Lip movements matched to dialogue track — must be frame-accurate. Critical in ADR, dubbing, and digital post. Millisecond drift kills credibility.
When the mouth doesn't match the sound, the audience immediately notices something — even if they can't consciously articulate what. Lip sync is the frame-accurate synchronization between mouth movement and the dialogue audio track. This sounds trivial, but it becomes a technical challenge as soon as the audio track isn't recorded on set or needs post-production work.
On set, the problem is minimal: the sound is recorded along with the live lip movements. It becomes critical with ADR (Automated Dialogue Replacement) or full dubbing. The actor sits in the studio in front of the monitor, sees their close-up, and must speak the new audio track with such precise timing that every phoneme matches the corresponding lip and jaw movement. This is physically demanding — not because it needs to be loud, but because it needs to be precise. A syllable shifted by 2-3 frames is noticeable in a close-up.
In digital post-production, there are several levels: First, the manual check — the sound designer or editor overlays the new track onto the footage and compares frame by frame. Tools like Premiere or DaVinci allow for frame-accurate shifting of audio clips. With significant discrepancies, re-dubbing or changing the camera position is necessary (a half-profile shot instead of frontal increases the margin for error). Some facilities use semi-automatic algorithms based on speech recognition, but these don't replace critical control — they only speed it up.
Language and cultural differences complicate cross-dubbing: English requires different lip positions than German or Mandarin. An English "P" is a different mouth formation than a German "P". This sometimes forces reframing for International Versions — the camera moves closer or further away to make the mouth less visible. In extreme cases, off-voice is used, or the character is shown from the back.
The most common practical rule: a four- to six-frame tolerance is considered acceptable when cuts are involved. In a pure close-up — a standard interview situation — it must be frame-accurate. For the director, this means planning for alternative shots already on set. An over-the-shoulder shot is more forgiving than an absolute frontal close-up if post-production is likely.