[FFmpeg-user] Possible to speed up/down video while keeping audio pitch?

[Bo Berglund]

On Mon, 25 Mar 2024 01:47:27 +0100, Paul B Mahol <onemda at gmail.com> wrote:

>On Mon, Mar 25, 2024 at 12:54?AM Laine Lee <llee040 at sbcglobal.net> wrote:
>
>>
>>
>> > On Mar 24, 2024, at 5:31?PM, Bo Berglund <bo.berglund at gmail.com> wrote:
>> >
>> > ?I know how to use ffmpeg to adjust the video/audio sync and that is a
>> real
>> > simple command not requiring any remuxing or such. This is what I use in
>> my
>> > audiosync script and it runs very fast:
>> >
>> > ffmpeg -i $SOURCEFILE -itsoffset $DELAY -i $SOURCEFILE -map 1:v -map 0:a
>> -c copy
>> > $TARGETFILE
>> >
>> >
>> > But is it possible also to modify an mp4 video file to *play*
>> faster/slower than
>> > original while keeping the accompanying audio pitch the same?
>> >
>> > It *can* be done for instance in the VLC video player on playback, but
>> can one
>> > also modify the video file itself using ffmpeg so it will play at the
>> new speed
>> > while keeping audio recognizable (not Donald Duckie)?
>> >
>> > If so how is it done?
>> >
>> >
>> > --
>>
>> Sounds to me as though you want to change tempo of the audio stream. I
>> believe there’s a pipe from ffmpeg to Sox to handle it, but for that you
>> will extract audio, apply the tempo change, then remux. Changing tempo by a
>> factor greater than one reduces duration, and by a factor less than one
>> increases duration.
>>
>
>FFmpeg have tempo audio filter called atempo. No need to use obsolete SoX.
>
>

Thanks a bunch!

I found the syntax as an example at

http://trac.ffmpeg.org/wiki/How%20to%20speed%20up%20/%20slow%20down%20a%20video 

so I made a test.

Note: Newsreader is doing the line wrap, it is on a single line of course:

ffmpeg -hide_banner -i input.mp4 -filter_complex
"[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" output.mp4

It took 6 s to encode a 76 s mp4 video into a 38 s sped up video.

So it seems like one must apply the inverse factor on setpts and atempo, i.e.
change audio tempo by the inverse of the video speed.

I tested it with speedups of 2.0 and 1.25 and both worked OK.

So I will package this into a speedchange script among the other video utilities
scripts I have created.


-- 
Bo Berglund
Developer in Sweden