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The solution guaranteed to correct "lip-sync":


Before we address how to correct lip-sync error let's explain why it is so easily misunderstood:  Many "experts" - even video editors - have believed the fact that video is comprised of discrete frames meant the sound accompanying each frame was in alignment if it fell anywhere in that frame's 33 to 40 ms duration. They overlooked the reason motion pictures work in the first place! "The fact that repeated static frames create the perception of continuous motion in our brain.  Sound needs to align with that "perception" just as it does in the real world!"

With a DD740 you can "see" this transformation from being out of sync as you add delay in small increments of 1 ms (or even 1/3 ms)  to being in perfect sync when sound aligns with video.  If you know this is what you need skip to our BUY Now link at the bottom of this page. If not, read on: We'll probably tell you more than you want to know about lip-sync correction.

Many people are confused by HDMI's "automatic lip-sync correction" claim and think it can solve this problem. It can't and only allows a TV to request a "fixed" delay which is set for that particular TV and has nothing to do with the actual lip-sync error present in the signal. It won't correct lip-sync error any better than any other fixed delay and often makes it worse!

Furthermore, many TV's (most Sony, Panasonic and Toshiba TV's for example) delay audio in their HDMI input and don't allow a user to stop it. Intercepting digital audio before a TV is allowed to touch it is the only way to completly correct lip-sync on those TV's. Samsung, Vizio. etc. usually allow turning any audio delay OFF but don't accept Dolby Digital in their HDMI inputs so intercepting audio is the only way to get Dolby Digital from HDMI inputs on those TV's. The DD740 supports both digital audio formats (optical and coax digital) and automatically converts between them.

Our HD940 is the only HDMI lip-sync corrector ever developed as far we know.  After building  300 of them we stopped production and never released it.  For more info on "why" see our Digital Audio Delay page under "Using HDMI".  HDMI is great for video but not for synchronized audio.

There is no way to automatically correct lip-sync because there is nothing to define when video and audio are in sync. From content creation until delivery by your large screen there are many places along the broadcast chain where audio video sync error - lip-sync error - creeps in.  An audio delay must be adjusted while viewing the out of sync video to offset the cumulative lip-sync error.


When sound is out of sync with the picture on your TV and voice does not match lip movement you have begun to notice lip-sync error or av sync error.  It plagues all TV's and projectors and is worse on larger 4K and 8K screens which is probably why you have just noticed it.


Lip-sync error causes audio to be heard "before" the action which produced it is seen. You hear speech before you see the lip movement that produced it. That's impossible in the real world but not on your TV!


If you have tried adjusting a fixed audio delay to correct  lip-sync you have already discovered it's impossible. A variable delay is required.  

To achieve perfect lip-sync error correction you need to adjust sound delay in small steps when the video delay changes. 1 ms usually works well and 1/3 ms is even better for the most sensitive but the main requirement is to be able to change the audio delay in small steps without upsetting the image as you watch.  Pressing the DD740's + and - buttons accomplishes that.  Its numeric pad can be used for large delay changes.




Most TV's, projectors and STB's provide a digital audio output - usually Toslink optical - which the DD740 will delay before it goes to your sound system. The DD740 will also delay coax digital audio but that's used less often.


The DD740 features "bit-perfect" reproduction so its digital output is identical to its digital input eliminating any chance of degradation.

Synchronizing audio with video is important even if lip-sync error doesn't consciously bother you:

You want a realistic home theater, right?  You've spent a lot and may feel you've achieved it  but the "image" is not all that matters when creating an immersive environment.

Sound "before" the action that creates the sound  is impossible  in the  real world. Our brain can't reconcile this conflicting visual and aural information.

There is strong evidence based on research done at Stanford that our subliminal defense mechanism to this impossibility is to look away from the faces and moving lips so as not to notice this contradiction of reality. Force yourself to focus on the lips in a "talking head" scene and you will probably be amazed at the error you have been able to ignore by subconsciously looking away.

Just as we have less trust for those who do not make eye contact we get that same negative impression when "we" are the ones looking away from the characters faces. Even when we do it subliminally.

To read the actual research report from Stanford that statistically proved lip-sync error interferes with viewer perception causing content to be less believable click HERE.  It's a PDF report you can save. Note that their results were the same whether viewers consciously noticed the lip-sync error or not. 

If you think the industry should have solved this we agree and SMPTE has a solution but Tektronix's AVDC100  could have solved it almost 20 years ago.  The Stanford research has been known for over 28 years and still no solution. Why? Because it is a "chicken or the egg" problem. The audio and video must be synchronized at content creation and however it is marked (meta data in the current proposal) must be maintained throughout the entire broadcast chain to be used to restore sync. No one entity wants to invest in the required equipment until "everyone" has plus it won't solve the problem for any preexisting content.


So, lip-sync error is not just an annoying distraction noticed by some of us but it fundamentally disrupts the immersive home theater environment even when viewers aren't consciously aware of it.  It's ironic that these almost perfect 4K and 8K displays  demand such a large amount of data increasing the video delay which allows the sound to arrive even earlier and creates the most unrealistic spot on the planet - a place where sound comes before the action that created it!  The silver lining is that you can fix it.

Correcting lip-sync error "restores reality"!

With the DD740 you can easily align sound and picture and realize the realistic environment you've already spent so much to achieve.

Felston DD740 IR remote controlled audio delay
DD740 lip-sync explanation
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                               At InfoCom we demonstrated how the DD740 can shift audio putting it in sync with video and restoring reality.

The only way to correct lip-sync error caused by delayed video is to delay audio an equal amount whenever it changes. 

Most people initially only notice lip-sync error when it exceeds 40 to 75 ms.  We call the value at which it is noticed consciously their "threshold of recognition" and it varies enormously from person to person depending upon the individual's defense mechanism - how far he can look away from the moving lips so as to ignore the lip-sync error.

An individual's "threshold of recognition" falls greatly once it has been reached and lip-sync error has been noticed.  At that point their defense mechanism can no longer compensate and the sync problem enters their conscious mind.  The same person who was never bothered by a 40 ms lip-sync error may, after noticing 120 ms error, become far more sensitive and notice errors of a few milliseconds - a small fraction of their previous "threshold".   Most people can "see" lip-sync error as small as "one millisecond" when focusing on lip movement.  Some can even detect 1/3 ms error.

Even some expert sound editors have thought getting sound within one frame of the video (33 ms for 30 fps TV for example) was all you needed for perfect lip-sync. It is easy to make that mistake until you realize the repeating static frames create the perception of continuous motion “within our brain” and sound needs to align with that “perception” just as it would in the real world.


 Here's our "guaranteed" solution:

The Felston DD740  digital audio delay solves lip-sync error by letting you delay audio (including Dolby Digital) in 1 ms  (or 1/3 ms in fine mode) steps to compensate for the cumulative and changing video delays - no matter what their cause - at the touch of a button on its remote while watching TV. We provide a money back guarantee it will solve your lip-sync problem and we allow 30 days for you to decide.


Is it possible it might not work for you? Unfortunately, yes, but that's what we have the guarantee for. In cases where a broadcaster transmits "already delayed" audio there is NO product that can delay 4K or 8K video to correct it. But if it's not more than your system's inherent  video delay a DD740 can.  Click HERE for more detail.

The DD740 connects between up to four digital audio sources and your AV receiver (or Sound -Bar) allowing you to delay the audio to match the video achieving "perfect lip-sync".  Because lip-sync error changes between sources and even between programs on the same source the DD740's hand held remote is an essential part of true lip-sync error correction. It allows moving the audio back and forth at the touch of a button without any disturbance in the video

To buy the DD740 click the link to PayPal below. Our products are also on Amazon and eBay at slightly higher prices.(eBay because they offer an international shipping program beneficial to our foreign customers.)

Save $70: We have a few with water stained cartons for $179. Their packaging was discolored by a flood but they were not wet and carry full warranty as well as the guarantee above. Email if interested.


Click HERE to buy the Felston DD740 for $249


                                                 For immediate answers call: 803-738-1922

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