7.1 How is the frame-rate difference worked out when film is displayed on television?
Description
This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
7.1 How is the frame-rate difference worked out when film is displayed on television?
7.1.1 European Television Standard
European television conforms to the PAL (Phase Alternation by Line)
standard, which runs at 25 frames (50 fields, or half-frames) per
second. This is close enough to the film standard of 24 fps, that 24
fps films are often simply run at 25 fps, with possibly a bit of
pitch-shifting on the soundtrack to make it sound less `screechy.'
Films shot for television broadcast are often shot at 25 fps, and many
cameras have an option of a 25 fps crystal, and tape recorders are
made with 50hz (rather than 60hz) crystals for syncing to 25 fps film.
Both PAL and SECAM (another television standard, used mostly in
Eastern Bloc nations) use 625 scan lines, running at 50 fields per
second. These standards are able to provide higher-quality images than
the U.S. standard described below.
7.1.2 U.S./Canada/Japan Television Standard
In the United States, Canada, and Japan, modern color television
conforms to the NTSC (National Television Standards Committee)
standards, which were devised in an attempt to make color television
signals compatible with black-and-white receivers. The standards
provide for a frame rate of 29.97 frames (59.94 fields) per second
(versus the film standard of 24 fps), and 525 scan lines. These scan
lines are `interlaced,' meaning that every other line (one `field') is
scanned once, and then the alternate lines are scanned in another
`field.' Thus 262.5 lines are scanned once, then another 262.5 line
are scanned. The two fields combine to form one `frame,' which is the
full set of 525 lines, and is analogous to a `frame' of film (although
there are more of them per second in television).
It should be noted that the original U.S. television standard for
black-and-white transmissions provided for 30 frames/60 fields per
second, but had to be revised to allow for color. When black-and-white
shows are broadcast by a color station, the TV station can either
broadcast at 30 fps, or broadcast a color burst signal at 29.97 fps.
In practice, though, this standard is now ignored.
Early broadcast setups were designed to simply repeat every fourth
film frame when a film was to be shown on television. This method
comes very close to showing the film at the proper speed (it makes the
film about 5% longer (with respect to running time) when it is shown
on television, because this method assumes that television runs at 30
fps, rather than the actual 29.97). This results in the following
frame relationships:
Television Film
Frames # Frame #
1 1
2 2
3 3
4 4
5 4
6 5
7 6
... ...
Modern film-broadcast setups work by making each film frame reproduce
alternately on two or three consecutive fields. This scheme provides
more-accurate representation of motion, and leaves fewer motion
`artifacts' of the film on the television display. This results in the
following frame relationships (with fields designated by half-frames).
Television Film
Frames # Frame #
1 1
1.5 1
2 1
2.5 2
3 2
3.5 3
4 3
4.5 3
5 4
5.5 4
6 5
6.5 5
7 5
... ...
Continue to:
My Books
