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 however 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
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
29.97fps, the definition of
Like all
Note: | |
| 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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