2Choose a multiplier — 0.5x for half speed, 2x for double — staying roughly inside 0.75x to 2x if the content is speech.
3Run it; the time-stretch preserves pitch, so voices stay natural instead of transposing upward.
4Download the retimed FLV file with picture and sound still in sync.
Change FLV Speed FAQ
How does Change FLV Speed change the playback rate?
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Concretely, the presentation timestamps are rescaled and the audio is time-stretched to match with pitch preserved, so picture and sound stay in sync; large speed-ups also drop redundant frames rather than just playing them faster. Choose a multiplier — 0.5x for half speed, 2x for double — and the whole file is retimed.
Will speeding it up make voices sound high-pitched?
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No. The time-stretch preserves pitch, so a 1.5x speech track sounds like the same person talking faster rather than like a chipmunk. That is the difference between a proper time-stretch and simply playing the samples faster.
Does the source format affect the result?
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It does. FLV is the retired Flash container — nothing plays it natively any more, which is normally the reason it is being converted. That determines whether the retimed result can be written back cheaply or has to be fully re-encoded.
Anything specific to FLV here?
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Yes — nothing plays FLV natively any more, so the practical end of any FLV job is a modern container. Worth knowing before you commit to a large speed change.
How far can I push the speed before it sounds wrong?
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Roughly 0.75x to 2x is transparent for speech. Beyond about 2.5x the stretch artefacts become audible on any algorithm, and below 0.5x the reconstruction has to invent too much. Music tolerates far less than speech does.
Do picture and sound stay in sync?
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Yes. Both are retimed by the same factor in one operation, so they cannot drift apart — which is the usual failure when speed is changed by editing the two streams separately.
What can I upload to Change FLV Speed?
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MP4, MOV, MKV, WebM, AVI, WMV, FLV, 3GP, M4V, MPEG and MPG. The container and codec are probed before anything runs, so a mixed batch is handled correctly per file rather than assumed to be uniform.
Is there a file size limit on Change FLV Speed?
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Yes: free accounts process video up to 1 GiB per file, by far the most generous cap on the site; ffmpeg with x264, x265 and libvpx does the work, which is the same toolchain behind most desktop video software. Video is the one family where the cap is high enough that upload time, not the limit, is what you will actually notice.
Will Change FLV Speed lower the quality of my FLV videos?
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It depends on whether the job needs a re-encode. Operations that only touch the container or the timeline are done as a stream copy and the picture is bit-identical to the source. Anything that has to redraw frames goes through x264/x265 at a quality level you set, where CRF 23 is visually transparent on most footage.
Can I run Change FLV Speed on several FLV videos at once?
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Yes. Uploads queue in parallel, which matters more here than anywhere else because video jobs are minutes rather than seconds. Free accounts run one file at a time; a pass lifts that.
What happens to audio, subtitle and chapter tracks?
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They are carried across. Multi-track audio, embedded subtitle tracks and chapter markers survive wherever the target container can represent them — that is the main thing that separates a proper job from a naive re-encode that silently keeps only track one.
Does Change FLV Speed cost anything?
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No. Change FLV Speed is free without an account, and the output carries no watermark, no corner logo and no intro card. Uploaded video is deleted from the workers shortly after the job completes.
