Difference between revisions of "Compensate for PAL Speedup in Digital Video"

Revision as of 09:30, 14 October 2023

If you are not familiar with PAL Speedup then you should read about it to understand the problem and implications of how it adversely impacts digital video.

It is best to encode in the correct framerate. 25 FPS Telecine video mastered from film or NTSC source is bad. Delete it when you come across it. If the video you are looking for is remastered with 25 FPS Telecine, suffering from PAL Speedup, and no other video source is available, you can use one of various methods to compensate for the distortion during playback.

Play PAL movies at correct speed

VLC

• SOLUTION FOUND AND VERIFIED

This guide comes from Mac OS Hints on the Macworld web site and applies to VideoLAN Player on Windows, MAC, and Linux. The article in the Macworld forum is Play PAL movies at correct speed in VLC.

In Video LAN Player / VLC Player do the following:

1. Go to the 'VLC' menu and select 'Preferences…'
2. In the radio buttons in the lower-left corner of the window, change 'Basic' to 'All'.
3. Click on 'Input / Codecs' in the left-hand side list.
4. In the right-hand side frame look for 'Playback speed' and change it to 0.96.
5. Click on 'Audio' in the left-hand side list.
6. In the right-hand side frame disable 'Enable time stretching audio'.
7. Click the 'Save' button on the lower-right corner of the window.
8. Restart VLC.

Adjusting playback speed of PAL video to 0.96 in VLC player preferences to compensate for PAL speedup works very well during VLC playback, and using the record function during the playback creates a VLC video clip file which properly shows the added length of the clip due to the speed change. However, the speed adjustment is not retained in the new video clip created by VLC, therefore, this is not a good method of remastering the video (don't waste your time.) Just use this method for viewing the PAL video.

• Make sure to set the Playback speed to 1.00 when watching NTSC encoded movies!

command line

vlc -f videofilename.avi --no-audio-time-stretch --rate=0.96

mplayer

• PARTIAL SOLUTION

To just slow it down 4%

mplayer dvd://1 -speed 96/100

Keep tempo, change pitch. Use [ ] keys to change pitch

mplayer file.mp3 -af scaletempo=scale=1.0:speed=pitch

• the temp adjustment necessary or if necessary is unknown at this time!

smplayer

Now smplayer might be using either mplayer or mpv as the backend video player...

mpv

• SOLUTION FOUND AND VERIFIED

working with something like this....

mpv --af=scaletempo --speed=.96 videofilename.avi

FFmpeg

Goal of slowing down the video to the desired frame rate of 23.976 fps.

Using ReClock to correct for PAL Speedup

The original purpose of Reclock, which is a Directshow Filter, is to allow for smooth playback on a PC video monitor for video from other sources. It turns out that ReClock is also excellent for reversing the PAL Speedup effect. It will also allow you to reverse PAL Speedup. Reclock is described as a DirectShow filter which is loaded in place of the default directsound audio renderer.

Older versions of ReClock do not include this very useful feature. Version 1.8.8.4 will include the feature. ReClock is only for Microsoft Windows. ReClock will not only stretch the video to the correct length, it will also fix the audio pitch problem associated with PAL Speedup. ReClock is also known by the full name: ReClock DirectShow Filter.

Discussion of reversing PAL speedup

I. You can use command line parameters to force a workable video player to compensate for PAL speedup, thus restoring the proper audio pitch and playtime duration.

VLC Player aka VideoLAN Player will easily accomplish this and has been able to for many versions to present. Example:

vlc -f videofilename.avi --no-audio-time-stretch --rate=0.96 --no-embedded-video --no-sub-autodetect-file vlc://quit 

If you install zenity on your linux system you can use the following shell script called [altplayer] which will give you a dialog and allow you to choose NTSC or PAL when launching VLC to play a video.

See Also: Compensate for PAL Speedup in Digital Video.

VLC can be made to correct for the speed and audio pitch problems introduced by crappy PAL video in multiple ways besides using command line parameters.

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.

Here is a command line switch example using VLC:

"C:\Program Files\VideoLAN\vlc.exe" "c:\palvideo.wmv" --no-audio-time-stretch --rate=0.96

Question: Can a flag be set in the video file such as a tag in an mkv file to instruct a video player that it shall apply PAL speedup compensation?

Answer: Theoretically this is very possible. As to practical application it is unknown if any current video players or flag/tag standard exists to accomplish this. The digital video community has mostly tended to ignore this problem although something of the sort might exist by the time you read this.

II. You can re-encode or convert PAL speedup videos back to NTSC with the correct audio pitch and playtime.

There are different tools to accomplish this. There are also some side effects such as the introduction of slight jerkiness in video from things like duplicated frames depending on the process you use. The sample rate of the audio needs to be changed so the audio is stretched to the correct playtime. Then the video has to be stretched (which may involve the replication of frames) to match the audio runtime.

Stretching the audio so that the audio tract is at the correct pitch and total length in runtime involves the formula calculated by ((24000/1001) / 25) * 48000. Then the video can be stretched which involves extracting the video stream then a mux to put it back with the corrected audio stream. (1). Separate audio and video into 2 files. (2). process audio to correct pitch and runtime (3). process video to correct play length (4). mux correct audio and video into new file.

For the audio track the process is lossy. For the video track this depends on the software and method you use which may either introduce a slightly noticeable jerkiness in the video or some loss in quality.

From a video discussion forum a bash/shell script that uses ffmpeg:

ffmpeg -loglevel info -y -i "$file" -map 0 -c copy -c:a ac3 -filter:a aresample=resampler=soxr,asetrate=46033.92 -ar 48000 tmp.mkv
mkvmerge -o "$outfile" --default-duration 0:23.976fps tmp.mkv

In this person's example the change in video speed is lossless, only the metadata gets updated.

Here is another example:

ffmpeg -i input.mkv -filter_complex "[0:V:0]setpts=25/24*PTS[vout];[0:a:0]asetrate=46080,aresample=resampler=soxr:osr=48k[aout]" -map "[vout]" -map "[aout]" -r:v 24/1 -vsync vfr output.mkv

A new problem is introduced when you have subtitles. If the subtitle track has already been timestamped to match the PAL video, the subtitles will lose synchronization with the newly corrected video. This is especially troublesome with Forced Subtitles that are in their own track, like an srt embedded into an mkv file. If the track is forced subtitles you might find that editing the timestamps in the file by hand with a text editor your only remedy. This is less practical with a full line by line subtitle track. Additional software is required or a clever bash script involving regex.

III. You can adjust the audio pitch and ignore the faster playtime (not recommended).

The audio file is not stretched, but processed so that the pitch is decreased to match the original pitch of the sound. However, speech and audio will still be delivered faster than the original. Imagine the actor being perceived as delivering a line faster than the original and yet not sounding higher pitched, just seeming to talk faster. Human perception is not likely to notice the subtle time shift as long as the actor does not seem higher pitch in sound.