Refresh rate
From Wikipedia, the free encyclopedia
The refresh rate (or "vertical refresh rate", "vertical scan rate" for CRTs) is the number of times in a second that a display is illuminated. This is distinct from the measure of frame rate in that the refresh rate includes the repeated illumination of identical frames, while frame rate measures how often a display can change from one image to another. For example, a movie projector advances from one frame to the next 24 times each second. But each frame is illuminated twice or three times before the next frame is projected. As a result, the movie projector runs at 24 frames per second, but has a 48 or 72 Hz refresh rate.
For Computer Spreadsheets, the term could also be applied to how frequently a cell is updated with a new external value from another source (for example; a Shared public spreadsheet or other external data feed).
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[edit] Cathode ray tubes
In a CRT, the scan rate is controlled by the vertical sync signal generated by the video controller, ordering the monitor to position the electron gun at the upper left corner of the raster, ready to paint another frame. It is limited by the monitor's maximum horizontal scan rate and the resolution, since higher resolution means more scan lines. Increasing the refresh rate decreases flickering, reducing eye strain.
The refresh rate can be calculated from the horizontal scan rate by dividing by the number of horizontal lines and multiplying the result by 0.95 (since about 5% of the time it takes to scan the screen is spent moving the electron gun back to the top). 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 × 0.95 = 89 Hz (rounded down).
[edit] LCD displays
Much of the discussion of refresh rate does not apply to LCD monitors. This is because while a phosphor on a CRT will begin to dim as soon as the electron beam passes it, LCD cells open to pass a continuous stream of light, and do not dim until instructed to produce a darker color. Pertaining to LCDs, see also response time and Input lag.
[edit] Computer displays
On smaller CRT monitors (14") few people notice any change above 60–72 Hz. On larger CRT monitors (17", 19") most people would experience mild discomfort unless the refresh is set to a more comfortable 85 Hz or higher. 100 Hz is comfortable for most people. However, LCD monitors do not suffer from the same problems as their CRT predecessors and the refresh rate simply does not mean the same. LCD or TFT monitors will provide excellent quality and resolution at 60–Hz. The more important issue for a TFT or LCD monitor is its Response Time, Image Brightness and Image Contrast Ratio.
Different operating systems set the default refresh rate differently. Windows 95 and Windows 98(SE) set the highest possible refresh rate. Windows NT based OS's such as Windows 2000 and its descendant Windows XP, however, by default set the refresh rate to the lowest supported, usually 60 Hz. And the many variations of Linux usually have the user set up the display manager during installation and set the preferred settings. Although with xfree86 a default option is usually included.
Old monitors could be damaged if a user set the video card to a higher refresh rate than supported by the monitor. Nowadays most monitors would simply display a notice that the video signal uses an unsupported refresh rate.
[edit] Stereo displays
When LCD shutter glasses are used for stereo displays, the effective refresh rate is halved, because each eye needs a separate picture. For this reason, it is usually recommended to use a display capable of at least 120 Hz, but 170 Hz is optimal. Unfortunately most monitors cannot handle this rate, especially if the screen resolution is high.
[edit] Televisions
When the cathode ray tube was developed in the 1920s, technology limitations of the time made it difficult to run monitors at anything other than a multiple of the AC line frequency used to power the set. Thus producers had little choice but to run sets at 60 Hz in America, and 50 Hz in Europe. These rates formed the basis for the NTSC (60 Hz) and PAL & SECAM (50 Hz) sets used today. It was widely perceived that this accident of chance gave European sets an advantage, because the slower 50 Hz refresh rate gave the CRT time to scan more detail. However this rate also introduced more flicker, and exacerbated the negative effects of interlace, so sets that use digital technology to double the refresh rate to 100 Hz are now popular.
Another problem with 50 Hz standards is that motion pictures cannot be presented in the typical 24 fps rate used for 35 mm film. These must be accelerated by 4% - with an accompanying slight shift in the pitch of the audio. NTSC sets can display both 24 fps and 25 fps material without speed shift by using a technique called 3:2 pulldown.
Unlike computer monitors and some HDTVs, television broadcasts use interlace, which can increase flicker compared to a progressive scan image at the same refresh rate. The amount of extra flicker is largely dependent on the content of the image, and the brightness of the screen. Many newer televisions are flicker-free.
[edit] Computer spreadsheets
For Computer Spreadsheets, the term could also be applied to the frequency of update for each cells' external data value in a sheet. This might be expressed in minutes, hours or days or perhaps as a formula dependent on other data value(s) in the same spreadsheet or even other Shared public spreadsheets. It determines how often a new data value should be "fetched" afresh from its source. A stock price may require updating frequently during trading times but a population statistic would probably be sufficiently accurate for most purposes on a yearly basis or when the cell has "focus" and has been selected.
[edit] References
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
