To lock framerate, or not to lock framerate.


Shawn Purdy On May 4, 2014
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Framerate is an extremely important aspect of simulation racing. What we will cover in here is somethings from a technical point of view, that not everyone is aware of. So first, I will cover a couple of different sim-racers. They are usually in three categories.
 

  1. Users that want a smooth driving experience, and as much eye candy as possible.
  2. Users that want as much framerate as possible, but will still try have some eye candy.
  3. Users that want as much framerate as possible, but also don't care how the game looks graphically.
So below is some of the reasons that sim-racers fall into these categories.

1. What it boils down to is the users that intend on having a smooth driving experience. They hate seeing what is known as screen tearing. This happens because the monitor is unable to keep up with the amount of data it's being fed. Basically at times you might see data from one frame mixed with another frame, or worse. For some people this drives them crazy and they can't drive. Some people are just convinced that it is overworking your video card for no reason. Personally I've had many video cards, and I personally fall into Category 2 but sometimes 3. I will explain later why sometimes 3. I've never had a video card failure. So I really don't see this as a valid argument. With proper cooling these cards are designed to run full power. The monitors however are not.

2.They can't stand seeing graphical lag off in the distance. Usually they are looking further down the road than other drivers. Or they are just more sensitive to those glitches. When you look to the side of the road out your drivers side window in real life, things are very smooth. So when you see it's not smooth you want to get more framerate.

3.This is by far the most complicated category. The reason these drivers don't care about eye candy is not because they don't want to see it. It's because they are convinced it's faster to drive this way. Being convinced however isn't really a knowledge on why it can be faster. So I will try to explain the technical reasons on why it can be.

Technical reasons why it can be faster to allow maximum framerate that's possible with your computer.

Not all simulations are made equal. Depending on how the software is designed will make a difference. For example the original rFactor when it first came out. The steering hertz was tied directly to the framerate. So if you had say 30 fps. You were only getting 30 Hertz of steering resolution. That means every one second, your steering information is updated 30 times. Lets think about this for a second, because 30 times a second is no where near enough. A driver can make 100's of corrections in a second in many cases. After awhile someone did bring up this fact, and rFactor patched this by putting a dedicated thread in the software. This locked the Steering hertz. The advantage in locking it is that it is consistent. So if you lock it at say 120 Hertz. Than you always have 120 Hertz. where if your steering is consistently changing every few seconds. You may not be aware of it, but it will have a negative effect on your driving.

Update: According to David Tucker at iRacing my information below is not valid for iRacing. As it is fixed at 60hz. The information however when it relates to other simulations can still apply. I've highlighted in red what is not correct. What this means is that iRacing has 60 updates per second. He goes into much detail about the design. He also explains why vsync is not that useful. You can read his full report on this information on the iRacing forums.

Lets use another example. iRacing has this exact problem. Whatever framerate you have, that's how much steering hertz you have. You can even test this yourself if you don't beleive me. To help you understand what is happening. The code in the software is going something a long the lines of:

Please update frame, Oh, while you are it, please update the steering position as well.

So by going this route, you can see clearly that the only way the steering gets updated is when the new frame comes in. That simply means if you lock your framerate to 60, which is most common. You are limiting yourself to 60 steering updates per second. The chances are a lot of sims are designed this way. It's not that uncommon. So no matter what simulation you are driving. Try it out because it just might make you faster! Sometimes it is extremely obivous. I've seen videos where you see the person turn the steering and the onscreen game doesn't actually respond right away.

With newer 120 hertz monitors, I beleive this problem will slowly become less important. Personally I have found a difference as high as 300FPS still making a difference. You have to take into account when your driving with 30 cars on the track. So while I may have 455 fps in solo practice. I only really have 297 in a full blown race, and it's a good idea to keep a reserve of 50-75 FPS for any kind of heavy graphical moment.The amount of car control you gain will be less the more framerate you have. So this is why I fall into category three at times. Because I do recoginize the difference, but sometimes I just find that I feel like I'm driving a sim from the 1990's when I take all the graphics off. I tend to lean toward upgrading my video card so that I can push it further without giving up steering resolution.





 

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Comments

Tim Chitwood On 2014-06-08 16:34:25
Tim Chitwood

I have a GTX780 3Gb video card and a 120 Hz tv, I have been torn between the iRacing V-synch which may slow controls and 164 fps myself. Now I did find that I had a "micro-stutter" which is caused by one or more cores "sleeping." I found this in iRacing it is helpful and ez fix.

 https://www.youtube.com/watch?v=pl3u9eiskM4&feature=youtu.be

Reply
Tim Chitwood On 2014-07-05 18:31:57
Tim Chitwood

After more experience with my GTX 780 video card, I can get 450- 550 fps on iRacing and the picture is so realistic it can fool even guys that have actually raced there. I have a friend that has raced at Lime Rock and he said it looks like the real thing to him. But, not all games are made the same, others may max out below 140 fps, some still "tear" unless "v-synched" or turn everything down. So there is really no single setting set up for me.

Reply

Jason Spencer On 2014-05-18 21:54:06
Jason Spencer

I tend to be one of those people who don't like screen tearing and I like to turn the eye candy up as much as I can when I play IRacing. I personally use an AMD 7870 card and a while back I discovered a nice little program called RadeonPro that lets you adjust your graphics card settings more in depth than you can with the regular Catalyst Control Center that comes with the AMD drivers. Nobody ever told me about this program and I had owned my video card for over a year until I found out about it surfing the web for graphics tweaks one day. I'm not sure you can even find this program on the AMD website.

The first thing that is nice about that program is that you can inject FXAA in conjunction with the regular anti-aliasing effects. This gives a little boost to the anti-aliasing quality without much loss in performance, if at all. It somewhat blurs the anti-aliasing like morphological filtering would but without as much blurring of the text in the menu screens. Note: there is also an SMAA injection option, which from my understanding is of little higher quality. These two anti-aliasing methods you can't find in the Catalyst Control Center software!

But the second and most important point I would like to make is that there is a Dynamic V-sync option in the RadeonPro software that allows you to use vsync without the performance drop that you might find while using the *regular* v-sync in the Catalyst Control Center. I believe NVidia may have this option too in one form or another, but basically, the RadeonPro software monitors your frames in real time and when they are above your montiors refresh rate(ex. 60hz) v-sync is turned on, and when your real time frames per second are below the montiors refresh rate v-sync is then turned off. So it doesn't totally eliminate screen tearing but it doesn't allow your frames to drop like normal v-sync would. So I guess you could think of it as a form of vsync that maintains the framerate at 60 basically. I personally find this to be a better option than setting a framerate limit alone and what used to be screen tearing when looking at the tire/suspension camera in IRacing is now gone.

Keeping in mind what David Tucker mentions about the 60hz monitor and wheel refresh rates in IRacing, this may be a viable option for you other AMD owners out there who want to minimize screen tearing while maintaining 60 frames. So if you're an AMD GPU owner then give this a shot as I think you might be pleasantly surprised. smile
 

Reply
Randy Eliason On 2014-06-16 15:23:11
Randy Eliason

Suprised indeed! The tool works great for my little old 5750. Been working for a long time trying to put an end to the tearing, jaggies and stutters. All have been removed. Only took a short time in learing how to use it! Thank you for the post Jason.

Reply

Christien Hoogveld On 2014-05-14 02:33:18
Christien Hoogveld

I tried to lock my frame rate at 73 and things got really chunky. Set it back to 60 and it was smooth sailing. I also tried 169 but that didn't work either. My GPU was capable of 120 plus FPS but not 169. Thought I would share this. I don't know why, but at 60 FPS my iRacing runs much smoother.

Reply

Rob Cobb On 2014-05-07 18:01:16
Rob Cobb

Thanks for the article and the SimPit is looking great!

Locking the framerate is something we advise drivers to do for two reasons:

1) Reduces screen tearing. Tearing occurs when the refresh rate of the monitor and the refresh rate presented to it do not match. You can reduce tearing by setting the framerate to something that will hold steady (something the video card can handle easily) that is equal to or a multiple of the monitor refresh rate. 120Hz is usually a good choice because many monitors are 60Hz and most game/card combinations can handle 120FPS.

2) Reduces CPU lag. Allowing a high-performance video card to have it's head (push as many FPS as possible) can overrun the capabilities of the CPU. There's no value in running hundreds of FPS, so why push the CPU? It has other jobs to do including game logic and processing network packets.

Reply
Daven Aki On 2014-05-07 18:36:39
Daven Aki

"A better workaround is to make sure that the 'tear' does not show up in the same spot each time, that way your eyes will have a hard time seeing it. You can achieve this by setting your render rate to some number that is not cleanly divided by your monitors refresh rate. In other words, if you render at 60 FPS with a 60 Hz monitor you will end up with a tear line that appears relatively stable on the display and you will not have much trouble finding it, and if you render at 120 FPS you will end up with two tears that are relatively stable but only show up every other frame. However if you pick a number like 73 FPS then your tear line will move about from frame to frame and you will hardly notice it at all." 

Reply
Rob Cobb On 2014-05-07 18:52:31
Rob Cobb

Good info. I think I see the phenomanon as described in your post. The other benefit to locking the frame rate is that you know what you are going to get at all times. Allowing the framerate to bounce around moves any "tears", but it also seems to introduce a jitter as wild swings in framerate are encountered. Not sure what that is called...


Daven Aki On 2014-05-07 11:36:38
Daven Aki

I recommend capping fps to 169 even with 60hz screen. That way you got very 'fresh' picture and no tearing at all because tear is always at different level. You cant see single tear... only continuing tears at same place. Worst numbers are those which are closely divided with refresh rate. 

Reply

Niel Hekkens On 2014-05-06 17:43:04
Niel Hekkens

Mr Tuckers response on this article.
http://members.iracing.com/jforum/posts/list/1575/1473510.page#8027315

Reply
Shawn Purdy On 2014-05-07 05:24:13
Shawn Purdy

Thanks, I've updated the article. smile

Reply

Marc De Loose On 2014-05-05 08:26:25
Marc De Loose

I don't think iracings steering/ffb update loop is tied to the framerate.
To my knowledge from reading on the comments on the iracing forum the update rate is 60hz.

So if you have a fps that is super high should not make a difference other than maybe a quicker screen update in some situations.

I might be wrong but I remember reading comments from D. Tucker that stated something different than what is claimed here.

Reply
Shawn Purdy On 2014-05-05 17:29:54
Shawn Purdy

You sure about that? Is that something new? I apologize if it's invalid. The article still applies though because it's something that is done. The screen however won't update faster than the refresh rate of 60Hz or 120Hz depending on the monitor. I'm just suprised if it's limited to 60 because it's not enough.

Reply
Matt Kossler On 2014-05-05 19:42:11
Matt Kossler

Shawn,

Sorry is you know all this, but it's important in the context of some of the points you made in your article.

A screen refresh is the completion of re-drawing every horizontal line of resolution on the screen.  This happens 60 times per second on a 60Hz monitor.  However, when the graphics card refreshes at a faster rate (say, 150 frames per second), then the line information it sends to the screen is from a different frame for every line.  This phenomenon is what is called screen tearing - it is impossible to see any completely rendered frame on the screen, because the graphics card has "moved on" to new screen renders.  

If there is a scene change mid-screen-refresh, then you can even get "tears" that momentarily appear to show the newer screen on top and the older screen draw on the bottom.  The whole screen tearing effect can create some very weird artifacts, especially with quickly changing scenery like in a sim racer.


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