Frame rate

Frames per second
A measure of the number of frames displayed per second in a video.

Frame rate (expressed in frames per second or fps) is the frequency (rate) at which consecutive images called frames appear on a display. The term applies equally to film and video cameras, computer graphics, and motion capture systems. Frame rate may also be called the frame frequency, and be expressed in hertz.

Due to the mains frequency of electric grids, analog television broadcast was developed with frame rates of 50 Hz (most of the world) or 60 Hz (US, Japan, South Korea). Hydroelectric generators, due to their massive size, developed enough centrifugal force to make the power mains frequency extremely stable, so circuits were developed for television cameras to lock onto that frequency as their primary reference.

The introduction of Color Television technology made it necessary to lower that 60 fps frequency by .1% to avoid "dot crawl", an annoying display artifact appearing on legacy black-and-white displays, showing up on highly-color-saturated surfaces. It was found that by lowering the frame rate by .1%, that undesirable effect was highly minimized.

Today's North America, Japan, and South Korea's video transmission standards are still based on 60÷1.001 or ≈59.94 images per second. Two sizes of images are typically used: 1920x540 (1080i) and 1280x720 (720p); Confusingly, interlaced formats are customarily stated at 1/2 their image rate, 29.97 fps, and double their image height, but these statements are purely custom; in each format, 60 images per second are produced. 1080i produces 59.94 1920x540 images, each squashed to half-height in the photographic process, and stretched back to fill the screen on playback in a television set. The 720p format produces 59.94 1280x720 images, not squeezed, so that no expansion or squeezing of the image is necessary. This confusion was industry-wide in the early days of digital video software, with much software being written incorrectly -- the coders believing that only 29.97 images were expected each second, which was incorrect. While it was true that each picture element was polled and sent only 29.97 times per second, the pixel location immediately below that one was polled 1/60th of a second later -- part of a completely separate image for the next 1/60 second frame.

Film, at its native 24fps rate could not be displayed without the necessary pulldown process, often leading to "judder": To convert 24 frames per second into 60 frames per second, every odd frame is repeated, playing twice; Every even frame is tripled. This creates uneven motion, appearing stroboscopic. Other conversions have similar uneven frame doubling. Newer video standards support 120, 240, or 300 frames per second, so frames can be evenly multiplied for common frame rates such as 24 fps film and 30 fps video, as well as 25 and 50 fps video in the case of 300 fps displays. These standards also support video that's natively in higher frame rates, and video with interpolated frames between its native frames. Some modern films are experimenting with frame rates higher than 24 fps, such as 48 and 60 fps.

Frame rate in electronic camera specifications may refer to the maximum possible number of frames per second, where, in practice, other settings (such as exposure time) may reduce the frequency to a lower number.

glossary_ FPS - 400
  • fps
also known as
  • Frames per second
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