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LIPS AND VOICE SLIDE APART OVER A 20-MINUTE INTERVIEW. WHAT'S GOING ON??

audioframe-ratesyncvfr3 replies · 1 views · started
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  1. Yara Celik
    @yarac30

    not gonna lie this is driving me nuts hey, scenario: a screen recording with a separate voice-over track. Everything matches for the first few minutes, then my mouth and teh words visibly drift apart:

    Timeline 1 | 30 fps | 48 kHz

    Timeline 1   |   30 fps   |   48 kHz
A1  camera
A2  recorder   +8 fr   +16 fr
Clap marks line up at the start, then slowly drift apart

    i tried re-importing, same result.

    Is it the recorder, the editor, or me? seriously need help here 😭

    2
  2. Dev Olsen
    @devo153

    oof gradual drift (not a fixed offset) has two common causes:

    1. Variable frame rate (VFR) video. phones and screen recorders often vary frame timnig. editors assume a constant rate, so picture and sound slowly disagree. check with MediaInfo: it will say "Frame rate mode: Variable".
    2. Sample rate msimatch. Audio recorded at 44.1 kHz but interpreted as 48 kHz (or teh reverse), or two devices whose clocks are slightly different.

    Fix VFR by converting to constant frame rate before editing, for example:

    ffmpeg -i interview.mp4 -fps_mode cfr -r 30 -c:v libx264 -crf 16 -c:a copy interview_cfr.mp4

    fingers crossed 🤞

    0 3 helpful
  3. Liv Silva
    @liv_quiet

    Even two devices both set to 48 kHz drift a little, cuz their internal clocks are never perfectly identical 🙏 Over a long take the difference becomes visible. Pro setups solve this with timecode sync or a shared clock. Without that, the practical fix is to stretch the external audio slgihtly to match: sync teh start, find teh ofsfet at the end, and change teh clip speed by the tiny amount needed :) Many editors have an auto-sync by waveform feature that does well on shorter takes. fingers crossed 🤞

    2 6 helpful
  4. Nico Walsh
    @nwalsh_5

    ok so also check the clip attributes in ur editor. if a 29.97 fps clip was interpreted as 30 fps (or 23.976 as 24), u get exactly this kind of slow drift. make the clips frame rate match how it was actually shot.

    0 1 helpful

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