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Quite often i'll come across movies or television shows that were originally 24fps (as all movies, and most U.S. television shows are), but have been converted to 25fps for PAL playblack.

The conversion of 24fps source material to 25fps is pretty simple:

  • speed up the playback rate by 1.0466%

This has the effect of turning a 2h 20m movie into a 2h 14m 24s movie. It also has the undesirable side-effect of increasing the pitch by a full note. This gives everyone speaking a chimpmunk quality that i find objectionable.

i uploaded a sample PAL (25fps) video clip to youtube:

You can compare it to the original 24fps film version:

Warning: If you've never been able to hear the difference between original and PAL versions then don't start now; don't watch those clips.

Fundamentally i don't want to re-encode the video, incurring the penalty of another lossy compression. i simply want the player to be instructed to play the video slower. The progressive frames do not need to be re-compressed, and the audio doesn't need to be re-sampled.

                           25fps    24fps
                           =======  =======
Frames per second:         25       24
Display each frame:        40.0ms   41.6ms
Audio samples per second:  44,100   42,336

Is there a way to modify the "framerate" of an AVI so that it is tagged as being 24fps rather than 25fps, so that the player will:

  • play fewer video frames per second
  • play fewer audio samples per second

Although it's not a viable solution (because it suffers a video and audio recompress), i can get the required conversion using AVISynth:

Aliens - restored to 24fps.avs 

DirectShowSource("Aliens - 25fps.avi")
AssumeFPS(24, true)

Which works; except now i have to re-encode everything.

There is a Frame Rate Changer utility:

enter image description here

It does change the video playback rate, but the audio rate is unchanged. This causes the audio to no longer be in sync with the video.

It should also be pointed out that this exercise is only required for material that was originally 24fps (i.e. theatrical movie, television show), and then converted to 25fps for the PAL market, which is when i got my hands on it. Now i would like to restore it to the original 24fps.

Ian Boyd
  • 23,066

2 Answers2

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The simplest solution would be the Reclock. It won't change your files for you but it will undo the PAL speedup on playback.

Mr Alpha
  • 6,837
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after using AVI Framerate Changer (I use Virtualdub to retime the video without re-encoding) you only have to recompress the audio and mux it with the new video file. If the audio is mp3, you might be able to change it without recompressing - I know mp3gain can change the volume levels without recompressing it, so maybe there's a way to change playback speed.

guest
  • 16