The
refresh rate (most commonly the "vertical refresh rate", "vertical scan rate" for
CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen...
s) is the number of times in a second that display hardware draws the data. This is distinct from the measure of
frame rateFrame rate, or frame frequency, is the measurement of the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
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 projectorA movie projector is an opto-mechanical device for displaying moving pictures by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.-Physiology:...
s 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
flickerFlicker is visible fading between cycles displayed on video displays, especially the refresh interval on cathode ray tube based computer screens. Flicker occurs on CRTs when they are driven at a low refresh rate, allowing the screen's phosphors to lose their excitation between sweeps of the...
ing, 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.
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
In a CRT, the scan rate is controlled by the
vertical blankingThe vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the next. It is present in analog television, VGA, DVI and other signals. During the VBI the incoming data...
signal generated by the video controller, ordering the monitor to position the beam at the upper left corner of the
rasterA raster scan, or raster scanning, is the rectangular pattern of image capture and reconstruction in television. By analogy, the term is used for raster graphics, the pattern of image storage and transmission used in most computer bitmap image systems....
, ready to paint another frame. It is limited by the monitor's maximum
horizontal scan rateHorizontal scan rate, or horizontal frequency, usually expressed in kilohertz, is the frequency at which a CRT moves the electron beam from the left side of the display to the right and back, and therefore describes the number of horizontal lines displayed per second...
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 crystalLiquid crystals are a state of matter that has properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...
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
backlightA backlight is a form of illumination used in liquid crystal displays . Backlights illuminate the LCD from the side or back of the display panel, unlike frontlights, which are placed in front of the LCD...
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 cathodesA fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light....
, which commonly operate at ~200 Hz) have a separate figure known as
flickerThe flicker fusion threshold is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer...
, 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 rateFrame rate, or frame frequency, is the measurement of the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
, 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 95Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
and
Windows 98Windows 98 is a graphical operating system by Microsoft. It was released on June 25, 1998, and is the successor to Windows 95. Like its predecessor, it is a hybrid 16-bit/32-bit monolithic product based on MS-DOS...
(First and Second Editions) set the refresh rate to the highest rate that they believe the display supports.
Windows NTWindows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was originally designed to be a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was...
-based operating systems, such as
Windows 2000Windows 2000 is a line of operating systems produced by Microsoft for use on business desktops, notebook computers, and servers. Released on February 17, 2000, it was the successor to Windows NT 4.0, and is the final release of Microsoft Windows to display the "Windows NT" designation...
and its descendants
Windows XPWindows XP is a line of operating systems produced by Microsoft for use on personal computers, including home and business desktops, laptops, and media centers. The name "XP" is short for "eXPerience"...
and
Windows VistaWindows Vista is a line of operating systems developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, set the default refresh rate to the lowest supported rate, usually 60 Hz. The many variations of
LinuxLinux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed,...
usually set a refresh rate chosen by the user during setup of the display manager (although a default option is usually included with
xfree86XFree86 is an implementation of the X Window System. It was originally written for Unix-like operating systems on IBM PC compatibles and is now available for many other operating systems and platforms. It is free and open source software under the XFree86 License version 1.1. It is developed by the...
). 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".
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
stereoStereoscopy, stereoscopic imaging or 3-D imaging is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph, movie, or other two-dimensional image is created by presenting a slightly different...
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
NTSCNTSC, named for the National Television System Committee is the analog television system used in most of the Americas, Japan, South Korea, Taiwan, Burma, and some Pacific island nations and territories . is also the name of the U.S. standardization body that developed the broadcast standard...
(60 Hz) and
PALPAL, short for Phase Alternating Line, is an analogue television encoding system used in broadcast television systems in large parts of the world. Other common analogue television systems are SECAM and NTSC. This page primarily discusses the colour encoding system...
&
SECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
(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
interlaceIn the domain of mechanical television, the concept of interlacing was demonstrated by Léon Theremin. He had been developing a mirror drum-based television, starting with 16 lines resolution in 1925, then 32 lines and eventually 64 using interlacing in 1926, and as part of his thesis on May 7, 1926...
, 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-freeFlicker-free is a term given to televisions that operate at a 100 or 120 hertz field rate to eliminate flicker, compared to standard televisions that operate at 50 Hz or 60 Hz...
in the form of 100 Hz technology.
Many high-end LCD televisions now have a 120 or 240 Hz (current and former
NTSCNTSC, named for the National Television System Committee is the analog television system used in most of the Americas, Japan, South Korea, Taiwan, Burma, and some Pacific island nations and territories . is also the name of the U.S. standardization body that developed the broadcast standard...
countries) or 100 or 200 Hz (
PALPAL, short for Phase Alternating Line, is an analogue television encoding system used in broadcast television systems in large parts of the world. Other common analogue television systems are SECAM and NTSC. This page primarily discusses the colour encoding system...
/
SECAMSECAM, also written SÉCAM , is an analog color television system first used in France....
countries) refresh rate. The rate of 120 was chosen as the
least common multipleIn arithmetic and number theory, the least common multiple or lowest common multiple or smallest common multiple of two integers a and b is the smallest positive integer that is a multiple both of a and of b. Since it is a multiple, it can be divided by a and b without a remainder...
of 24
frame/sFrame rate, or frame frequency, is the measurement of the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
(cinema) and 30 frame/s (NTSC TV), and allows for less distortion when movies are viewed due to the elimination of
telecineTelecine is the process of transferring motion picture film into video form. The term is also used to refer to the equipment used in the process....
(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
24pIn video technology, 24p refers to a video format that operates at 24 frames per second frame rate with progressive scanning . Originally, 24p was used in the non-linear editing of film-originated material...
-source video output (e.g.
Blu-ray DiscBlu-ray Disc is an optical disc storage medium designed to supersede the standard DVD format. Its main uses are for storing high-definition video, PlayStation 3 games, and other data, with up to 25 GB per single layered, and 50 GB per dual layered 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.
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 interpolationMotion interpolation is a form of video processing in which intermediate animation frames are generated between existing ones, in an attempt to make animation more fluid.-HDTV:...
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
Television is a widely used telecommunication medium for transmitting and receiving moving images, either monochromatic or color, usually accompanied by sound. "Television" may also refer specifically to a television set, television programming or television transmission...
- Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen, with internal or external means to accelerate and deflect the electron beam, used to create images in the form of light emitted from the fluorescent screen...
- Plasma display
A plasma display panel is a type of flat panel display common to large TV displays . Many tiny cells between two panels of glass hold a mixture of noble gases. The gas in the cells is electrically turned into a plasma which then excites phosphors to emit light...
- Comparison of display technology
- General characteristics :- Temporal characteristics :Different display technologies have vastly different temporal characteristics, leading to claimed perceptual differences for motion, flicker etc....
- Frame rate
Frame rate, or frame frequency, is the measurement of the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...