RTSS Scanline Sync HOWTO

Everything about latency. Tips, testing methods, mouse lag, display lag, game engine lag, network lag, whole input lag chain, VSYNC OFF vs VSYNC ON, and more! Input Lag Articles on Blur Busters.
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Re: RTSS Scanline Sync HOWTO

Post by Chief Blur Buster » 05 Mar 2021, 05:15

Front Edge / Back Edge Sync

These new additions were developed without my involvement, but I think it's a great way of making Quick Frame Transport much easier -- especially if you have a large VBI (large vertical total). Where a back-edge sync would reduce "VSYNC ON" latency significantly. I do not yet have experience testing these features as I'm currently focussed on a different project (at the moment)
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Re: RTSS Scanline Sync HOWTO

Post by Winterblossom » 06 Mar 2021, 21:57

I'm using RTSS for a while now and some time ago I also tried the newer scanline sync feature. But what I disliked about it was that there were still chances for tearing to occur, especially in situations with higher GPU load. I have a 60 Hz TV and like to cap my games at 60 FPS to reduce GPU load and have a more silent system.

I learned that I can use RTSS to improve frametimes with v-sync on so that they don't fluctuate anymore between lower and higher values around 16.7 ms but stay at a perfectly flat line at 16.7 ms. I thought that this is the best possible outcome when it comes to 60 FPS. But now, after updating RTSS and reading about the new hybrid scanline sync mode and thinking about scanline sync again I'm a bit confused. If 16.7 ms input lag (as shown by RTSS) at 60 FPS is the best possible outcome and this can easily be achieved by enabling classic v-sync and putting RTSS as a framerate limiter on top (simply set to 60), why is scanline sync even necessary? So I wonder if there's more to it than meets the eye.

While comparing both modes (classic v-sync with RTSS set to 60 against v-sync off and RTSS scanline sync set to 1) I'm not sure if I notice a difference. There might be a slight difference but I could also just fool myself by my expectations. So the result is not so clear. I enabled SyncInfo=1 and noticed the so called "present latency" value which is around 11-12 ms with v-sync on (RTSS limiter off) that can be reduced to 5-6 ms by setting the FPS cap in RTSS to 60. Now the interesting part: I can reduce this value even further to 0.1 ms by setting scanline sync to 1, even when v-sync is enabled, which is the same value when v-sync is off while scanline sync is set to 1, with the major difference that v-sync on prevents screen tearing.

Now I wonder: What does this "present latency" actually mean? Do I gain anything by reducing this? Can anything even be gained in my scenario when RTSS already shows a flat line of 16.7 ms input lag? I don't know if I'm chasing phantoms here but there has to be more to this scanline sync thing, otherwise I don't get the usefulness of it, at least not in my scenario. Can you please explain it to me?

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Re: RTSS Scanline Sync HOWTO

Post by Chief Blur Buster » 07 Mar 2021, 18:00

Winterblossom wrote:
06 Mar 2021, 21:57
I learned that I can use RTSS to improve frametimes with v-sync on so that they don't fluctuate anymore between lower and higher values around 16.7 ms but stay at a perfectly flat line at 16.7 ms. I thought that this is the best possible outcome when it comes to 60 FPS.
Life is not that simple.

The input lag chain is long, long, long. RTSS doesn't measure the WHOLE latency chain. Here's a massively simplified diagram:

Image

But it can actually be way more than 100x more complex than the above diagram, as seen in:

1. Not all pixels of a display refresh at the same time. See High speed videos of LCD refresh cycles
2. The numbers in RTSS is not button-to-photons time. RTSS can't always predict latency of each pixel. The different sync technologies can change the latency gradient (TOP < CENTER < BOTTOM versus TOP = CENTER = BOTTOM versus TOP > CENTER > BOTTOM) depending on how it interacts (sync setting, strobe setting, etc).
3. Quick Frame Transport like large vertical totals and long VBIs. Some tweaks allow 60Hz refresh cycles show faster, e.g. transmit them over cable in 1/240sec and refresh the first-to-last pixels in 1/240sec. Scanline Sync helps the Quick Frame Transport effect, to properly time the game's frame presentation more optimally for DIY Quick Frame Transport (like Large Vertical Total + RTSS Scanline Sync near the end of VBI).

The newtonian thinking of 60Hz=16.7ms doesn't reveal the complex physics behind latency in the art of serializing 2D images over a 1D cable, creating latency differences for each different pixel on the screen.
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Re: RTSS Scanline Sync HOWTO

Post by Winterblossom » 08 Mar 2021, 03:43

Thanks for the thorough reply. So, would you say that in my specific scenario (60 Hz Samsung LCD TV from 2012 over HDMI, no VRR support obviously) I could benefit from scanline sync in comparison to the settings I currently use (v-sync on, low latency mode set to ultra in NVIDIA CP, RTSS limiting to 60 FPS)? Quick frame transport doesn't seem to apply in my case as the TV is only capable of 60 Hz, am I correct? And what exactly is "present latency" I mentioned in my previous post? Is there any significance to input lag when it shows 10 ms, 5 ms or 0.1 ms in the RTSS overlay? Does it mean that input lag is 9.9 ms shorter in the latter than in the first case?

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Re: RTSS Scanline Sync HOWTO

Post by Chief Blur Buster » 09 Mar 2021, 00:22

Winterblossom wrote:
08 Mar 2021, 03:43
I could benefit from scanline sync in comparison to the settings I currently use (v-sync on, low latency mode set to ultra in NVIDIA CP, RTSS limiting to 60 FPS)? Quick frame transport doesn't seem to apply in my case as the TV is only capable of 60 Hz, am I correct?
VSYNC ON + NULL Ultra + RTSS cap about 0.01fps below what you see at www.testufo.com/refreshrate is pretty good but can create latency sawtoothing effects.

Image

RTSS Scanline Sync can prevent latency sawtoothing effects, since it's a perfect framerate=Hz, as CPU clocks are never synchronized perfectly with GPU clocks, which is why www.testufo.com/refreshrate#digits=5 probably shows 59.99275Hz or 60.00243Hz on your computer.

Thus, for VSYNC ON:
(A) If refresh rate is slightly below RTSS cap, you will get VSYNC ON latency of what NULL does
(B) If refresh rate is slightly above RTSS cap, you will get latency sawtoothing effect. Since since your cap is never hit perfectly, your framerate will be perpetually slewing against refreshrate if you're using a non-scanline-sync'd RTSS framerate cap.

If you are seeing a glassfloor in RTSS, then you're probably situation (A) at the moment.

Right now, for a non-large-VT 60fps 60Hz, it's hard to say if for your specific game that NULL Ultra versus RTSS Scanline Sync is currently superior. There were some tests that showed Scanline Sync was slightly lower lag, but that NULL Ultra was more forgiving. You may wish to try both ways and see which "feels" better.

Also, some TVs support undocumented QFT / 120 Hz. Try testing 120Hz PC to TV trick and see if it works.

Image

Test both 1080p and 720p. Some TVs, even back to year 2012, occasionally successfully does 120Hz.
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Re: RTSS Scanline Sync HOWTO

Post by Winterblossom » 09 Mar 2021, 09:05

Again, thank you very much for the effort you put in your posts! Much appreciated.

Unfortunately I couldn't get 120 Hz to work, nor 70, 75 or 100 for that matter. Sometimes the TV simply switched to a different resolution to accommodate for the higher refresh rate.

The refresh rate reported by the UFO test is around 60.0001... (three zeros) after some time waiting, so a bit above 60 but not as much as your example.

Regarding this sawtooth effect, would this be visible in the RTSS frametime graph? Because in my case after setting RTSS to 60 it is a completely flat graph (just a one pixel line). Without RTSS, depending on the game, the graph can fluctuate wildly with v-sync on, but RTSS always tames it and makes it a perfectly flat line, simply by setting the limiter to 60.

So, what would be the conclusion of this? I guess there isn't any real room for improvement? I could either use v-sync on with RTSS set to 60 or I could use scanline sync but risk tearing with maybe absolutely minimal improvements in lag.

Oh, and there's a third thing I tried out: Using v-sync on while enabling scanline sync instead of setting the normal limiter to 60. It seems to have the same results (only one particular game had exactly 20 FPS afterwards), the frametime graph ist flattened completely and there is no tearing. I tried different scanline sync values and some values seemed to increase the "present latency" value but simply using 1 always yielded good results in all the games I tried (around 0.1 ms). Can you explain what is happening there? Is that a viable way to "flatten the curve" and not having to deal with a fixed limit being a bit too low or too high?

And one last thing: My TV seems to draw the frames from the bottom to the top. I used my iPhone slo-mo camera with 240 FPS to clearly notice this (higher numbers at the bottom on pause). Is this relevant?

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Re: RTSS Scanline Sync HOWTO

Post by Startrek66 » 13 Mar 2021, 08:11

I try to get rtts scan-line sync to work with nvidia fast sync. What I didn't understand is if this subscription is beneficial and when to apply it. I have a 60Hz monitor and my gtx1050 card is capable of creating 120fps with no fps lock and no vsync enabled. I play only on airplane simulator. My goal is to achieve smooth and smooth gameplay even at 60fps. Can you help me?

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Re: RTSS Scanline Sync HOWTO

Post by Chief Blur Buster » 13 Mar 2021, 17:41

Winterblossom wrote:
09 Mar 2021, 09:05
And one last thing: My TV seems to draw the frames from the bottom to the top. I used my iPhone slo-mo camera with 240 FPS to clearly notice this (higher numbers at the bottom on pause). Is this relevant?
Do you have that 240fps video of www.testufo.com/scanout ? Would be interesting to see!

It won't affect RTSS Scanline Sync behavior since GPU delivery is still top-to-bottom at the GPU output, and you can still have latency reductions (deliver frames faster to the TV's buffer), if the TV somehow supported quick frame transport behaviors.

Some TVs do reverse scan, and all TVs that do this adds extra input lag -- because buffering is needed for scan conversion of any kind (whether to reverse scanned LCDs or plasma subfields or DLP subfields, etc). The top-to-bottom GPU scan is buffered by the TV motherboard, then output bottom-to-top to the screen.
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Re: RTSS Scanline Sync HOWTO

Post by Winterblossom » 16 Mar 2021, 03:17

Chief Blur Buster wrote:
13 Mar 2021, 17:41
Do you have that 240fps video of www.testufo.com/scanout ? Would be interesting to see!
Here: https://filebin.net/42sjnjss7qxkuqao

hl3.exe
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Re: RTSS Scanline Sync HOWTO

Post by hl3.exe » 21 Mar 2021, 11:49

Weird problem I noticed. Maybe someone could help me understand this issue.

Some of my games stutter, even though no real inconsistent framerate is detected. Using Scanline Sync 2/x seemed to finally fix this issue. All the frames are nicely paced! But when I checked my performance profile I noticed that 1 thread is running unusually high load when Scanline Sync is toggle on. Reinstalling RTSS and MSI Afterburner did nothing.

https://imgur.com/a/MB6jbCw

I'm a missing something here? Or is this how it's supposed to work? Thanks for the help! :)
Last edited by RealNC on 21 Mar 2021, 12:49, edited 1 time in total.
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