DVD Demystified

DVD Technology Primer
Figure 3.25
Converting film to
video
137
actually plays at 59.94 Hz, so the film runs 0.1 percent slow and the audio
must be adjusted to match. Displaying film at PAL rates of 50 video fields
per second is simpler and usually is achieved by showing each film frame as
two fields and playing it 4 percent faster. 27 This is sometimes called 2—2
pulldown.
Most video is encoded from videotape. The videotape is created by a
telecine machine, which performs 2—3 pulldown when making an NTSC
tape. Since it would be inefficient to encode the extra fields, they are not
duplicated in the MPEG-2 stream. A good encoder recognizes and
removes the duplicate fields. This is called inverse telecine (no, it is not
27 Since the video is sped up 4 percent when played, the audio must be adjusted before it is
encoded. In many cases the audio speedup causes a semitone pitch shift that the average viewer
will not notice. A better solution is to digitally shift the pitch back to the proper level during the
speedup process.