Refresh rate
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The frame rate or refresh rate is a term used in film, television and computer technology. It refers to the number of individual images per second that are projected onto a cinema screen, for example, or written onto a television screen or monitor by its electron beam. It is usually expressed in the unit Hertz (Hz) or as fps (frames per second). A distinction is made between the frame rate of the medium/input signal and the refresh rate of the screen/projector.
Effect
In video or moving image reproduction, the physiological property of human vision is exploited that changes in the image content are superimposed on the previous image content, which continues to exist for milliseconds, and the individual images thus merge with each other. [Evidence?]
It is important to distinguish between frame rate and refresh rate, as they do not have to be identical. The frame rate should not fall below a value of approx. 30 frames/second in order to be able to convey a fluid image impression to the human eye with moving image content. The limit for the perception of a flowing movement depends on the scene and also varies slightly from person to person. The frame rate of 24 Hz, which is common in cinema today, limits the moving image representation, which leads to objects moving at an unfavourable, medium speed on the screen being perceived as "jerky" or, due to synthetic motion blur, as "washed out". A good director knows how to avoid this, for example by making camera pans only very slowly or sufficiently fast.
The refresh rate of a screen makes a decisive contribution to the flicker impression. However, this also depends on the screen technology used. In cinemas, for example, each of the 24 images per second (= 24 Hz frame rate) is projected twice with a shutter, i.e. alternately switched on and off at a refresh rate of 48 Hz. With liquid crystal screens, on the other hand, the refresh rate has hardly any effect on the flicker impression, since the brightness of each pixel remains almost constant between the image changes due to the principle, which avoids light-dark changes. It is worth noting that human vision is more sensitive to changes in brightness in the outer areas due to the rods. Flickering is therefore more likely to be perceived when viewing a CRT screen from the corner of the eye, for example. To avoid this flicker, the refresh rate (50/60 Hz) is doubled compared to the frame rate (25i/30i fps) when reproducing on CRT screens. In this case, the interlaced scanning method is used, whereby the 25 or 30 frames per second are reproduced as fields with 50 or 60 motion phases per second, respectively, optimized for reproduction on a CRT screen. With the interlaced method, an i is added to the frame rate to identify it. Often the old notation is still used, in which the interlaced frame rate is not specified as the full frame rate, but as the field rate (here: 50i/60i fps).
In the best case, the refresh rate is an integer multiple of the played back frame rate, because otherwise the source cannot be played back completely smoothly. This fact also explains the frequent refresh rate 144 Hz, which is a multiple of the two cinema standards 24 and 48 fps. The also common 120 Hz and 240 Hz refresh rates additionally take into account the compatibility with the widely used 60 fps standard.
It must also be considered that the advantages of a high rendered frame rate are lost if the display has high response times. The same applies to synthetic motion blur, which should be reduced as the frame rate increases.
Some filmmakers, such as American director James Cameron, have been advocating the introduction of higher frame rates in motion pictures for several years.
Standards
In the case of interlaced display, as used in PAL or NTSC (analogue television), the frame rate corresponds to the transmitted field rate, i.e. 50 Hz for PAL and approx. 59.94 Hz (exactly 60000/1001 Hz) for NTSC. The actual frame rate is only half as high in each case. The picture produced in this way with picture tubes is nevertheless perceived by most people as flickering. In many modern devices, the refresh rate is therefore increased to 100 Hz with the aid of a picture memory.
Some display devices (LCD, projectors) are not refreshed with the frequency output by the graphics card, which requires a frame buffer in the device for frequency conversion. These devices can handle different refresh rates in the input signal, but the best picture is achieved when the input frequency matches the refresh rate, which is 60 Hz for almost all of today's devices. Techniques like Nvidia's G-Sync or AMD's FreeSync solve this problem by not using a fixed refresh rate, but refreshing a displayed image only when the graphics card outputs a new image.
Usual frame rates:
- MDA/Hercules: 50 full frames/second, monitors used were often persistent
- CGA/EGA: 60 full frames/second
- VGA: 60 or 70 full frames/second, sometimes also 56 full frames/second
- IBM 8514: 86 fields/second, 60 frames/second
- Computer monitors (tube): 86 fields/second, 56 to 160 full frames/second
- Computer monitors (LCD): usually 50, 60, 120, 144 or 240 full frames/second
- Television (tube): 59.94 fields/second (NTSC), 50 fields/second (PAL)
- TV (LCD): 48-800 frames/second (especially with high frequencies mostly using interframe calculations, with 3D TVs the frame rates for left and right eye are added)
