Audio video sync

Audio-to-video synchronization (also known as audio-video sync, audio/video sync, lip sync, or by the lack of it: lip sync error, lip flap) refers to the relative timing of audio (sound) and video (image) parts during creation, post-production (mixing), transmission, reception and play-back processing. When sound and video have a timing related cause and effect, AV-sync can be an issue in television, videoconferencing, or film.

Lip sync errors are most commonly noticed by average viewers, (i.e. persons not professionally involved in the broadcast television industry) when a close up of the face (also known in the broadcast industry as a head shot) of a performer such as a newscaster is viewed. In home television viewing experiences involving high definition programs displayed on a flat panel TV the sound heard by the viewer most commonly leads the video seen by the viewer by a significant and often noticeable amount of time. This timing error (i.e. lip sync error) can range from near zero up to several seconds. The error typically slowly varies by a significant amount throughout a television program and frequently varies from noticeable to unnoticeable amounts several times per hour. In industry terminology the lip sync error is expressed as an amount of time the audio departs from perfect synchronization with the video where a positive time number indicates the audio leads the video and a negative number indicates the audio lags the video. This terminology and standardization of the numeric lip sync error is utilized in the professional broadcast industry as evidenced by the various professional papers, standards such as ITU-R BT.1359-1, and other references below.

Digital or analog audio video streams or video files usually contain some sort of explicit AV-sync timing, either in the form of interleaved video and audio data or by explicit relative time-stamping of data. The processing of data must respect the relative data timing by e.g. stretching between or interpolation of received data. If the processing does not respect the AV-sync error, it will increase whenever data gets lost because of transmission errors or because of missing or mis-timed processing.

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