lag, IF the panel can scanout faster. In that situation, sometimes it's favourable to raise the refresh rate support of the display, so it's not uselessly idling between refresh rates.

In that sense, a true-864Hz VRR display (i.e. 48Hz-864Hz VRR range with 1/864sec delivery) is MORE LIKELY to happen than a 48Hz-240Hz range with 1/864sec QFT -- because if a panel can refresh faster, why hold back the max Hz? Manufacturers want to be able to advertise higher Hz.

[Chief Blur Buster]

This Year 2021, Blur Busters Will Start a Plug-And-Play Quick Frame Transport Initiative

Ideally, monitors need to be able to support Quick Frame Transport in the menus, where turning ON/OFF that setting, automatically enables an alternate set of EDIDs with plug-and-play Large Vertical Totals.

FreeSync ON/OFF toggle already modifies the monitor's EDIDs in real time (causing Windows to re-detect the monitor), so naturally, Quick Frame Transport ON/OFF should also modify the monitor's EDIDs in real time, for user friendly lower strobe lag + user friendly lower-lag VSYNC ON.

All FreeSync-compatible panels already support fixed-Hz QFT in the scaler/TCON level, because VRR is a superset of QFT (Because VRR is essentially QFT for all lower refresh rates / frame rates below max Hz). What QFT is simply a fixed-Hz equivalent.

[Kamen Rider Blade]

When do you think we'll get 1 Hz as the low end of the VRR spectrum to be common?

Apple pulled it off with a low end 1 Hz on their Apple Watch Series 5 in late 2019

https://www.wired.com/review/apple-watch-series-5/

Where's the rest of the Display Panel industry?

The gold standard for Low End of VRR is obviously 1 Hz.

[Chief Blur Buster]

1 Hz is not easy to do with LCDs without flicker, and things can get so bad that LFC actually improves image quality at a certain point.

[Chief Blur Buster]

There's a new HOWTO for creating Quick Frame Transport:

HOWTO: Quick Frame Transport (QFT) - Large Vertical Totals (reduce lag, reduce crosstalk)

[2mg]

Also -- "Quick Frame Transport" equivalent is already built into FreeSync/GSYNC. Variable refresh rate displays have have been doing this since 2012. Low frame rates shows large input-lag-reducing benefits on high-Hz variable refresh rate dispays, since those refresh cycles are delivered at full dotclock velocity of maximum Hz, even if you're just doing low frame rates (ala 40fps / 40Hz). The lag-reduction benefits show really clearly in the various Blur Busters GSYNC tests (including GSYNC 101). Quick Frame Transport is simply bringing these lag-reducing to fixed-Hz displays (benefits of faster scanout of low refresh rates).

Regarding QFT:

1. Is it HDMI only?

2. It's known that ie 60FPS @ 60Hz is more laggy than 60FPS @ highest_refresh_rate_hz, but you mention the "40fps / 40 Hz" so I'm confused if QFT is working at delivering frames at panel's max speed if you use your example or 60FPS @ 60Hz?

3. Does QFT work when not using VRR?

4. Does capping FPS for example to put less stress on the GPU (heat/noise) when using max refresh rate (with or without VRR) introduce noticeable input lag (and which cap limiter is the best for low lag, RTSS, RTSS's Reflex, NVIDIA driver...)?

[Chief Blur Buster]

Regarding QFT:

1. Is it HDMI only?

For the official "quick frame transport" brand name, yes.

For the actual technique, no.

You can do the QFT technique on any video cable, even VGA, SVGA, XGA, HDMI, DP, USB-C, etc.

It's just merely simply speeding up the raster scanning (more pixel rows per second) and adding a longer pause between refresh cycles (longer blanking interval).

2. It's known that ie 60FPS @ 60Hz is more laggy than 60FPS @ highest_refresh_rate_hz, but you mention the "40fps / 40 Hz" so I'm confused if QFT is working at delivering frames at panel's max speed if you use your example or 60FPS @ 60Hz?

Remember, not all pixels transmit simultaneously.
Remember, it takes time between first and last pixels.

60Hz is not always the same signal.
60Hz signals may take 1/60sec between first and last pixel
60Hz signals may take 1/120sec between first and last pixel
60Hz signals may take 1/240sec between first and last pixel
60Hz signals may take 1/77.5423sec between first and last pixel
60Hz has infinite methods of transmitting 60Hz
"60Hz" does not always mean the latency between first/last pixels is the same.
"60Hz" does not give enough information about the signal.
See Custom Resolution Glossary

3. Does QFT work when not using VRR?

QFT is for non-VRR.

VRR already includes QFT built into VRR. All refresh cycles are transmitted over the cable at max-Hz velocity, so a "60fps 60Hz" frame on a 240Hz VRR display (when display settings is Hz=240 VRR=ON) will transmit in 1/240sec, instead of the usual 1/60sec. It looks like 60Hz, but is lower latency than 60Hz.

4. Does capping FPS for example to put less stress on the GPU (heat/noise) when using max refresh rate (with or without VRR) introduce noticeable input lag (and which cap limiter is the best for low lag, RTSS, RTSS's Reflex, NVIDIA driver...)?

Yes, capping can reduce GPU load, but most people (>99%) here do not use it for that purpose.

The usual method of capping is to improve motion fluidity and/or to reduce latency, or to solve a different problem.

One exception: Mobile gaming. However, using a laptop + capping, can allow you to play games for longer on battery, if you cap at a low frame rate, so it can be used as a battery management method. Some smartphone games have a reduce-power option that acts as a defacto 20-30fps cap, so it's more of a power management method of capping.

[2mg]

...

Thanks for explaining!