For what it's worth, here on Ubuntu 21.04 :

ldd /usr/bin/ffmpeg |igrep rubber:
librubberband.so.2 => /lib/x86_64-linux-gnu/librubberband.so.2

dpkg -l ffmpeg:
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version                  Architecture Description
+++-==============-========================-============-================================================================
ii  ffmpeg         7:4.3.2-0+deb11u1ubuntu1 amd64        Tools for transcoding, streaming and playing of multimedia files



On Sat, Jun 19, 2021 at 4:28 PM Andrew Randrianasulu via Cin <cin@lists.cinelerra-gg.org> wrote:


On Saturday, June 19, 2021, Andrew Randrianasulu <randrianasulu@gmail.com> wrote:


On Saturday, June 19, 2021, Phyllis Smith via Cin <cin@lists.cinelerra-gg.org> wrote:
Gorge,
About crashing:

That's odd, it's a tweaked version
Not the main.cpp program but the F_atempo plugin inside of Cinelerra.  Andrea is much more expert at testing the
ffmpeg plugins and it did not crash for him as stated in his email.  I did get the plugin to not crash but I do not know
what it is doing. 


random internet search says:

The parameter for atempo must be between 0.5 and 2.0. If you want to slow it down even more, you can use the atempo filter more than once.
Eg. to slow it down by 4:
ffmpeg -i input.mp4 -filter:a "atempo=0.5,atempo=0.5" -vn output.aac


https://muzso.hu/2015/04/25/how-to-speed-up-slow-down-an-audio-stream-with-ffmpeg


more random internet search unearth those finds:


https://patchwork.ffmpeg.org/project/ffmpeg/patch/20171202191342.25358-1-onemda@gmail.com/#21029

adds scaletempo filter, probably not in mainstream due to discussion about how useful or not this new filter might be. 

for rubberband apparently you need librubberband installed and pass configuration flag to ffmpeg (cingg's build system allow this) 
----
Apply time-stretching and pitch-shifting with librubberband.

To enable compilation of this filter, you need to configure FFmpeg with --enable-librubberband.
----
 


 

When it runs , a terminal comes up to show you the technical info.  But the outfile is created.
I am still trying to compile the program because I have to put the     include <libavcodec/avcodec.h>  in the right place.
Then I will test it.  ...Phyllis
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