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Audio to video synchronization (also known as audio video sync, audio/video sync, AV-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, as well as and other references below. Digital or analog audio video streams or video files usually contains 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. ==Incorrectly synchronized== There are different ways in which the AV-sync can get incorrectly synchronized: *During creation AV-sync errors happen because of * *Internal AV-sync error: Different signal processing delays between image and sound in video camera and microphone. The AV-sync delay is normally fixed. * *External AV-sync error: If a microphone is placed far away from the sound source, the audio will be out of sync because the speed of sound is much lower than the speed of light. If the sound source is 340 meters from the microphone, then the sound arrives approximately 1 second later than the light. The AV-sync delay increases with distance. *During mixing of video clips normally either the audio or video needs to be delayed so they are synchronized. The AV-sync delay is static, but can vary with the individual clip. *Video editing effects. Examples of transmission (broadcasting), reception and playback that can get the AV-sync incorrectly synchronized: *A video camera with built-in microphones or line-in may not delay sound and video paths by the same number of milliseconds. A video camera should have some sort of explicit AV-sync timing put into the video and audio streams. Solid state video cameras (e.g. Charge-coupled device (CCD) and CMOS image sensors) can delay the video signal by one or more frames. *An AV-stream may get corrupted during transmission because of electrical glitches (wired) or wireless interruptions - this may cause it to become out of sync. The AV-sync delay normally increases with time. *There is extensive use of audio and video signal processing circuitry with significant delays in television systems. Particular video signal processing circuitry which is widely used and contributes significant video delays include frame synchronizers, digital video effects processors, video noise reduction, format converters and MPEG pre-preprocessing. *The video monitor processing circuit may delay the video stream. Pixelated displays require video format conversion and deinterlace processing which can add one or more frames of video delay. *A video monitor with built-in speakers or line-out may not delay sound and video paths by the same amount of milliseconds. Some video monitors contain internal user-adjustable audio delays to aid in correction of errors. *Some transmission protocols like RTP require an out-of-band method for synchronizing media streams. In RTP's case, each media stream has its own timestamp using an independent clock rate and per-stream randomized starting value. A RTCP Sender Report (SR) is needed ''for each stream'' in order to synchronize streams.〔RFC 3550〕 The necessary RTCP packets might be lost (since RTP/RTCP does not guarantee delivery) or not sent until at least several seconds after the stream has begun. Many software clients do not send RTCP at all or send non-compliant data. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「audio to video synchronization」の詳細全文を読む スポンサード リンク
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