Refresh rate

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The refresh rate (most commonly the "vertical refresh rate", "vertical scan rate" for CRTs) is the number of times in a second that display hardware draws the data. This is distinct from the measure of frame rate in that the refresh rate includes the repeated drawing of identical frames, while frame rate measures how a video source can feed an entire frame of new data to a display.

For example, most movie projectors advance from one frame to the next one 24 times each second. But each frame is illuminated two or three times before the next frame is projected using a shutter in front of its lamp. As a result, the movie projector runs at 24 frames per second, but has a 48 or 72 Hz refresh rate.

On CRT displays, increasing the refresh rate decreases flickering, thereby reducing eye strain. However, if a refresh rate is specified that is beyond what is recommended for the display, damage to the display can occur.^[1]

For computer programs or telemetry, the term is also 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).

Cathode ray tubes

A high speed exposure shows an electron beam in the process of scanning an image

In a CRT, the scan rate is controlled by the vertical blanking signal generated by the video controller, ordering the monitor to position the beam 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.

The refresh rate can be calculated from the horizontal scan rate by dividing by the number of horizontal lines multiplied by 1.05 (since about 5% of the time it takes to scan the screen is spent moving the electron beam 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 × 1.05) ≈ 89 Hz (rounded down).

Liquid crystal displays

Much of the discussion of refresh rate does not apply to the liquid crystal portion of an LCD monitor. This is because while a CRT monitor uses the same mechanism for both illumination and imaging, LCDs employ a separate backlight to illuminate the image being portrayed by the LCD's liquid crystal shutters. The shutters themselves do not have a "refresh rate" as such due to the fact that they always stay at whatever opacity they were last instructed to continuously, and do not become more or less transparent until instructed to produce a different opacity. Most of the TFT LCDs used in portable devices and computer monitors need a continuous refresh. The driving voltage determines the transmittance of the liquid crystal.

The closest thing liquid crystal shutters have to a refresh rate is their response time, while nearly all LCD backlights (most notably fluorescent cathodes, which commonly operate at ~200 Hz) have a separate figure known as flicker, which describes how many times a second the backlight pulses on and off. However they also have a refresh rate that governs how often a new image is received from the video card (often at 60 Hz).

Computer displays

On smaller CRT monitors (up to about 15"), few people notice any discomfort below 60–72 Hz. On larger CRT monitors (17" or larger), most people experience mild discomfort unless the refresh is set to 85 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 Hz. 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 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, such as Windows 2000 and its descendants Windows XP and Windows Vista, set the default refresh rate to the lowest supported rate, usually 60 Hz. The many variations of Linux usually set a refresh rate chosen by the user during setup of the display manager (although a default option is usually included with xfree86). Many full-screen applications, such as games, allow the user to reconfigure the refresh rate before entering full-screen mode. But some poorly designed applications launch directly into full-screen mode in an out-of-range setting and force the user to reconfigure the setting "blind".^{[citation needed]}

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. Currently most monitors simply display a notice that the video signal uses an unsupported refresh rate.

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 200 Hz is optimal. Unfortunately most monitors cannot handle this rate, especially at higher resolutions.

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. This allowed PAL sets to have higher resolution and detail than NTSC counterparts. (640x480 NTSC and 768x576 for PAL/SECAM) However, the lower scan rate can introduce more flicker on high speed motion, so sets that use digital technology to double the refresh rate to 100 Hz are now very popular.

Another difference between 50 Hz and 60 Hz standards is the way motion pictures (film sources as opposed to video camera sources) are transferred or presented. 35 mm film is typically shot at 24 frame/s. For PAL 50 Hz this allows film sources to be easily transferred by accelerating the film by 4%. The resulting picture is perfectly smooth, however, there is a slight shift in the pitch of the audio which cannot normally be noticed. NTSC sets display both 24 frame/s and 25 frame/s material without any speed shifting by using a technique called 3:2 pulldown, but at the expense of introducing unsmooth playback in the form of telecine judder.

Unlike computer monitors, HDTV and some DVDs, analog television systems use interlace, which decreases the apparent flicker by painting first the odd lines and then the even lines (these are known as fields). This doubles the refresh rate, compared to a progressive scan image at the same frame rate. This works perfectly for video cameras, where each field results from a separate exposure - the effective frame rate doubles, there are now 50 rather than 25 exposures per second. The dynamics or a CRT are ideally suited to this approach, fast scenes will benefit from the 50 Hz refresh, the earlier field will have largely decayed away when the new field is written, and static images will benefit from improved resolution as both fields will be integrated by the eye. Modern CRT-based televisions may be made flicker-free in the form of 100 Hz technology.

Many high-end LCD televisions now have a 120 or 240 Hz (current and former NTSC countries) or 100 or 200 Hz (PAL/SECAM countries) refresh rate. The rate of 120 was chosen as the least common multiple of 24 frame/s (cinema) and 30 frame/s (NTSC TV), and allows for less distortion when movies are viewed due to the elimination of telecine (3:2 pulldown). For PAL at 25 frame/s, 100 or 200 Hz is used as a fractional compromise of the least common multiple of 600 (24 x 25). Until a 600 Hz refresh rate becomes available, PAL video will speed up cinema by a small percentage (currently 1 to 4 percent). These higher refresh rates are most effective from a 24p-source video output (e.g. Blu-ray Disc), and/or scenes of fast motion.

Displaying movie content on a tv

As movies are usually filmed at a rate of 24 frames per second, while tv-sets operate at different rates, some conversion is necessary. Different techniques exist to give the viewer an optimal experience.

The combination of content production, playback-device, and display device processing may also give artifacts that are unnecessary. A display device producing a fixed 60frame/s rate cannot display a 24frame/s movie at an even, judder-free rate. Usually, a 3:2 pulldown is used giving a slight uneven movement.

While common multisync CRT computer monitors have been capable of running at even multiples of 24 Hz since the early '90s, recent "120Hz" LCD displays have been produced for the purpose of having smoother, more fluid motion, depending upon the source material, and any subsequent processing done to the signal. In the case of material shot on video, improvements in smoothness just from having a higher refresh rate may be barely noticeable. ^[2]

In the case of filmed material, as 120 is an even mutiple of 24, it is possible to present a 24frame/s sequence without judder on a well-designed 120 Hz display (i.e., so-called 5-5 pulldown). If the 120 Hz rate is produced by frame-doubling a 60frame/s 3:2 pulldown signal, the uneven motion could still be visible (i.e., so-called 6-4 pulldown).

Additionally, material may be displayed with synthetically-created smoothness with the addition of motion interpolation abilities to the display, which has an even larger effect on filmed material.

"50Hz" tv-sets (when fed with "50Hz" content) usually get a movie that is slightly faster than normal, avoiding any problems with uneven pulldown.

Computer data and telemetry

For computer data and telemetry, the term is also used to refer to the frequency of updates to a piece of data from an external source. This might be expressed in any unit of time.

See also

• Television
• Cathode ray tube
• Plasma display
• Comparison of display technology
• Frame rate

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.