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FAQ: Understanding HDMI Quick Frame Transport (lower lag)

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FAQ: Understanding HDMI Quick Frame Transport (lower lag)

Postby Chief Blur Buster » 21 Apr 2018, 16:39

People who may have heard of a new method of delivering refresh cycles faster. We're very familiar with this, but few people are.

See HDMI Version 2.1 on HDMIFORUM.org which says:

HDMI Forum wrote:HDMI Quick Frame Transport (QFT) reduces latency for smoother no-lag gaming, and real-time interactive virtual reality.


Image

But what the hell is Quick Frame Transport? Well, it's simply a large blanking interval.

Around here, we sometimes call this the Large Vertical Total trick, which also has other benefits such as reducing strobe crosstalk.

Normally, refresh cycles are transmitted one after the other, in tight fashion with a tiny blanking interval:

Image


However, it's possible to scanout quicker, such as delivering 100Hz refresh cycles in 1/144sec:

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It's possible to go even further, such as delivering 60Hz refresh cycles in 1/240sec! Basically, a frame-delivery acceleration of 4x factor, for supported platforms.

HDMI Quick Frame Transport, while specified by HDMI, the fundamental technique also works on DisplayPort and DVI connections, since it's simply a large blanking interval. A refresh cycle is transmitted faster, with a longer pause between refresh cycles.

Also, some 240Hz monitors can only scan-out their panels at full velocity (1/240sec). So they have to buffer an incoming slow-scanning 60Hz refresh cycle over the cable, before scanning-out in 1/240sec. By using Quick Frame Transport, you can do realtime concurrent LCD panel scanout in sync with cable scanout, reducing the input lag of 60Hz or 120Hz signals (e.g. XBox One consoles) on a 240Hz displays.

Ideally, a display has to advertise this feature correctly via the correct EDID/DisplayID info, to inform a computer that it supports a Quick Frame Transport mechanism. However, many existing 144Hz and 240Hz monitors support Custom Resolution Tweaking to create large VBIs, so it would be very easy to add Quick Frame Transport capabilities to these displays, for supported signal sources. Monitor manufacturers should add information to their HDMI EDIDs to include the Quick Frame Transport feature. FreeSync compatible LCDs are relatively easy to make QFT compatible.

(Currently, most 240Hz monitors are very bad at 60Hz consoles, since they only do bufferless scanout at 240Hz -- because they buffer a 60Hz scanout signal and does a full-velocity scanout on 240Hz LCD panels. Unless they're made to support a Quick Frame Transport at 60Hz with a large Vertical Total of >4000 scanlines for a 1080p signal).

Now you understand better!
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Re: FAQ: Understanding HDMI Quick Frame Transport (lower lag

Postby Chief Blur Buster » 21 Apr 2018, 17:35

Also -- the XBox One reportedy supports an upgrade path to HDMI Quick Frame Transport (along with 120Hz, FreeSync, etc). While further testing will be needed, as of today (April 21st, 2018), there is now an XBOX ONE Forum at Blur Busters which will grow over time.

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).
Head of Blur Busters - BlurBusters.com | TestUFO.com | Follow @BlurBusters on Twitter!
To support Blur Busters: Official List of Best Gaming Monitors | G-SYNC | FreeSync | Ultrawide
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Posts: 4934
Joined: 05 Dec 2013, 15:44


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