Question about vsync , tearing and crossfire

[shevanel]

I was under the impression that vsync was mainly used for eliminating screen tearing mostly found when using lcd monitors with 60hz rr but I have some questions about vsync when used with crossfire.

I notice that whenever I am playing "some" games that there is a bit of stutter and this is all while having vsync on and the framerate is steadily locked at 60 however if I disable vsync I eliminate all stutter in the game I'm playing.

What has me confused is that I notice zero screen tearing while experiencing frames over my refresh rate and on any single card setup Ive ever had I would notice screen tearing on this monitor instantly.

So my questions are:

why can I run crossfire with vsync off and not have any screen tearing?

what is the top main reason for vsync other than for screen tearing?

why does vsync cause this stutter?

Here are some screen shots and you can noitice there is not any tearing, unless you see something I don't.

thanks for the info.

 

[gvblake22]

I may be wrong here, but I don't think tearing is something you can capture in a screenshot. I think it is something only visible by the end user looking at the screen because it is due to how the monitor is able to display the information, rather than the quality of frame data/information being sent to it (which is what a screenshot captures).

As for CrossFire vs. single card setups, I have no idea. Please list what specific games you are playing.

 

[shevanel]

I do not notice any screen tearing on this setup, but I have before when vsync would be off on a single card setup. If I enable vsync on CS:s fortress forever or MW2 I get stutter.. the stutter is minor but noticeable in MW2 but it's severe in CS:s + double firing whenever I shoot.

I just want to know what causes tearing and is having vsync on mainly to reduce tearing? What other benefits does vsync have?

 

[gvblake22]

I've not heard of another use for vsync other than to prevent tearing (due to the refresh rate and framerate issue you mentioned). Whenever I had tearing in a game, I would just turn on vsync and the problem went away. If you get stuttering, try reinstalling or changing your video driver.

 

[shevanel]

So what causes screen tearing exactly? And why does it only happen sometimes?

 

[gvblake22]

The tearing is caused by the issue you described: the framerate of the game is significantly higher than the refresh rate of the monitor and the monitor can't keep up so the frames get mushed together and appear "torn".

You could also try using triple buffering to see if that makes any difference.

 

[SummerDays]

Your monitor has a vertical refresh rate. For example, this screen is working at 1920 x 1200 @ 60 Hz.

Vertical Synchronisation forces the output of a graphics card to match that of the monitor.

With V-synch off the graphics cards tries to update the screen more than 60 times per second, there is tearing. You can do this by turning Vsynch off and using a graphics card that can support a higher frame rate than 60 fps in a game.

With LCD monitors the number of FPS is based on how quickly the monitor can respond to a change in signal. For example, this monitor has a 4 ms rapid response time. So, it should be able to provide a higher rate than 60 fps. In fact, the manufacturer advertises a maximum frame rate of up to 240 fps.

The stuttering may be caused by crossfire itself, or if you're playing a game online by the network or server.

Right now nearly all the LCD monitors support 60 hz. Soon more 120 Hz monitors will come out. You can also increase your refresh rate by lowering the resolution of the monitor sometimes.

So all that expensive graphics card equipment is going to waste, unless you like to watch screen tearing or you turn up the AA settings etc..

 

[shevanel]

Thats the thing. I'm running vsync off and I do not see any screen tearing. I was wondering why.

 

[phanbuey]

4ms refers to the time it takes a pixel to change color, even at 2ms, if your refresh rate is 60Hz, you will display 60FPS.

Hz=frames per second.

response time = how quick the monitor's pixels adjust color from one frame to another.

So if your running 60Hz and have a 25ms GTG time, you will see a 'blur' or 'ghosting' (or even straight up stutter) between the colors when looking at something like a soccer ball flying across the screen, because the pixels are having trouble switching fast enough from one frame to another - but you are still running at 60FPS.

Here is a good summary from annand tech:
"Response Time: Response time is an unusual preference and always a trade off. Typical response time (TrTf - Time rising, Time falling) refers to the time that it takes the LCD subpixel to twist from the fully "on" position to the fully "off" position and then back again. Response time has absolutely nothing to do with framerate. Pixel response times are independent of each other, and it does not take the entire screen 25ms to refresh if a monitor is labeled as a 25ms response time LCD. The time that it takes the LCD to go from black to white may be 15ms while the time that it takes the LCD to go from black back to white may be 10ms. Furthermore, your monitor is generally rendering a color that is not on end of the color spectrum. The time that it takes your LCD subpixel to twist from one half of a tone to another may be more or less than 15ms. The TrTf response time is normally a pretty useless measurement - but it makes for an easy specification in which to market LCDs."

Screen tearing can happen if the gfx card is rendering either faster OR slower than the FPS of the monitor. Even if youre rendering 50FPS and your display is rendering 60... there will come a time when half a frame will be rendered because the frequency of the refresh isnt aligned to the rendering card. Thats why Vsync works with multiples of the refresh rate and not just "below 60".

also, some ppl are more sensitive to it than others... it is usually noticeable on the edges of the screen, at higher rez, and when turning... so if you are walking look at the things rendered around the edges and you will see a slight amount of tearing, that would otherwise be unnoticeable.

also some games tear more than others (dragon age cutscenes), and higher resolutions tend to tear more noticeably.

 

[Wrigleyvillain]

I thought triple buffering was only for OpenGL games so I googled it and came across this which I guess is still useful?
http://www.ocworkbench.com/2006/articles/DXtweaker/

Hey Shev congrats on your first ATI cards btw and cool to see you play Fortress Forever! I'll add you to my Steam friends tonight.

You can also see if changing Catalyst AI setting makes any difference but it has to be set to standard at least for Crossfire to work.

 

[phanbuey]

Crossfire has sometimes something called "microstutter" which is different from vsync but might be caused by it.

 

[Wrigleyvillain]

SLI as well, to be fair.

 

[shevanel]

well ive been testing games today and so far the only game to stutter was cs:s... I don't even play that game I just installed to use it's bench ap.

for some reason I havent noticed any issues with vsync on or off and things are running very nicely. knock on wood.

Thanks for the info in this thread though, Ive always wondered about some of the things listed here ie 5ms response time etc

 

[phanbuey]

http://www.youtube.com/watch?v=ZuaFrFnfZ1U&feature=related

here is a good video to show some micro stuttering so you know what it looks like

 

[shevanel]

No, Ive never experinced that. It's normally a little skip while walking. It hasnt been doing it now though, like I said only in cs:s. MW2 I have not noticed it at all since yesterday

 

[Goodman]

http://www.youtube.com/watch?v=ZuaFrFnfZ1U&feature=related

here is a good video to show some micro stuttering so you know what it looks like

Video seems fine to me

 

[phanbuey]

Video seems fine to me

look at how the gun jerks when it moves... its hard to see on a video, but in real life the entire screen does that and its basically stuttering when fraps is showing an average of 60 fps.

It makes the game feel and look like 20fps when its running at 60.

 

[Goodman]

look at how the gun jerks when it moves... its hard to see on a video, but in real life the entire screen does that and its basically stuttering when fraps is showing an average of 60 fps.

It makes the game feel and look like 20fps when its running at 60.

Then don't use fraps

It could be driver problem , overheat , mouse battery...

But in any case that would not be enough for me to stop playing the game unless it was much worst then the video is showing...?

 

[Wrigleyvillain]

For whatever it's worth I never experienced any microstuttering when I had 4850 crossfire. Not saying it doesn't exist just that it's not really as pervasive as some seem to think. At least anymore.

 

[phanbuey]

Then don't use fraps

It could be driver problem , overheat , mouse battery...

But in any case that would not be enough for me to stop playing the game unless it was much worst then the video is showing...?

Drivers can help, but its a hardware phenomenon... and you shouldnt game with mice that use batteries anyways . It isnt as common as it used to be, for sure.

You have to play it to get how annoying it is. Especially on an fast paced shooter since you cant really hit anything thats moving between 70fps and 15fps alternating every half a second.

Its not about fraps - its SLI or CFX setup isnt synchronizing frames...

basically if you only have one card: it renders the image, sends it off to the monitor, then renders another, and sends it off... no dance necessary.

but if you have two cards using AFR, what happens is one card renders one frame sends it off and then the other card renders the other frame, so they alternate. If theyre not timed right (like a really badly mixed techno transition), you will get two frames in rapid succession, and then nothing, then two more frames, then nothing.

 

[SummerDays]

Suppose this little . here is a pixel.

If you can switch that little dot on and off in 4 ms then it will be 250 Hz.

It may take some finite amount of time to get the signal to the dot, input lag. So there will be a lag from when the graphics card sends a signal to change to when the monitor actually displays it.

Yes, the monitor refresh rate is different from CRTs which simply used a gun to scan down the entire screen. They're independant of each other.

But using basic logic you should be able to figure out that if the entire screen has to refresh itself, then it can't refresh itself any faster than its individual pixels can.

From Nec's Site:

"Provides for uninterrupted display of full motion video with response times as quick as 4ms. It delivers streaming video without noticeable ghosting or blurring, while achieving as many as 250 frames per second (fps). This remarkably quick motion makes displays better than ever for gaming and video applications such as presentations and streaming web video."

The blurring or ghosting takes place because the pixel isn't changing fast enough. Our eyes can tell the difference because the protein can switch on and off fast enough.

 

[phanbuey]

Suppose this little . here is a pixel.

If you can switch that little dot on and off in 4 ms then it will be 250 Hz.

It may take some finite amount of time to get the signal to the dot, input lag.

Now if you add the time it takes to get that dot on and off to the input lag you should be able to figure out the maximum FPS your monitor is.

Yes, the monitor refresh rate is different from CRTs which simply used a gun to scan down the entire screen. They're independant of each other.

But using basic logic you should be able to figure out that a screen can't refresh itself any faster than the individual pixels can.

LCD's have a different system. But they still have to "refresh".. i.e. check when the video card has updated the pixel... how often the check is how many fps you can display.

you can have a 60hz screen with 2ms delay or a 240hz tv with a 4ms delay. response time is unrelated to FPS.

I agree that refresh rate is a misnomer and that an 8ms screen should theoretically be able to render 120 FPS - but they still have an update rate at which they update the pixels so they can really only accept a certain amount of FPS (in the case of the mass market this is 60).

"Myths & Stigma:
There are many myths associated with LCD monitors, and many ideas that people have that are simply false. Unfortunately, this misinformation gets passed along all too often so I will attempt to put as much of it to rest here as possible.

Perhaps the biggest piece of misinformation is the common belief that a monitor or TV can display more frames per second than its refresh rate allows. In actuality, a monitor can truly only display as many frames per second as its refresh rate (e.g. a 60Hz monitor can only display 60fps), so providing a higher framerate than that is virtually worthless. Thus, the idea that a higher framerate is better even when the upper limit of monitor refresh rates is reached is entirely false! In fact, doing so can often lead to the tearing phenomena mentioned earlier, and is the reason vertical sync is an important tool.

A lot of hype has also been made about 120Hz refresh rates on TVs and some monitors. It is important to realize that although the TV or monitor can display 120 frames per second, it cannot accept 120 frames per second - it simply receives the signal at x frames per second and then interpolates to make the remaining 120 frames. When implemented properly this can lead to a perceived reduction in motion blur and studder and is indeed a useful technology; however, the display still only accepts a 60fps signal at most. The same applies for the upcoming 240Hz and 480Hz displays. "

http://www.overclock.net/faqs/94872-info-explanation-lcd-response-time.html

 

[Goodman]

Drivers can help, but its a hardware phenomenon... and you shouldnt game with mice that use batteries anyways . It isnt as common as it used to be, for sure.

You have to play it to get how annoying it is. Especially on an fast paced shooter since you cant really hit anything thats moving between 70fps and 15fps alternating every half a second.

Its not about fraps - its SLI or CFX setup isnt synchronizing frames...

basically if you only have one card: it renders the image, sends it off to the monitor, then renders another, and sends it off... no dance necessary.

but if you have two cards using AFR, what happens is one card renders one frame sends it off and then the other card renders the other frame, so they alternate. If theyre not timed right (like a really badly mixed techno transition), you will get two frames in rapid succession, and then nothing, then two more frames, then nothing.

Not using Fraps or mouse battery was meant as a joke
but drivers or heat could do that in games i would think

 

[SummerDays]

In any case, we're all waiting for 120 Hz monitors to come along, so that our true refresh rate will be increased.

 

[phanbuey]

When you are setting Vsynch off though, you're giving permission for the graphics card to update the screen at a higher rate, which results, in tearing. That is dependant on the response time of the individual pixels.

Yes, it's going to be less than 250 fps.

no... when you turn vsync off you stop synchronizing the output of the video card with the rate at which the monitor accepts frames *its 'refresh' rate, if you will*.

You allow the video card to send frames as fast as it can, but the monitor does not "accept/draw" the frames any faster than it did initially, and it is instructed to draw another frame before it finishes the former.

The pixel response time has 0 to do with that. All pixel response time does is measure how fast a pixel switches when it is instructed to change- how often it is instructed to change (the refresh rate) is a constant rate which has nothing to do with its response time... and yes - you CAN update the screen (instruct the pixels to change) faster than the pixels can switch - if the pixels dont switch in time you have ghosting.

edit"
the NEC quote looks like the marketing department talking out of their @ss. Theoretically the film used in the screen can display 250fps without ghosting, but the screen will not display more than 60 or 120 (or w/e its refresh rate is)