Audio-to-video synchronization

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Development of the music video

Audio-to-video synchronization (AV synchronization, also known as

videoconferencing, or film
.

In industry terminology, the lip-sync error is expressed as the 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.[1] This terminology and standardization of the numeric lip-sync error is utilized in the professional broadcast industry as evidenced by the various professional papers,[2] standards such as ITU-R BT.1359-1, and other references below.

Digital or analog

timestamping
of data.

Sources of error

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 due to different signal processing delays between image and sound in video camera and microphone. The AV-sync delay is normally fixed. External AV-sync errors can occur 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 can delay video causing it to lag the audio.

Transmission (

compression systems
.

Processing circuits

format conversion
and deinterlace processing in video monitors 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 equally. 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 some RTP systems each media stream has its own timestamp using an independent clock rate and per-stream randomized starting value. A RTCP Sender Report (SR) may be needed for each stream in order to synchronize streams.[3]

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:

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 may even cause the viewer to not enjoy the program, decrease the effectiveness of the program or lead to a negative perception of the speaker on the part of the viewer.[6] The potential loss of effectiveness is of particular concern for product commercials and political candidates. Television industry standards organizations, such as the Advanced Television Systems Committee, have become involved in setting standards for audio-video sync errors.[4]

Because of these annoyances, AV-sync error is a 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, has moved the problem into the viewer's home and beyond the control of the television programming industry alone. Consumer product companies now offer audio-delay adjustments to compensate for video-delay changes in TVs and A/V receivers, and several companies manufacture dedicated digital audio delays made exclusively for lip-sync error correction.

Recommendations

For television applications, the

ITU performed strictly controlled tests with expert viewers and found that the threshold for detectability is 45 ms lead to 125 ms lag.[1] For film, acceptable lip sync is considered to be no more than 22 milliseconds in either direction.[5][7]

The

Consumer Electronics Association has published a set of recommendations for how digital television receivers should implement A/V sync.[8]

SMPTE ST2064

SMPTE standard ST2064, published in 2015,[9] provides technology to reduce or eliminate lip-sync errors in digital television. The standard utilizes audio and video fingerprints taken from a television program. The fingerprints can be recovered and used to correct the accumulated lip-sync error. When fingerprints have been generated for a TV program, and the required technology is incorporated, the viewer's display device has the ability to continuously measure and correct lip-sync errors.[10][11]

Timestamps

Presentation time stamps (PTS) are embedded in MPEG transport streams to precisely signal when each audio and video segment is to be presented, to avoid AV-sync errors. However, these timestamps are often added after the video undergoes frame synchronization, format conversion and preprocessing, and thus the lip sync errors created by these operations will not be corrected by the addition and use of timestamps.[12][13][14][15]

The

timestamps on an arbitrary timeline. A real-time clock such as one delivered by the Network Time Protocol and described in the Session Description Protocol[16] associated with the media may be used to synchronize media. A server may then be used to for final synchronization to remove any residual offset.[17]

See also

References

  1. ^ a b c "ITU-R BT.1359-1, Relative Timing of Sound and Vision for Broadcasting" (PDF). ITU. 1998. Retrieved 30 May 2015.
  2. ^ Patrick Waddell; Graham Jones; Adam Goldberg. "Audio/Video Standards and Solutions A Status Report" (PDF). ATSC. Archived from the original (PDF) on 17 February 2016. Retrieved 4 April 2012.
  3. RFC 3550
  4. ^
    ATSC, 2003-06-26, archived from the original
    on 2012-03-21
  5. ^ a b "The relative timing of the sound and vision components of a television signal" (PDF).
  6. ^ Byron Reeves; David Voelker (October 1993). "Effects of Audio-Video Asynchrony on Viewer's Memory, Evaluation of Content and Detection Ability" (PDF). Archived from the original (PDF) on 2 October 2008. Retrieved 2008-10-19.
  7. SMPTE
    . Appropriate A/V sync limits have been established and the range that is considered acceptable for film is +/- 22 ms. The range for video, according to the ATSC, is up to 15 ms lead time and about 45 ms lag time
  8. ^ Consumer Electronics Association. "CEA-CEB20 R-2013: A/V Synchronization Processing Recommended Practice". Archived from the original on 2015-05-30.
  9. SMPTE
    , 2015
  10. SMPTE, 10 December 2013, archived
    from the original on 2021-12-15
  11. SMPTE, 10 December 2013, archived from the original
    (PDF) on 2016-08-26, retrieved 2016-06-09
  12. ^ "MPEG-2 Systems FAQ: 19. Where are the PTSs and DTSs inserted?". Archived from the original on 2008-07-26. Retrieved 2007-12-27.
  13. ^ Arpi (7 May 2003). "MPlayer-G2-dev: mpeg container's timing (PTS values)".
  14. ^ "birds-eye.net: DTS - Decode Time Stamp".
  15. ^ "SVCD2DVD: Author and burn DVDs: AVI to DVD, DivX to DVD, Xvid to DVD, MPEG to DVD, SVCD to DVD, VCD to DVD, PAL to NTSC conversion, HDTV2DVD, HDTV to DVD, BLURAY". www.svcd2dvd.com.
  16. RFC 7273
  17. RFC 7272

Further reading
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