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March 9, 2012

Experts' Experts: Lip-Flap — Why your expensive TV's audio and video are out of sync

I've noticed for years now, especially when watching shows like PTI, that the sound and video are way out of sync on both my HDTVs.

Sometimes the lips stop moving and words keeping coming for up to a second, an awfully long delay when you're watching.

When it gets that disturbing I stop looking at the screen and just listen.

I've noticed this doesn't happen when I watch a DVD, so I figure it must not be the TV's fault but, rather, something to do with the way the TV signal makes its way to me.

Sure enough, that's the case, according to Wikipedia.

I've read that a lot of people think their TV is broken when this asynchrony happens — but it's not.

Excerpts from Wikipedia's entry on lip-flap appear below.

Audio to video synchronization (also known as audio video syncaudio/video syncAV-synclip sync, or by the lack of it: lip sync errorlip-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 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.

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. The lack of effectiveness problems are of particular concern when product commercials and political candidates are viewed. 

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

• 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, 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 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.

• In broadcast television, it is not unusual for lip-sync error to vary by over 100 ms (several video frames) from time to time.

What a pleasure it is to have Wikipedia as a resource from which I can quote as extensively and as often as I wish without any fear of blowback. 

Indescribably liberating.

Good luck, by the way, with trying to fix the lip-flap problem with a little brown box like the guy in the video up top: from what I've read, these aftermarket add-ons work some of the time on some TVs, but it's anyone's guess whether they'll work on yours.

Caveat emptor.

March 9, 2012 at 04:01 PM | Permalink


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Here's your problem: http://www.atsc.org/cms/standards/a_78a.pdf

Posted by: 6.02*10^23 | Mar 10, 2012 11:56:57 AM

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