PAL's 4% Speedup

    Have you ever watched a movie on TV or VHS or DVD and enjoyed the music so much that you went and bought the soundtrack CD? Have you then noticed that the soundtrack CD sounds a bit different to the movie?

    Have you ever watched a movie on TV or VHS or DVD and then found out that the US version is longer than the Australian version?

    Have you ever compared the specifications of a DVD in Australia with a DVD in the US and noticed that the US version runs longer than the Australian version?

    Are we being ripped off? Are our movies censored?

    The short answer is no. It is true that the running times of movies on TV, VHS and DVD are shorter than their theatrical running times, but this is not because of censorship. It is because of the way in which movies are transferred to video.

24 Frames Per Second

    Movies are projected at 24 frames per second. 24 full images are projected off the film onto the screen every second. This is a world wide standard. This is all well and good, but a problem arises when we want to transfer a movie to video.

50 Fields Per Second

    The PAL TV system shows images at 50 fields per second. One half of an image is displayed and then the other half of the image is displayed. The nett result of this is that 25 full images are shown per second.

24 Into 25 Doesn't Go

    How do you take a film which has 24 full images per second and display this on a TV which shows 25 full images per second? There are two ways you can do this.

    The first way, which is not commonly used, is to show 24 images from the film on the TV in sequence and then repeat the 24th film image. This fits film's 24 images into the 25 images needed by TV. This approach has one major drawback, which is that once every second there is a noticeable pause (judder) in the video because of the extra inserted image. Most people find such an artefact extremely objectionable.

    The second way, which is commonly used, is to show 25 images from the film every second. This fits the film nicely into the TV format of 25 full frames per second, but the nett result is that 25 frames from the film are being shown in the same time as 24 frames were supposed to have been shown. This means that the film is being shown 4% faster than it was originally intended to be shown. This approach also has a series of disadvantages, but these are less objectionable than the judder introduced with the first-mentioned approach.

Effects of the 4% Speedup

    The most obvious effect of this 4% speedup is that the film runs for 4% less time. Take as an example the recent movie The Mask of Zorro. The theatrical running time for this movie was 136 minutes according to the Internet Movie Database. The running time of this movie on Region 4 DVD is 132 minutes, 4% less.

    A less obvious effect of this 4% speedup is that the audio for the film is both 4% faster and 4% higher in pitch. In musical terms, this equates to a rise in pitch of a little under one semitone.

    Another less obvious effect of this 4% speedup is that the on-screen action occurs 4% faster.

    For the majority of us, this 4% speedup is of no consequence, and is something that we are blissfully unaware of. For a small minority of movie and music buffs, the 4% speedup is objectionable.

What Can Be Done About It?

    There is very little that can be done about this issue at present.

    One possible partial solution is for the movie's soundtrack to be digitally processed so that it plays back at the correct pitch on video. This can be done either whilst the movie is being mastered for video or whilst the movie is playing back. This is very rarely done in practice, as it does not solve the problem of the movie and the music still playing back 4% too fast, even if it is now at the correct audio pitch.

    The advent of DVD and digital projection devices has opened up another possibility to solve this problem. In theory, a DVD player could be constructed which plays back PAL movies at the correct speed and hence at the correct pitch. At present, the only devices capable of doing this are DVD-ROM drives. Such a device could be mated with a display device that is capable of displaying images at 24 frames per second. It is likely that in the future such solutions to this problem will become more readily accessible, particularly with the advent of HDTV.

Does It Really Matter?

    For most of us, the answer is no. However, it is important to be aware of this issue so as to more fully appreciate the movie watching experience, and to explain some apparent anomalies in movie running times that may have troubled you in the past.

© Michael Demtschyna
2nd November 1999