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All video assists are based either on PAL or NTSC video
systems, with a fixed video frequency of either 25 full video
frames per second with PAL or 29.97 full video frames per
second with NTSC. The speed of the film camera can how-
ever be selected over a wide range. At every film speed
which is different from the video frequency, the IVS has to
add repeated video fields to compensate for the different
frame rates.
In practice the most important case is a film camera speed
of 23.976 fps and a NTSC video assist.
This creates the situation described in the drawing.
From the first film frame two video fields are derived,
from the second film frame three video fields are derived,
from the next film frame two video fields, and so on.
The Pull-Down information is created as follows:
Every time the video field corresponds to a new film frame,
the letter will change either from A to B or B to A and the
number will be set to 1. As long as no new film frame is
taken, the video fields are counted, beginning from 1.
Consequently A2 is the first repetition of A1. B2 would be
the first repetition of B1, B3 would be the second repetition.
In spite of the fact that the name Pull-Down comes from the
working practice in the NTSC systems, where on a telecine
the film is running on 23.976 fps and gets converted to
29.97 fps, the definition of Pull-Down information on the
IVS can also be applied to PAL and to film speeds other
than 23.976 fps.
Like all man-readable information, the data is inserted as a
window on the monitor image. The window can be switched
on and off independently. Background, position and cha-
racter format can be altered without affecting the settings
of other windows.
Note: Pull-Down information is only inserted when
time code is actually recorded on film. If there is
no time code recording, for example because
the camera is not running on a time code
speed, only A1 will be displayed.
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