[FFmpeg-user] How to slow down a video

[Michael Koch]

Am 05.05.2020 um 18:27 schrieb Ulf Zibis:
>
> Am 05.05.20 um 15:07 schrieb Edward Park:
>> Hi,
>>
>>> But I'm lost how use it correctly. I want to slow down the video by 
>>> 1.4 and then increase the audio frequency (pitch) by 1.25.
>>
>> So if you slow the video down by 1.4, You are also slowing down the 
>> audio by the same amount right? I think 1/1.4, and to add to the 
>> pitch to 1.25 ratio you'd have to multiply by 1.75, using filters to 
>> change frequency/pitch, librubberband has a simple pitch ratio factor 
>> you can multiply to whole spectrum. Or you could resample to your 
>> output format rate * 0.8 (1/1.25) then asetrate to the output sample 
>> rate after strectching/shrinking the audio to be 1.25 times the video 
>> length I think it will have a similar effect.
>>
>> For example, slowing the video down could be -vf setpts=1.4*PTS, and 
>> for audio -af atempo=1/1.25/1.4,aresample=36000,asetrate=48000
>>
>> Regards,
>> Ted Park
>
> Swapping aresample with asetrate is an interesting aproach. It seems 
> to have very little better quality.
>
> Aside I have a question: Where can I find a list of properties, which 
> can be used as parameter in commands. E.g. I can use "PTS" for current 
> picture time stamp and "h" for the heigth.

Either in the documentation, or in the built-in help function (which 
does sometimes contain more details).
ffmpeg -h filter=asetrate

> In my case interesting: What is the parameter for the audio rate to 
> use with asetrate=RATE*1.25 ?
>

I don't think such a variable exists for asetrate filter. Or it's 
undocumented.

Michael