The refresh rate or frame rate is a term used in film, television and computer technology. It describes the number of frames per second that z. B. be projected onto a cinema screen or written onto a television screen or monitor by its electron beam . These are usually given in the unit Hertz ( Hz ) or also as fps (English: frames per second) . A distinction is made between the frame rate of the medium / input signal and the refresh rate of the screen / projector.
In the case of video or moving image reproduction, the physiological property of human vision is used that changes in the image content are superimposed on the previous image content that continues for milliseconds and the individual images thus merge with one another. [Receipt?] .
It is important to differentiate between frame rates and refresh rates, as these 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 to the perception of a flowing movement is scene-dependent and also slightly different from person to person. The frame rate of 24 Hz that is common in today's cinema limits the display of moving images, which means that objects that move at an unfavorable, medium speed in the picture are perceived as "jerky" or, due to synthetic motion blur , as "blurred". A good director knows how to avoid this, for example by panning the camera very slowly or with sufficient speed.
The refresh rate of a screen contributes significantly to the flicker impression. However, that also depends on the screen technology used. In the cinema, for example, each of the 24 images per second (= 24 Hz frame rate ) is projected twice with a shutter, i.e. switched on and off alternately with a refresh rate of 48 Hz. In the case of 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, which avoids light-dark changes. It is noteworthy that the human eyesight in the outdoor areas reacts more sensitively to changes in brightness due to the rods . Flicker is therefore more likely to be perceived when z. B. viewed a tube screen from the corner of one eye. In order to avoid this flickering, the refresh rate (50/60 Hz) is doubled compared to the frame rate (25i / 30i fps) when playing on CRT monitors. The interlace method is used here, with the 25 or 30 images per second being reproduced as fields with 50 or 60 movement phases per second, optimized for reproduction on the tube screen. In the case of interlacing, the frame rate is appended with an i to identify it. Often you can still find the old way of writing, in which the interlaced method does not specify the full frame rate but the field rate (here: 50i / 60i fps).
In the best case, the refresh rate is an integral multiple of the reproduced frame rate , since otherwise the source cannot be reproduced completely smoothly. This fact also explains the frequent refresh rate of 144 Hz, which is a multiple of the two cinema standards 24 and 48 fps. The refresh rates of 120 Hz and 240 Hz, which are also widely used, also take into account compatibility with the widely used 60 fps standard.
It must also be taken into account that the advantages of a high reproduced 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.
In the interlaced display , as used in PAL or NTSC ( analog television ), the refresh 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. Most people still perceive the picture generated with picture tubes as flickering. In many more modern devices, the refresh rate is increased to 100 Hz with the help of an image memory .
Some display devices ( LCD , projectors ) are not refreshed with the frequency output by the graphics card , which requires an image memory in the device for frequency conversion. These devices can process different refresh rates in the input signal, but the best picture is achieved when the input frequency corresponds to the refresh rate, which is 60 Hz in almost all devices today. Techniques such as Nvidia's G-Sync or AMD's FreeSync solve this problem by not using a fixed refresh rate, but rather refreshing a displayed image only when the graphics card outputs a new image.
Usual refresh rates:
- MDA / Hercules : 50 full frames / second, monitors used were often photoluminescent
- CGA / EGA : 60 frames / second
- VGA : 60 or 70 full images / second, sometimes 56 full images / second
- IBM 8514 : 86 fields / second, 60 full frames / second
- Computer monitors ( tube ): 86 fields / second, 56 to 160 full images / second
- Computer monitors ( LCD ): usually 50, 60, 120, 144 or 240 full frames / second
- TV set ( tube ): 59.94 fields / second (NTSC), 50 fields / second (PAL)
- TV ( LCD ): 48 to 800 frames / second (even at high frequencies usually using intermediate image calculations in 3D TVs refresh rates for left and right eyes are added)
- 100 Hz technology
- Frame rate
- Flicker fusion frequency
- PAL acceleration
- Rainbow effect
- Vertical frequency
- Line frequency
- Hollywood's Most Powerful Nerd , March 27, 2012, Spiegel online
- Jerky cinema undesirable: Cameron wants more than 24 frames per second , January 21, 2010, heise online