We begin with "FILM" in the center of the diagram (fig. 2). Film material is shot in 24 fps. A film frame is nominally displayed for 1/24 s ≈ 41.67 ms. One second of the film contains 24 frames and will be disassembled ("telecined") into 48 fields.
For the transfer onto PAL the film is simply accelerated. PAL needs 50 fields/s. Two fields (: n1 and n2) of the 2nd second of the film are still "pressed" into the first second - marked-off with the red dotted line - of the PAL video. Thus the so-called PAL Speedup emerges. PAL films are played 25/24 ≈ 4,167 % faster than the original film in the cinema. We can also say that the PAL length is 24/25 times longer compared to the original film. That means that the PAL length is 4 % shorter. Due to the acceleration of the film some people notice that the tone pitch is increased with PAL films. Two fields are displayed within 1/25 s = 40 ms.
50 fields/s are needed for an interlaced display and these fields will be shown in the order a1, a2, b1, b2, etc. Concerning a progressive display the fields will be added up first and then displayed in 25 fps (a1+a2, b1+b2, c1+c2, etc.). In the case of a progressive display with 50 fps each field is converted into one frame by adding missing lines of each field due to special deinterlacing. Thus a1, a2, b1, b2, etc. become A1, A2, B1, B2, etc.
The transfer from film to NTSC is a more complicated matter. In the chart (fig. 1) we recognize that the "x2 field" already overlaps the first second (: x2). The NTSC refresh rate is 29.97 fps corresponding to 59.94 fields/s. We note that there are not 60 but 59.94 fields per second transferred.
The first two fields (a1, a2) are displayed in 1/29.97 s ≈ 33.37 ms. The three following fields are displayed in 1/19.98 s ≈ 50.05 ms. Together the result for the first five fields is approx. 83.42 ms which represents a deceleration compared to the first two film frames (2*41.67 ms = 83.34 ms). The film frames A and B are displayed approx. 0.08 ms longer in the NTSC television.
The following section "telecining" describes the achievement of 59.94 fields/s from 24 fps film material.
