Lip-Sync Error Technical Details
Although the cause of lip-sync error is usually: "delay of video more than audio allowing sound to be heard "before" the lip movement that produced it" you may wish to delve deeper into what causes the video delays.
Links on this page will probably tell you more than you want to know about the video delays that created the lip-sync errors that brought you here, but first we'd like to share some of our observations in a little less technical format:
Why haven't you noticed lip-sync error before?
You may be here because you are plagued with lip-sync error in your home theater system and blame a recently acquired LCD, DLP, or plasma display because you did not have the problem before you acquired it.. That seems perfectly logical but in reality there are many cumulative sources of video delay, all of which contribute to lip-sync error and your display was likely "the straw that broke the camels back". If there were no other video delays to contribute to lip-sync error it is far less likely you would have noticed the error introduced by your display alone. But as you will see noticing lip-sync error is actually a "good thing" since it allows you to correct it. More on that below.
If you were to look at a graph showing video delays from content creation through the full broadcast chain you would see cumulative delay that can exceed the video delay your display introduces many times over. Note they measure delays in video "frames" which are approximately 33 Milli-seconds (ms) so when they talk about a "6 frame delay" they mean almost 200 ms which is about twice the video delay your display is likely to contribute. So don't be too hard on your display manufacturer since their delay may not be the largest you have now started noticing. But "the straw that breaks the camel's back" gets the blame most often!
How much lip-sync error does it take to be noticed?
We call that value the "threshold of recognition" and it definitely varies from person to person but it is generally assumed that a viewer will not notice audio leading the video if by less than 15 milliseconds or audio lagging the video if no greater than 45 milliseconds. Some people may not notice video delays of even 100 ms but we have observed once it goes high enough to be noticed the viewer's "threshold of recognition" drops precipitously and lip-sync error must then be corrected to within a much smaller margin to again be transparent to that particular viewer. This phenomenon aggravates the problem of lip-sync correction and almost negates the industry assumption that keeping the audio within the range of 15 ms leading and 45 ms lagging will solve the problem.
Our theory to explain this is fairly simplistic. Our brains can't process the physical contradiction of sound arriving "before" the action that created it so we develop a defense mechanism to ignore it: Subliminally looking away from the lips but when the lip-sync error is so large we can no longer compensate it becomes noticeable and after that the defense mechanism no longer works.
The industry lip-sync error tolerance assumptions (-15 ms to +45 ms) are based on average viewer's whose thresholds have never been exceeded and are therefore meaningless for those who now are sensitive to much smaller variations in lip-sync error.
If you are now "sensitized" to lip-sync error because your high end display pushed it past your "threshold of recognition", take a close look at lip-sync on your ordinary TV where it never bothered you before and see if you don't see lip-sync errors you never noticed. I'll bet you do.
As mentioned above, noticing lip-sync error rather than masking it will now allow you to correct it completely and enjoy the realistic immersive home theater environment you've spent so much to achieve. If you read the Stanford research below you will see the lip-sync error you could previously ignore (50 to 80 ms in their study) was still having its negative impact on perception - subliminal interference you are now conscious of and can therefore correct.
Don't most a/v receivers already correct lip-sync?
For true lip-sync correction the user needs interactive on-the-fly adjustment without disturbing the image but most a/v receivers treat it as a "set-and-forget" fixed delay as if it never changed. Most relegate the delay to an obscure set-up menu overlaying the screen making frequent adjustment intolerable. Lip-sync error changes between sources and even between programs on the same source.
But the largest problem is "too large a delay increment". We've seen some equipment claim to correct lip-sync but only allow adjustment in 50 ms steps. Even with the 10 or 20 ms delay increments offered by many TV's many viewers never feel they can get it corrected.
When you are adjusting the audio delay to match the video delay you really need to be able to adjust in 1 ms steps while watching the video. All Felston products, DD340 - DD540 and DD740 have adjusted in 1 ms steps and the newest DD740 even offers an optional "fine" mode adjustment in 1/3 ms steps.
No a/v receiver offers the lip-sync correction features of the Felston DD740.
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Stanford research proves lip-sync error negatively impacts viewer perception causing less trust and making presentations less believable.
Click HERE for the research paper done at Stanford University which statistically proves even consciously ignored lip-sync error adversely impacts presentations making them less believable and diminishing the viewer's trust. Lip-sync must be precisely corrected if realism and the immersive home theater environment is to be achieved.
A real-world report by a top broadcaster whose goal is ZERO added lip-sync error:
Click HERE for an excellent real-world white paper written by Steven Smith, Vice-president of Engineering at Liberty Corporation, describing how they implemented their goal of adding zero lip-sync error at their 15 television stations. If more broadcasters were this serious about lip-sync the problem would be limited to video delays in the feeds they receive and those introduced by video processing within home theater equipment. (such as video scalers which may be built into DVD players and LCD, DLP, and plasma displays)