How to change playback speed & duration (Film>PAL)

[bergerado]

Problem: I have several videos which play at too low speed, approximately by 4%. The assumption is these were encoded from NTSC sources. Although their FPS are set to PAL speed in most cases (i.e. between 23-25 fps) the encoder apparently did not change the playback duration, thus leading to longer videos. (For example, a movie that should originally be 100 minutes long will play for 104 minutes instead...)

Now, Avidemux has an option to re-encode audio tracks from "Film to PAL" (and vice versa), which delivered good test results: Extracted audio tracks were playing at the correct speed (eg. 100 minutes instead of 104...).

Question: In order to get the same effect for the video images, can I re-encode the visual track from "Film to PAL" so that video and audio are in sync, resulting in the same duration? Is there any setting under "Video > Framerate" or otherwise to achieve this?

Many thanks for your helpful insights in advance!!

[bernd_b]

There are two video filters to change the frame rate:

One keeps duration - this one you shouldn't use for your purpose.
The other one changes frame rate and(!) duration - this is the filter you have to use.

What troubles me:
Most of the stuff is originally ntsc/film. If the video is PAL already, the speed up should already be done - so the PAL version (=25fps) should be shorter than the movie/ntsc - versions.

By the way: PAL is not between 23-25fps, it is 25fps. 23fps is not common at all. There is
23.976fps (ntsc film)
25 fps PAL

So the sources you are describing seem really strange to me ...

[Jan Gruuthuse]

as bernd_b says
Film 23.976 fps = cinematic FPS and not pal or NTSC fps (some dvd's and mostly found on blu-ray)
Use Change FPS see if that works.
or resample FPS
Probably you have to test for best result.

[bergerado]

There are two video filters to change the frame rate:

One keeps duration - this one you shouldn't use for your purpose.
The other one changes frame rate and(!) duration - this is the filter you have to use.

Thank you so much for this super quick reply, but where do I find this filter? I've already checked the whole list of video filters, and found the one to change FPS ("but keep duration")... Where's the other one I need? Do I need to install that externally perhaps?

Regarding the sources, I was only assuming. Not knowing their exact formats, or what issues might arise, I used to encode by keeping the original FPS of the source... (resulting in what you specified more accurately as 23.976 or 25 FPS).

[Jan Gruuthuse]

with loaded video and video output set to something else then copy:
Filters: Transform: Change FPS (position 4 )
click green cross: pop-up window where you can select in source and destination fps:
- Custom
- 25 (PAL)
- 23.976 (Film)
- 29.97 (NTSC)
- 50 (PAL)
- 59.93 (NTSC)

Further down you should find Resample FPS (position 8 )
in mode (source fps) you can select:
- Custom
- 25 (PAL)
- 23.976 (Film)
- 29.97 (NTSC)
- 50 (PAL)
- 59.93 (NTSC)
in New frame rate the wanted fps

[bergerado]

Thanks a lot Jan, finally found the filter (after installing the latest version, 2.6 ;-)

Now, with my video file being 25 FPS, in order to obtain the original speed & duration I guess I need to resample it back to 23,976. (Should make sense, assuming the mistake was to encode a 23,976 source at/to 25 FPS, resulting in a stretched video...)

And what about the audio track(s), will that be automatically adjusted so I can just use "copy" mode for  that? Or will that create a video where the visual plays for 100, but the audio track for 104 minutes?
My first idea was to do as I had tested, which was using the audio filter option "Film to PAL"...
Has anyone ever tried this before?

[bergerado]

Further down you should find Resample FPS (position 8 )
in mode (source fps) you can select:
- Custom
- 25 (PAL)
- 23.976 (Film)
- 29.97 (NTSC)
- 50 (PAL)
- 59.93 (NTSC)
in New frame rate the wanted fps

Sorry, but what exactly does "Resample FPS" do, sure that I really need both filters?
As said, I need to ensure my audio track be adjusted to the new 23,976 video frame rate... is that what it's for?

Please help, I'd like to be sure of what I'm doing before starting a re-encoding process of who knows how many hours...

Many thanks again!!

[Jan Gruuthuse]

No only one or the other. Mark more or less 1 minute section with some speech. See what works best for you.

[bergerado]

OK, thanks for making this clear! In the meantime I have tried to re-encode using the following settings:

Video stream: "Mpeg4 (AVC)" (same as source format), plus video filter "Change FPS" set to the original 25fps of my video as "source", and 23,976 as "destination" FPS.
Audio stream: "AC3 (Aften)" (same as source), plus filter "Film to PAL" (assuming this would correct the source FPS of 25 back to 23,976 and make the video run slightly faster/ at original speed).

Unfortunately, the result was opposite to what I wanted - (after 5 hrs encoding time,) video duration now is ca. 5 minutes longer than the original, and video/audio streams completely out of sync...

Will now try setting the video filter vice versa: 23,976fps for source and 25fps for target frame rate. Can you recommend whether the audio filter "Film to PAL" should be used as before or not? (Remember, extracting one audio stream using this filter setting had produced the desired of 100 mins runtime.)

[bernd_b]

You can read before or try out yourself 

As told above: Mark a short passage in your video with the help of the A/B marker to reencode only a short test clip. This saves you hours of time while testing. If it works as you expected, then process the whole video/audio.

And again Number 2: Changing ntsc/film to PAL shortens(!) the video, it does not(!) stretch it. The same number of frames is kept, but more frames per second are to be shown. This is while the resulting video and audio is shortened after this process. Therefore it is called PAL speed up(!).

[bergerado]

Right, shortening is what I aim for! And indeed, a test run with the reverse setting for the video filter did the trick:
"Resample from 23,98 to 25 FPS" (instead of 25 to 23,98)! The audio stream was filtered as before using "Film to PAL" - and the result is perfect, both in terms of V/A runtime and sync!

Still irritating though, as "MediaInfo" reads the source video as 25fps (but perhaps this information is wrong so has in fact 23,9...)

Thank you!

[bergerado]

Add. for reference only:
As I've just discovered (thread http://www.avidemux.org/smf/index.php/topic,10183.0.html), version 2.6 is the only one to date capable of correcting video runtimes or a/v sync (obviously without removing/adding frames ;-)!
The reason, and vital development news, is that 2.6. analyses video sources based on their inherent time stamps, if present, while earlier versions used frame numbers.