Refresh rate

In reference to terminology originating with cathode-ray tubes (CRTs), the refresh rate, also known as vertical refresh rate, vertical scan rate or vertical frequency, is the number of times per second that a raster-based display device displays a new image. This can be independent from frame rate, which describes how many images are stored or generated every second by the device driving the display. On CRT displays, higher refresh rates produce less flickering, thereby reducing eye strain. In other technologies, such as liquid-crystal displays, the refresh rate affects only how often the image can potentially be updated. Non-raster displays may not have a characteristic refresh rate. Vector displays, for instance, do not trace the entire screen, only the actual lines comprising the displayed image, so refresh speed may differ by the size and complexity of the image data. For computer programs or telemetry, the term is sometimes applied to how frequently a datum is updated with a new external value from another source (for example; a shared public spreadsheet or hardware feed).

Physical factors While all raster display devices have a characteristic refresh rate, the physical implementation differs between technologies.

Cathode-ray tubes

Raster-scan CRTs by their nature must refresh the screen since their phosphors will fade and the image will disappear quickly unless refreshed regularly. In a CRT, the vertical scan rate is the number of times per second that the electron beam returns to the upper left corner of the screen to begin drawing a new frame. It is controlled by the vertical blanking signal generated by the video controller, and is partially limited by the monitor's maximum horizontal scan rate. The refresh rate can be calculated from the horizontal scan rate by dividing the scanning frequency by the number of horizontal lines, plus some amount of time to allow for the beam to return to the top. By convention, this is a 1.05x multiplier. For instance, a monitor with a horizontal scanning frequency of 96 kHz at a resolution of 1280 × 1024 results in a refresh rate of 96,000 ÷ (1024 × 1.05) ≈ 89 Hz (rounded down). CRT refresh rates have historically been an important factor in video game programming. In early videogame systems, the only time available for computation was during the vertical blanking interval, during which the beam is returning to the top right corner of the screen and no image is being drawn. Even in modern games, however, it is important to avoid altering the computer's video buffer except during the vertical retrace, to prevent flickering graphics or screen tearing.

Liquid-crystal displays Unlike CRTs, where the image will fade unless refreshed, the pixels of liquid-crystal displays retain their state for as long as power is provided. Consequently, there is no intrinsic flicker regardless of refresh rate. However, the refresh rate still determines the highest frame rate that can be displayed, and despite there being no actual blanking of the screen, the vertical blanking interval is still a period in each refresh cycle when the screen is not being updated, during which the image data in the host system's frame buffer can be updated. Vsync options can eliminate screen tearing by rendering the whole image at the same time.

Computer displays

On smaller CRT monitors (up to about 15 in or 38 cm), few people notice any discomfort between 60–72 Hz. On larger CRT monitors (17 in or 43 cm or larger), most people experience mild discomfort unless the refresh is set to 72 Hz or higher. A rate of 100 Hz is comfortable at almost any size. However, this does not apply to LCD monitors. The closest equivalent to a refresh rate on an LCD monitor is its frame rate, which is often locked at 60 fps. But this is rarely a problem, because the only part of an LCD monitor that could produce CRT-like flicker—its backlight—typically operates at around a minimum of 200 Hz. Different operating systems set the default refresh rate differently. Microsoft Windows 95 and Windows 98 (First and Second Editions) set the refresh rate to the highest rate that they believe the display supports. Windows NT-based operating systems and their descendants, including Windows 11, set the default refresh rate to a conservative rate, usually 60 Hz. Most Linux distributions follow the same behavior, but some are able to dynamically set a default based on the monitor's refresh rate. Since macOS is normally used exclusively on Apple devices and products, most versions of macOS are locked to 60 Hz with exceptions like later MacBook Pro models. Some fullscreen applications, including many games, now allow the user to reconfigure the refresh rate before entering fullscreen mode, but most default to a conservative resolution and refresh rate and let you increase the settings in the options. Old monitors could be damaged if a user set the video card to a refresh rate higher than the highest rate supported by the monitor. Some models of monitors display a notice that the video signal uses an unsupported refresh rate.

Dynamic refresh rate

Some displays support variable refresh rate (VRR), the ability to change their refresh rate on-the-fly. VRR can be used to save power by allowing the system to spend less performance rendering frames, a technique seen on some mobile devices like smartphones and laptops. Some computer monitors feature VRR to lower input latency, typically that of a video game, by delay refreshing of the display until a new frame is sent from the graphics card. FreeSync and G-Sync are implementations of VRR seen in AMD and Nvidia's graphics card, respectively.

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