Audio to video synchronization
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.
Digital or analog audio video streams or video files usually contains some sort of explicit AV-sync timing, either in the form of interlaced 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 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.
Recommendations
For television applications, audio should lead video by no more than 15 milliseconds and audio should lag video by no more than 45 milliseconds.[1] For film, acceptable lip sync is considered to be no more than 22 milliseconds in either direction.[2]
MPEG: Presentation Time Stamp (PTS), Decode Time Stamp (DTS)
Presentation time stamps (PTS) can be embedded in MPEG transport stream to avoid AV-sync drift. Unfortunately these time stamps are often added after the video undergoes frame synchronization, format conversion and pre-processing, thus those delays remain uncompensated.[3][4][5][6]
Viewer experience of incorrectly synchronized AV-sync
The result typically leaves a filmed or televised character moving his or her mouth when there is no spoken dialog to accompany it, hence the term "lip flap" or "lip-sync error". The resulting audio video sync error can be annoying to the viewer and can even lead to the viewer's not enjoying the program, to the program's not being effective, and to the speakers being perceived negatively.[7] The lack of effectiveness problems are of particular concern when product commercials and political candidates are viewed. Television industry standards organizations, such as the Advanced Television Systems Committee, have become involved in setting standards for audio video sync errors.[1]
Because of these annoyances, AV-sync error is of concern to the television programming industry, including television stations, networks, advertisers and program production companies. Unfortunately the advent of high definition flat panel display technologies (LCD, DLP and plasma) which can delay video more than audio have moved the problem into the viewer's home and beyond control of the television programming industry alone. Consumer products companies now offer audio delay adjustments to compensate for video delay changes in TV's, a/v receivers, and several companies manufacture dedicated digital audio delays made exclusively for lip-sync error correction.
Effect of no explicit AV-sync timing
When a digital or analog audio video stream does not have some sort of explicit AV-sync timing these effects will cause the stream to become out of sync:
- In film movies these timing errors are most commonly caused by worn films skipping over the movie projector sprockets because the film has torn sprocket holes.
- Errors can also be caused by the projectionist misthreading the film in the projector, although this is rare with competent projectionists.
- Audio to Video Synchronization is commonly corrected and maintained with an audio synchronizer. Television industry standards organizations have established acceptable amounts of audio and video timing errors and suggested practices related to maintaining acceptable timing.[1]
- A/V sync errors are becoming a significant problem in the digital television industry because of the use of large amounts of video signal processing in television production, television broadcasting and pixelated television displays such as LCD, DLP and plasma displays.
- In the television field, audio video sync problems are commonly caused when significant amounts of video processing is performed on the video part of the television program.
- Typical sources of significant video delays in the television field include video synchronizers and video compression encoders and decoders. Particularly troublesome encoders and decoders are used in MPEG compression systems utilized for broadcasting digital television and storing television programs on consumer and professional recording and playback devices.
- A source of significant video delay is found in pixelated television displays (LCD, Plasma display, DLP) which utilize complex video signal processing to convert the resolution of the incoming video signal to the native resolution of the pixelated display, for example converting standard definition video to be displayed on a high definition display. "Lip-flap" may exceed 200 ms at times.
- In broadcast television, it is not unusual for lip-sync error to vary by over 100 ms (several video frames) from time to time.
- The EBU Recommendation R37 “The relative timing of the sound and vision components of a television signal” states that end-to-end audio/video sync should be within +40ms and -60ms (audio before / after video, respectively) and that each stage should be within +5ms and -15ms.[8]
Sources/references
See also
External links
- Further detailed information on lip sync error and audio synchronizer may be found by searching for these terms at the United States Patent and Trademark Office web site at [1].