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### Refresh Rate Control

Once you have found the identifier for the display mode you wish to use, you can then call the SetVideoMode function to set the display mode. One of the parameters to the SetVideoMode function is the refresh rate to be used for the mode. In most normal situations you would pass a value of 0 for this parameter, which will set the currently selected default refresh rate for the mode. However it is possible for the application to directly specify the refresh rate to be used via two different methods.

The first method is to pass the desired refresh rate to the SetVideoMode function as a specific value in Hz (ie: the integer value 60 for 60Hz etc). However this will only work if the refresh rate requested is actually supported by the display mode, so you need to first determine what refresh rates are supported. One of the bits of information returned by the GetVideoModeInfo function, is a complete list of all refresh rates for that display mode. This list of refresh rates will be properly filtered based on the capabilities of the underlying graphics hardware and the attached monitor (assuming a monitor has been selected by the user or detected automatically by Plug and Play). If you pass a value that is not listed in the information block returned by the GetVideoModeInfo function, the SetVideoMode function will return –1, indicating failure.

The second method allows for refresh rates that are not listed as part of the drivers standard timings, such that an application can provide an exact set of CRTC timings to be used for the display mode (useful for flat panels and fixed frequency displays). Alternatively if the application just needs a non-standard, custom refresh rate, the application can use the VESA Generalised Timing Formula (GTF) functions provided to compute a set of generic CRTC timings for any arbitrary display mode and refresh rate (see GA_computeCRTCTimings). Once the application has used the GTF functions to compute a set of CRTC timings for the display mode, those CRTC timings can then be passed directly to the SetVideoMode function along with use of the gaRefreshControl mode flag. This second method provides for the maximum versatility and support for specialised applications (such as fixed frequency monitors, LC stereo shutter glasses and head mounted displays).

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