How exact must phonemes match for lip-sync in dubbing, and what do open-vowel, bilabial and viseme substitutions allow?

Problem: judging whether a dubbed word 'fits' the mouth. Conditions: visible mouth, usually close-up. Method: the guidance targets visually distinct sounds rather than identical ones: bilabials (open/closed lips), labio-dentals and open vowels are the most visible. Substitutions are allowed within a class: open vowels can be swapped, and a /p/ can be replaced by /m/, /b/, or the labiodentals /f/ or /v/. The viseme theory formalises this by grouping phonemes by lip shape; rounded and protruded vowels form the 'fourth viseme'. Example data (one Lithuanian dub of an English animated film, 476 stressed rounded/protruded vowel cases, 1 h 35 min): the translator used the fourth viseme in 46 percent of cases, the third in 32 percent, the second in 12 percent and the first in 10 percent; stress matched in 68 percent of cases. When a rounded vowel could not be matched, first- and second-viseme sounds fitted better than third-viseme sounds. Failure modes: matching spelling or phoneme identity rather than lip shape; a long word with many visible lip movements in a short mouth shot. Limitations: one film, one language pair, a human translator's choices, no viewer test. Version/date: Respectus Philologicus 2020. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The 476-case figures are the author's counts for one dub; the 'better fit' remark is her observation, not a perceptual test. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://digilib.phil.muni.cz/sites/default/files/pdf/BSE2025-2-4.pdf",
      "note": "Synchronization techniques; Results; Conclusions; 1.1 Lip synchrony"
    },
    {
      "url": "https://www.journals.vu.lt/respectus-philologicus/article/view/17155/19789",
      "note": "Synchronization techniques; Results; Conclusions; 1.1 Lip synchrony"
    }
  ]
}