Difference between revisions of "Video Playback Slightly Too Fast"

Revision as of 16:22, 15 March 2013

The United States, like the rest of North America, use a video frame rate standard called NTSC. NTSC has a frame rate of 24 frames per second, of 24fps. PAL has a frame rate of 25fps. This relates to the electrical alternating current frequencies native to these regions. The U.S. is 60Hz while Europe is mostly 50Hz.

In NTSC television the rate is effectively slowed to 23.976 FPS (24×1000÷1001 to be exact), and when transferred to PAL or SECAM it is sped up to 25 FPS.

The difference in frame rate is causing problems in the digital video world. People in the NTSC world are finding videos that seem fine at first, but are subtly playing slightly too fast, and the pitch of all sound, including everyone's voice is slightly too high.

This issue has been called "PAL Speedup." Pal Speedup describes the increase of 4.2% in pitch and speed that results from the conversion of film (or NTSC-TV) to the european PAL format. This effect will cause the PAL version of a 2 hour movie will play in 1 hour 55 minutes.

Best described by a user on AfterDawn forums: "Anyone notice this problem? It seems to be with some xvid or divx videos I've downloaded. The speed difference is so slight that you really can't tell until you watch the original and hear that the theme song is in a lower key and the voices are lower."

source: AfterDawn > Forums > Video playback problems > video plays slightly too fast

Files playing at 25fps but were filmed at 24fps will cause this problem. For European PAL TVs the film is simply sped up the 4.16%. The sound is also sped up and is a half-step too high. People are not encoding the files properly, and the difference is just too slight to notice.

Judging by the running times, I would guess the source was probably 24fps film. The 25 fps AVI probably came from a PAL DVD where the film is sped up to 25 fps. The 25 fps AVI retained that frame rate and probably has smooth, though slightly fast playback (compared to the original film).

Film can be shot at either 24fps or 25fps for PAL. There are two ways to transfer the film to tape.

Telecine A: 25 FPS frame-for-frame from the negative or workprint. Either film that originated at 24 FPS or 25 FPS can be transferred this way. If the film was shot at 25 FPS, there is no modification in the playrate. If the film was shot at 24 FPS, it is being played 4.166% faster.

Telecine B: 24 FPS adding a field at the 12th and 24th film frame (24 FPS film is cine-expanded to 25 frames). Film shot at 24 FPS is played at that rate with an additional frame generated in the telecine process so one second of film still plays at that duration on the PAL tape.

A lot of bad video is being distributed by people who do not know how to properly encode. It goes unnoticed since the speed difference is so slight. However, it is bad quality video. People need to wise up and become aware of it.

Play PAL movies at correct speed in VLC

Have you been annoyed by PAL-encoded movies playing a little faster than usual, where the music's tempo is wrong and everyone talks a little faster?

If you have VLC, here's how to fix it:

   Open VLC.
   Go to the 'VLC' menu and select 'Preferences…'
   In the radio buttons in the lower-left corner of the window, change 'Basic' to 'All'.
   Click on 'Input / Codecs' in the left-hand side list.
   In the right-hand side frame look for 'Playback speed' and change it to 0.96 (use a comma if your system uses that as the decimal separator).
   Click on 'Audio' in the left-hand side list.
   In the right-hand side frame disable 'Enable time stretching audio'.
   Click the 'Save' button on the lower-right corner of the window.
   Quit and relaunch VLC.

The NTSC standard is 29.97 fps, and PAL is 25 fps. And if this hint is talking about what I assume it is, not all PAL videos are affected, and it has nothing directly to do with VLC (although the workaround here should work, in principle). The problem is that theatrical movies are normally filmed at 24 fps, not 25 or 30. A process called 3:2 pulldown (too lengthy to go into the details here) converts such movies to the nearly 30 fps rate required on NTSC systems, and the playback speed ends up very close to the original. On PAL systems, the common approach is to simply speed up the movie from 24 to 25 fps, shortening the playback time. Sometimes the soundtrack is processed to prevent the sound from raising in pitch, but that varies with the movie.

AGKnot request: Now how bout those pesky PAL DVDs

Convering FILM to PAL without 4% speed-up (frame/field blending?)

24fps NTSC films are sped up to 25fps when converted to PAL. The audio is sped up with the video so there will be a change in pitch. The speed difference is 4% which results in the PAL disc having a shorter duration, but all frames are still there.

Reclock

"For reference, fixing this nonsense is easily accomplished on playback (on a PC anyway) by using ReClock."

(using reclock is an ugly patch, not a good solution)

The following is an excellent discussion on the topic:

• How to convert x264 HD 25fps to 23.976fps

Fast dirty method with VirtualDub:

1. File -> Open AVI File.
2. Video -> Frame Rate... in Frame Rate Conversion section enable Convert To FPS and set 23.976 or 25.
3. Video -> Compression... select compression codec and settings (Xvid, target quantizer 3?)
4. File -> Save as AVI.

The result will be a little jerky.

source: Forum Video Video Conversion Changing AVI from 29fps to 25fps