625-24 VIDEO is a variant of 525-24 and NTSC formats.
It is often used by Video Image for playback purposes. It can also be easily
be synchronized with motion picture cameras shooting at 24.022 FPS or 24.00
fps.
Due to the fact that many pieces of video equipment utilize
digital memory, not all are suitable for modification for use in 24 frame
video playback. Quite simply, 655-24 video requires more memory than normal
NTSC or PAL video systems. Because we must use modified existing standards
converters to convert our 525-30, NTSC and PAL video formats to 24 frame
video, we are limited to the amount of line memory that a particular converter
may have.
Certain models of Standards Converter employed at Video
Image have this limited line memory. The maximum number of lines these
converters can utilize or output is 584 lines. This is the maximum number
of lines that the PAL system can display. Since all Standards Converters
were originally designed to convert between the normal world television
standards of PAL, NTSC and SECAM, there was no reason to design them with
more memory than was necessary to accomplish their primary task. Therefore,
some standards converters, when modified for 24 frame video, will only
display the maximum number of lines they can hold in their limited memory.
PAL which is 625 lines (625-25) uses 584 active picture
lines. When modifying a standards converter that only had this amount of
memory, we get the 625-24 format. Just like the 525-24 format, the remaining
unused lines to make up the 24 fps image frame which is 615 lines, will
be black. This appears as an expanded vertical interval.
The 625-24 format is preferred over the 525-24 format as it provides us a higher filmable resolution. For this reason, the 655-24 format is superior to all other composite 24 frame formats.
625-24 video has the following characteristics:
INTERLACED SCAN - identical to NTSC.
FRAME RATE - 24.022 frames per second, which equals
48.045 fields per second. Even though the frame rate is not exactly 24
FPS on many occasions, we generally refer to 24.022 video as 24 frame video.
NUMBER of HORIZONTAL SCAN LINES - 655 lines, which
consists of 586 lines of image information, 28 lines of black split on
the top and bottom of the image area, and 41 lines serving the same vertical
interval functions as in NTSC.