FAQ: Understanding HDMI Quick Frame Transport (lower lag)
Posted: 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:
But what the hell is Quick Frame Transport? Well, it's simply a large blanking interval.
First, if you have seen those numbers in a Custom Resolution Utility, they are just simply mapped spatially in signal layout:
Vertical Total = the entire height of this.
And VBI = Vertical Blanking Interval = (The sum of Vertical Front Porch + Vertical Sync + Vertical Back Porch).
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:
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!
______
EDIT: Want to do QFT on your computer? There's a new HOWTO for creating Quick Frame Transport:
HOWTO: Quick Frame Transport (QFT) - Large Vertical Totals (reduce lag, reduce crosstalk)
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.
But what the hell is Quick Frame Transport? Well, it's simply a large blanking interval.
First, if you have seen those numbers in a Custom Resolution Utility, they are just simply mapped spatially in signal layout:
Vertical Total = the entire height of this.
And VBI = Vertical Blanking Interval = (The sum of Vertical Front Porch + Vertical Sync + Vertical Back Porch).
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:
However, it's possible to scanout quicker, such as delivering 100Hz refresh cycles in 1/144sec:
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!
______
EDIT: Want to do QFT on your computer? There's a new HOWTO for creating Quick Frame Transport:
HOWTO: Quick Frame Transport (QFT) - Large Vertical Totals (reduce lag, reduce crosstalk)