Understanding Lip Sync problems

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Understanding Lip Sync problems

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Nexsen

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13. August 2007 @ 14:11

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This thread is to try to correct some misinformation about the common lip-sync problem now plaguing home cinema systems that I've seen scattered through many threads on the internet. Most discussions have focused on particular DLP, Plasma and LCD displays as if there were something wrong with these products and various users with identical equipment seem to confirm that when one reports his has a lip-sync problem and the other says his doesn't.

My point will ultimtely be that they are both viewing the same insidious problem and one doesn't consciously see it.

Digital image processing delays video but lets audio arrive too soon causing an impossible phenomenon - sound before the action that created it.

All digital image processing contibutes to the problem, not just the image engancement within the DLP, LCD, or plasma display. This starts with image capture and continues through post production and broadcasting or DVD enoding and small video delays creep in no mtter how hard they try to add compensating audio delays. These vieo delays may accumulate until they exceed the delay the display will finally add.

Because sound before the action that created it is impossible in nature our subconscious reaction to deal with this contradition of reality is to ignore it - to subliminally look way from the evidence confounding our senses - or mentally turn off the confusion. That subconscious defense mechanism allows us to ignore quite a bit of lip-sync error but at some point we exceed its ability to suppress it(usually when a new DLP, LCD or plasma display pushes us over our limit) and it enters our conscious mind.

It has been known for over 20 years that this phenomenon - sound ahead of video - causes even viewers who don't notice it to experience the same negative impact on their perception of the material as those who see it. It's insidious because the problem masks itself due to the defense mechanism that allows our senses to cope with the impossible - at the expense of undermining our perception.

Some users' defense mechanism can tolerate more lip-sync error than others and we see this in forum posts where identical equipment is described as having and not having this problem when in fact in few cases is that even possible.

From this you can also see that about half the lip-sync problem is created of random video delays occurring before the material is even brodcast or encoded onto DVD. The other half is of course the display's video delay. The random portion causes every program and every DVD to potentially have a different lip-sync error. Consequently users report lip-sync erors coming and going and that confuses the issue since most think of it as a fixed video delay problem which it is not.

Unfortunately there is no watermark or time stamp in the signals to define when they were ever in-sync so automatic lip-sync correction is impossible. Even HDMI 1.3's claim of automatic lip-sync correction doesn't address the constantly changing lip-sync error from source to source, program to program, and DVD to DVD.

HDMI 1.3 simply allows the display to tell a receiver what delay it will add so the receiver could "automatically" set that delay for you (which you could do manulayy in a minute or two) but it doesn't address variable lip-sync error at all.

The only way to correct lip-sync is to do it subjectively gradually changing the audio delay until lip-sync looks perfect. You have to overcome the defense mechanism that keeps you from looking at the lips and when you do you will see this clearly visible phenomenon and then you can eliminate it and its interference.

If a recently acquired DLP, LCD or plasma display has pushed this into your conscious mind you probably perceive this annoyance as a problem but the silver lining is that it has exposed a previously ignored problem that has been undermining your home cinema experience. The research at Stanford was done over 20 years ago (go to Wikipedia for "lip-sync error" for a link to the Stanford research) and it showed that even 41 ms of the unnatural leading audio which most people won't consciously notice causes the same negative impact on viewer perception as larger noticeable lip-sync error.

That says we should not just be satisfied correcting for the display's fixed video delay but we should take this opportunity to look closer at the lips and eliminate all the lip-sync error possible to eliminate the subconscious interference. I think it's well worth a few moments adjusting for perfect sync at the start of every program and every DVD.

If you want to prove this information to yourself - that there is a subliminal defense mechanism keeping you from viewing the lips and faces - force yourself to look at the lips and move your audio delay closer and closer to perfect sync. Do this even if you have already corrected for your display's video delay with a fixed audio delay. I'll bet you will find a spot with prefect lip-sync that was quite a ways from what you were previously viewing and ignoring - maybe even 41 ms away!

Doesn't that prove there's a subconscious defense mechanism causing us to ignore the contradiction of reality of sound "before" the action that created the sound?

This should help us interpret the various forum posts on the Internet if we keep in mind the lip-sync error members are discussing will depend upon not only the particular equipment but the material being watched and perhaps more importantly the user's defense mechanism causing him to ignore it.

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