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Overview
As VR becomes more widespread, more people are using VR devices, whether tethered headsets, mobile VR, or standalone headsets. Two common terms are refresh rate (Hz) and frame rate (fps). This article explains what those terms mean and how they affect the VR experience.
If you use VR frequently, you have likely experienced motion sickness. Addressing motion sickness requires reducing latency, and latency is closely related to refresh rate and frame rate.
Refresh Rate and Frame Rate
Refresh rate refers to how often the display receives an output signal from the graphics card. For example, 60 Hz means the display receives 60 signals per second. The refresh rate is determined by the display.
Frame rate refers to how quickly the image changes. For example, 60 fps means the graphics card generates 60 frames per second (in theory, each frame is a distinct image). Frame rate is determined by the GPU.
As long as the GPU is powerful enough, the frame rate can be high, and a high frame rate generally makes motion appear smoother. This is why some tethered headsets require a high-end PC—to provide sufficient GPU performance.
In simple terms, 60 Hz is like blinking 60 times per second, while 60 fps is like seeing 60 different images per second.
How Frame Rate and Refresh Rate Relate
Some headsets advertise very high refresh rates and claim to prevent motion sickness. However, increasing refresh rate alone has limited effect unless frame rate is matched to the refresh rate.
If the frame rate is half the refresh rate, the display will show the same frame for two refresh cycles. Conversely, if the frame rate is twice the refresh rate, only one of every two generated frames will be displayed. Frames generated above the display's refresh rate are effectively wasted and may even cause visual artifacts.
One useful analogy is a book that both the GPU and the headset are paging through. The GPU flips pages (frames); the faster it flips, the higher the frame rate. The display scans the pages it receives; the faster it scans, the higher the refresh rate.
Both systems must be synchronized to produce the final VR image. If the GPU produces 100 frames per second but the display can only refresh at 60 Hz, you will only see 60 frames per second; the remaining frames are discarded.
For VR, avoiding motion sickness typically requires both refresh rate and frame rate to reach at least around 90 Hz. Reaching 120 Hz on both can further reduce the likelihood of motion sickness. Many current VR systems still operate around 60 Hz, and some run at only 30 fps, which increases the risk of motion sickness.
