Feature Request: Add a timecompressor as postprocessing option

[fbuchinger]

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A common usage of youtube-dl is to download podcasts or instructional youtube videos for offline usage/archival. The people who listen to educative podcasts want to minimize their learning time, so they often increase the playback speed of the podcast mp3's or videos. On the other hand, they also want to save storage space.

Why can't we combine these two requirements in one feature? Youtube-dl could download the video and then re-encode it using ffmpeg at an increased framerate. With this "baked-in" playback speed increase, we also save storage space.

Example invocation:
youtube-dl https://www.youtube.com/watch?v=1n9KMqssn54 --playback-speed=2.0

As a proof-of-concept, I tried to reencoded the mentioned video to 2.0 playback speed using a manual ffmpeg call:

> youtube-dl https://www.youtube.com/watch?v=1n9KMqssn54
> dir *.mp4
[..]
02.03.2018  11:10       295.366.625 Using PyPy instead of Python for speed by Niklas Bivald-1n9KMqssn54.mp4
> ffmpeg -i ...1n9KMqssn54.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" timecompressed.mp4
> dir *.mp4
24.05.2018  22:08       133.155.279 timecompressed.mp4
02.03.2018  11:10       295.366.625 Using PyPy instead of Python for speed by Niklas Bivald-1n9KMqssn54.mp4

My PoC shows that the timecompressed version of the video saves about 56% storage space and about 12 minutes listening time (original duration ~24 minutes). I have to admit that the timecompressed version is not perfectly understandable and took a while to encode, but I haven't tweaked the settings yet and I am using a pretty old machine.

According to a recent lifehacker article, playback speeds from 1.4 to 1.7 are pretty common today: https://lifehacker.com/speed-up-your-podcasts-for-more-efficient-listening-1797029702

What do you think?

[fbuchinger]

Why is this a duplicate of #15840? My proposal is about resampling the video for increased playback speed, #15840 is about slicing a video at a given start/ end time.