How does Change 3GP 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. 3GP is the cut-down MP4 profile built for early mobile networks, usually very low resolution with AMR audio. That determines whether the retimed result can be written back cheaply or has to be fully re-encoded.
Anything specific to 3GP here?
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Yes — 3GP files are usually very low resolution with AMR speech audio, so there is little detail left to protect and a lot of size to reclaim. 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.
