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Large VT tweak on 60/75hz panels? (Quick Frame Transport)

PostPosted: 08 Mar 2019, 16:34
by kyubeQLF
Do they make a difference in input lag and image quality? I got a 75hz IPS 7ms GtG panel from Viewsonic that I'm temporarily using till I get a higher refresh rate monitor, so I want to make it the best experience possible.
I managed to nail these VT's, I only tweaked them by applying LCD reduced, then increasing the VT:
Image (default was 1093 VT)

Re: Large VT tweak on 60/75hz panels?

PostPosted: 08 Mar 2019, 18:25
by RealNC
A VT increase of 10 is not going to do anything.

Re: Large VT tweak on 60/75hz panels?

PostPosted: 08 Mar 2019, 19:08
by kyubeQLF
RealNC wrote:A VT increase of 10 is not going to do anything.

How much of a VT increase is noticable and worth doing?

Re: Large VT tweak on 60/75hz panels?

PostPosted: 08 Mar 2019, 19:28
by RealNC
At 1080p 75Hz, I'd say an increase of 300 is where you start getting some small latency benefits (4ms bottom-edge reduction, 2ms average reduction.)

On the majority of 60 and 75Hz displays though, these VT tweaks aren't possible. On high refresh rate monitors, they don't provide much latency benefit but are mostly used to reduce strobing crosstalk (for ULMB and other motion blur reduction modes.) Pretty much the only case where a VT tweak is used for latency reduction is driving 144Hz (or higher) monitors at 60Hz (useful for 60FPS games on monitors that don't support VRR.) There you can usually do large VT increases (because a high-Hz monitor can handle it at 60Hz.)

TL;DR: A VT increase of 10 is like a drop of water in the ocean :-) It doesn't do anything.

Re: Large VT tweak on 60/75hz panels? (Quick Frame Transport

PostPosted: 08 Mar 2019, 22:49
by Chief Blur Buster

I rate this topic Pandora Box because it's university-level Blur Busters refresh rate physics!

<Blur Busters Technical Pandora Box>

Quick Frame Transport (QFT) acceleration factor is Vertical Total Divided By Vertical Resolution
See Advanced Quick Frame Transport thread. Behind the scenes, new HDMI 2.1 specification actually use Large Vertical Totals to pull off Quick Frame Transport. It's essentially the same kind of thing; faster scanout over the cable followed by longer pauses (Bigger VBIs) between scanouts.

Now, what we're doing here is effectively a difficult kind of homebrew QFT.

Don't try it unlss you're already a ToastyX Expert (or can easily learn it -- e.g. a computer programmer experienced in Raster Interrupts), it's almost like Calculus 101.

Just stick to capped FreeSync or capped G-SYNC, that's a much easier method that does pretty much equivalent results.

To double the refresh cycle delivery speed, you need to double your Vertical Total.

VBI is only a tiny part of a refresh cycle, usually approximately 5% or less.
For 1080p, the default vertical total is approximately 1125. (1080p plus a 45 line VBI)

~VT1600 = 1.5x acceleration factor to refresh cycle delivery between computer & monitor
~VT2250 = 2.0x acceleration factor to refresh cycle delivery between computer & monitor

So adding 10 to your vertical total will not really help.

Imagine a video cable as an infinite stream of pixel rows (being transmitted continuously at the full Horizontal Scan Rate or Horizontal Refresh Rate in Kilohertz (KHz) -- scan lines per second). It's like 1080 rows of visible pixel rows and 45 dummy pixel rows separating the old/new refresh cycle.


Now, if you increased your vertical total to approximately 144/100ths the original size (like, approximately VT1400-VT1500), your signal looks like this:


Most monitors cannot do such large VTs except at their lower refresh rates. For example, a 240Hz monitor can gain a custom resolution tweak that creates a 120Hz mode with a 2x frame delivery acceleration factor, or a 60Hz mode with a 4x frame delivery acceleration factor.

FreeSync monitors are much more friendly to Quick Frame Transport tricks, but it's just simply much easier to use a 60fps or 120fps cap on a 240Hz FreeSync monitor -- much easier QFT for less work. Low-caps-on-high-Hz VRR is already a "defacto QFT'" system.

Homebrew QFT on fixed-Hz is
(A) Very advanced topic that requires you to understand what the numbers of a Custom Resolution Utility does
(B) Understand this mathematically. Understand how to edit numbers to make a "144Hz signal look exactly like a 240Hz signal with exactly the same Horizontal Scan Rate in number of scanlines per second". If you don't understand that concept, don't bother. Wink, Wink. ;)
(C) Do not blindly edit numbers without understanding it sufficiently
(D) Requires a monitor with very forgiving large vertical totals support (very few monitors are)
(E) Requires RTSS Scanline Sync to relocate the frame-presentation timing from beginning of VBI (default Microsoft Present() behaviour) to end of VBI (preferred latency-lowering behaviour for large VTs)
(F) Usually doesn't benefit VSYNC OFF because VSYNC OFF can bypass benefits of QFT

If you can't simultaneously meet (A)/(B)/(C)/(D)/(E)/(F), it's not yet time to play with large VTs as a method of reducing input lag.

Also, monitors that support QFT but doesn't support VRR is extremely few and rare -- so the existence of VRR means there's often existing a much easier method of QFT aka frame rate capping (e.g. 60fps cap at 240Hz for low-latency emulation, which can be much lower lag than switching refresh rate to non-QFT 60Hz).

Take your time to familiarize other simpler methods of reducing input lag, and slowly work your way to understanding how to make a fixed-Hz signal impersonate a FreeSync signal. (60Hz fixed-Hz at 1/240sec scanout non-FreeSync actually has nearly identical same signal structure as a perfect 60fps framerate cap on a 240Hz FreeSync VRR signal). That's why VRR frame rate capping is a much easier/simpler method of Quick Frame Transport.

If you're not scared away from the advanced calculus of Quick Frame Transport....
Some 75Hz non-FreeSync panels can do a 60Hz signal with a 75/60th acceleration factor.
1. Look at your 75 Hz mode.
2. Write down your Horizontal Scan Rate (or Horizontal Refresh Rate) and Dotclock
3. Create your 60Hz mode with identical numbers except increase your vertical total to lower the refresh rate. As you increase vertical total, your refresh rate will go down if you've locked your dot clock (pixel clock) -- (this is sometimes easier to do in ToastyX)
4. Now you've got a 60Hz mode that scans-out to panel in 1/75sec on a high speed camera. Basically, a 60Hz refresh rate with a 1/75sec refreshing velocity (Thanks to Quick Frame Transport tricks)

However, keep in mind a 75/60th acceleration factor (1.25x) will reduce your input lag only by (1/60sec) / (1.25) = approximately 3.3ms less input lag for screen bottom edge. (And half that for the lag reduction for screen middle -- e.g. approximately 1.6ms less lag for screen centre).

This benefit is very conditional -- it is assuming you're using VSYNC ON rather than VSYNC OFF which bypasses scanout latency (making sub-refresh latencies possible, see ....). ... Quick Frame Transport is mostly useless for VSYNC OFF except when the monitor is doing internal scan-rate conversion (e.g. buffering a slow-scanning non-QFT 60Hz signal for a fast-sweep scanout in 1/240sec on panels that are fixed-max-Hz scanout velocity, without real-time sync between cable scanout and panel scanout).

Now you want Quick Frame Transport at your max Hz? Unfortunately, very few monitors I've run into can do large VTs at their max Hz, while also maintaining realtime synchronous scanout (cable scanout = panel scanout velocity).

Eventually manufacturers will make QFT a simple checkbox toggle or such, but at the moment, doing a homebrew roll-your-own Quick Frame Transport (Large Vertical Totals) is a very advanced topic that is at the technical end of the Blur Busters educational venn diagram.

TL;DR: Today -- at the moment -- it is much easier and much more successful to reduce lag of low refresh rates simply by using a frame rate cap on a variable refresh rate monitor, as your homebrew QFT refresh rate. (Getting a "Low-Lag 60Hz VSYNC ON" via a 60fps frame rate cap on a 144Hz or 240Hz FreeSync or GSYNC monitor). That's currently a much easier method of "defacto" Quick Frame Transport (QFT) than using Large Vertical Totals.

</Blur Busters Technical Pandora Box>

Re: Large VT tweak on 60/75hz panels? (Quick Frame Transport

PostPosted: 10 Mar 2019, 13:53
by kyubeQLF
Amazing reply! Thank you for clearing things up!