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, or frame frequency, is the
measurement of the frequency (rate) 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.
Frame rate is most often expressed in
frames per second (FPS) and in progressive-scan monitors as hertz (Hz).
Frame rates in film and television
There are three main frame rate
standards in the TV and movie-making business.
- 60i (actually 59.94, or 60 x 1000/1001 to be more precise;
60 interlaced fields = 29.97 frames) is the standard video field rate
per second that has been used for NTSC television since 1941, whether
from a broadcast signal, rented DVD, or home camcorder. (When NTSC
color was introduced, the older rate of 60 fields per second was
reduced by a factor of 1000/1001 to avoid interference between the
chroma subcarrier and the broadcast sound carrier.)
- 50i (50 interlaced fields = 25 frames) is the standard
video field rate per second for PAL and SECAM television.
- 30p, or 30-frame progressive, is a noninterlaced format and
produces video at 30 frames per second. Progressive (noninterlaced)
scanning mimics a film camera's frame-by-frame image capture and gives
clarity for high speed subjects and a cinematic-like appearance.
Shooting in 30p mode offers video with no interlace artifacts. The
widescreen film process Todd-AO used this frame rate in
1954–1956. For video, this frame rate originated in the 1940s
for recording current events or recording shows that were not shot in
24p film as well as TV commercials. However, this frame rate became
popular in the 1980s, with the popularity of music videos.
- The 24p frame rate is also a noninterlaced format, and is
now widely adopted by those planning on transferring a video signal to
film. But film- and video-makers turn to 24p for the "cine"-look even
if their productions are not going to be transferred to film, simply
because of the "look" of the frame rate. When transferred to NTSC
television, the rate is effectively slowed to 23.976 frame/s, and when
transferred to PAL or SECAM it is sped up to 25 frame/s. 35 mm movie
cameras use a standard exposure rate of 24 frames per second, though
many cameras offer rates of 23.976 frame/s for NTSC television and 25
frame/s for PAL/SECAM. The 24 frame/s rate became the de facto standard
for sound motion pictures in the mid-1920s.
- 25p is a video format which runs twenty-five progressive
frames per second. This framerate is derived from the PAL television
standard of 50i (or 50 interlaced fields per second). While 25p
captures only half the motion that normal 50i PAL registers, it yields
a higher vertical resolution on moving subjects. It is also better
suited to progressive-scan output (e.g. on LCD displays, computer
monitors and projectors) because the interlacing is absent. Like 24p,
25p is often used to achieve "cine"-look.
- 50p and 60p is a progressive format used in high-end HDTV
systems. While it is not technically part of the ATSC or DVB broadcast
standards, it is rapidly gaining ground in the areas of set-top boxes
and video recordings.
- 72p is currently an experimental progressive scan format.
Major institutions such as Snell & Wilcox have demonstrated
720p72 pictures as a result of earlier analogue experiments, where 768
line television at 75Hz looked subjectively better than 1150 line 50Hz
progressive pictures with higher shutter speeds available (and a
corresponding lower data rate). Modern TV cameras such as the Red, can
use this frame rate for creative effects such as slow motion (replaying
at 24 fps). 72fps was also the maximum frame rate that gave the most
emotional impact to the viewer as measured by Douglas Trumbull that led
to the Showscan film format. Windows .WMV file format has a maximum
frame rate of 72p.
Even higher frame rates (~300Hz) have
been tested by BBC R&D from concerns over sports and other
broadcasts where fast motion with large HD displays could have an
effect with viewers. 300 fps can be converted to both 50 and
60Hz transmission formats without major issues.
Frame rate is also a
term used in real-time computer graphics systems. In a fashion somewhat
comparable to the moving-picture definition presented above, a
real-time frame is the time it takes to complete a full round of the
system's processing tasks. If the frame rate of a real-time system is
60 Hertz, the system reevaluates all necessary inputs and updates the
necessary outputs 60 times per second under all circumstances.
The designed frame rates of real-time systems vary depending on the
equipment. For a real-time system that is steering an oil tanker, a
frame rate of 1 Hz may be sufficient, while a rate of even 100 Hz may
not be adequate for steering a guided missile. The designer must choose
a frame rate appropriate to the application's requirements.
The article is based on materials from matroska.org, wikipedia.org.