[編輯] Sync 唔啱
- During creation AV-sync errors happens because of
- Internal AV-sync error: Different 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.
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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. 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.