How do I speed up video ffmpeg?
Changes video and audio speed together, which needs two filters because one alone desynchronises the file.
Speed up or slow down a video
Changes video and audio speed together, which needs two filters because one alone desynchronises the file.
ffmpeg -i https://cdn.rendobar.com/assets/examples/sample.mp4 -filter_complex "[0:v]setpts=0.5*PTS[v];[0:a]atempo=2.0[a]" -map "[v]" -map "[a]" -t 5 out.mp4- Encode
- 681 ms
- Output
- 413.6 KB
- Cost
- $0.0022
- Result
- 1280x720
job_d3a1fc6095a549e3
The part everyone gets wrong
The two factors are inverses of each other. Doubling speed is setpts=0.5*PTS for video and atempo=2.0 for audio. Getting them the same way round is the classic bug. atempo also caps at 2.0 per instance, so 4x needs atempo=2.0,atempo=2.0.
What the common version does
setpts speeds up the video only, and copying the audio leaves it at the original speed, so the two drift apart immediately.
ffmpeg -i https://cdn.rendobar.com/assets/examples/sample.mp4 -vf setpts=0.5*PTS -c:a copy -t 5 out.mp4It succeeds, and that is the problem
It did not error. It produced 433.3 KB against the correct command's 413.6 KB, a 1.0x larger file. A pipeline would accept this silently.
job_df145596e5764e36
Frequently asked questions
What goes wrong most often?
The two factors are inverses of each other. Doubling speed is setpts=0.5*PTS for video and atempo=2.0 for audio. Getting them the same way round is the classic bug. atempo also caps at 2.0 per instance, so 4x needs atempo=2.0,atempo=2.0.
What happens if I get it wrong?
We ran the naive version too. It did not error. It produced 433.3 KB against the correct command's 413.6 KB, a 1.0x larger file. A pipeline would accept this silently.
Do I need FFmpeg installed?
Not with Rendobar. The command on this page ran through the API against a URL, with no local FFmpeg and no server. The job id below it is the job that produced these numbers.
More recipes, all measured
Trim a video without re-encoding Cuts a section out of a video by copying streams, which is effectively instant and lossless. Remove audio from a video Strips the audio track and copies the video untouched, so there is no quality loss and almost no work. Resize a video with FFmpeg Scales to a target width while computing a height that stays even, which H.264 requires. Rotate a video with FFmpeg Rotates the picture in 90-degree steps and clears the rotation metadata so players do not rotate it again.
Run this without installing FFmpeg
This exact command ran through the API against a URL.