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What does "Optimal" Refresh Rate really mean?

Section 2 - Refresh Rates

Under the Refresh Rate drop down there are two common settings, Adapter Default and Optimal. Depending on the drivers used for the video card and monitor, there may be individual refresh rate settings (i.e. 60Hz, 70Hz, 72Hz, etc.) as well. However, if you choose one of the individual refresh rates listed, it will only be good for that resolution. If you change resolutions (common if you are a gamer), then the refresh rate will generally switch to 60Hz for that resolution and that isn't a good thing. When the game is exited, the prior resolution and refresh rate should be restored. But is there an alternative?  That is were the "Optimal" refresh rate comes into play. Theoretically, the "Optimal" setting should set the highest recommended refresh for a given resolution.  I say theoretically because, in practice that rarely happens. I've seen many newsgroup posts of people complaining that no matter what resolution they are using, the "Optimal" setting leaves them at 75Hz and that has been my experience as well.  So what gives?  My monitor can do up to 100Hz at 1152x864, why is it only doing 75Hz?  I decided to contact nVidia and get an explanation. Peggy Seltz, Software Manager Windows 2D/Video Development was kind enough to educate me on the fine points of refresh rates.

"Optimal" refresh rates are very confusing because it means different things to different vendors. Most people assume this is the maximum refresh rate that a monitor can support, but that is not always the case. Many system vendors have us cap the optimal refresh rates so they don't damage their customers' monitors. The problem we see from the driver perspective is we don't always have the information that we need. Sometimes we don't have a monitor .INF, and when we do, the information is not always reliable. The same thing goes for a monitor EDID. Even when we have enough information (and assume it is reliable) many system vendors don't want to risk damaging any monitors so they will cap the optimal refresh rate to a fixed value, for example, 75Hz.

So that's why so many of us are stuck on 75Hz maximum "Optimal" refresh rate. She goes on to say...

You can set a specific refresh rate in display properties (advanced tab). This lists all of the refresh rates that Windows has determined are safe for your monitor based on the information available to Windows (the driver .INF, the monitor .INF and the EDID). When setting a refresh rate, if the driver doesn't have enough info, it will use the information that it does have and make an intelligent but safe guess, which could be lower than the true maximum refresh rate supported by your monitor. We really go to great lengths to try to get this all correct for all cases, but as you can imagine, there are always some corner cases - for example an incorrect monitor .INF installed for a monitor, or EDID info that is not valid.

Many people complain that they can't get above 640x480 after installing new drivers.   Nine times out of ten, it's because the monitor wasn't detected properly.   When they choose the right monitor, then everything works again.  It is possible for a new monitor to work using a different monitor's drivers, however the new monitor's capabilities won't be properly recognized.

Another possibility is that Windows has determined (with the information available) that 75Hz is the max refresh that your monitor is capable of at this resolution. If you don't have a choice higher than 75Hz in display properties, then this is the case.

Finally, if you choose a less than standard resolution like 1152x864 with a refresh rate above 75Hz, you no longer are using VESA DMT monitor timings, but rather GTF timings.   The only DMT timing defined for 1152x864 is for 75Hz refresh. If you chose the monitor's maiximum refresh rate at any resolution, would it use GTF timings?

That depends on the refresh rates that your monitor is capable of. For 1152x864, if the driver determines that your monitor is capable of refresh rates other than 75Hz, the driver will use GTF timings for the other refresh rates. For the 1152x864 resolution, the only VESA DMT monitor timings that are defined are for 75Hz refresh rate.

So there you have it.  Most monitor manufacturers, while advertising ultra-high refresh rates, are more apt to error on the side of caution. Finally, this closing thought...

Monitor manufacturers are actually starting to do a good job of improving the EDID information and are giving us monitor range limits so we can determine the timings based on these values instead of fixed refresh rates.

Finally, after choosing the "Optimal" setting, you may want to see what value was selected. Most monitors offer the ability to check the refresh rate via the OSD (OnScreen Display). On my old Panasonic monitor, just hitting the number "2" button would show the refresh rate. On my new Viewsonic monitor, it's called "Viewmeter". The value you are looking for is FV or Vertical refresh rate.

Thanks again to Peggy and the fine folks at nVidia for taking some time out of their busy schedules to drop a few pearls of wisdom on me. As always, feedback on this is welcomed. The followup to this article also features refresh rates and the relationship with VSYNC. Click here for that article.

UPDATE: Since Windows XP came out, much of this article no longer applies. However, Win XP (and Windows 2000) bring their own problems with Refresh Rates.

Windows 2000/XP