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Digital Audio Lip-Sync Correction


Felston lip-sync correction devices solve lip-sync error by allowing you to delay the digital audio signal to match the delay* in the video signal restoring perfect lip-sync.

The DD740 is inserted in the digital audio path between your video source (STB, DVD, DVR, etc.) and your AV receiver (or sound bar)  as in the diagram above. Since the DD740's "bit-perfect reproduction" does not change the digital audio signal, it is compatible with PCM  (both Toslink Optical as well as coax digital) and all present and future s/pdif surround sound formats such as Dolby Digital and DTS at both 48 KHz and 96 KHz. 

Since there is nothing in the video or audio signal to define when they were ever in sync it is a subjective adjustment and this is where the remote control excels. The DD740 remembers the last delay setting used on each input and includes 36 presets where common delays can be stored for instant recall. But most importantly, the + and - buttons allow dynamic "on-the-fly" delay adjustments while watching with no image disturbance - an essential feature allowing tweaking for "perfect-sync". These are necessary features for true lip-sync correction and not generally available on even the most expensive AV receivers that claim a lip-sync delay feature. 

* Normally lip-sync error is due to video delays in both the arriving signal as well as in the display allowing audio to arrive too soon but when broadcasters over-correct for the video delay the arriving signal might have audio delayed instead of video. 

At first thought it might appear the DD740 audio delay could not correct for "already delayed audio in the arriving signal"  but in conjunction with the video delay of your TV or projector it actually can - up to the display's video delay. That is, if your TV delays video 100 ms your DD740 will correct lip-sync errors from 100 ms audio lagging to 580 ms audio leading.  For most projectors the video delay is even larger which is actually a good thing when used with a DD740 since it gives you more negative adjustment if sound arrives "already delayed.

For a more detailed explanation of how an audio delay can correct for audio that's already delayed CLICK HERE.


 Using HDMI

Felston developed an HDMI lip-sync corrector, the HD940, but by the time it was in production we discovered the way TV manufacturers handle audio in the HDMI data stream defeats the most important feature – delay of audio!  We discovered many TV's (top brands like Sony, Panasonic and Toshiba TV's for example) delay audio in the HDMI stream without allowing a user to stop it so intercepting digital audio and never allowing a TV to touch it is the only way to completely correct lip-sync on those TV's.


TV's from Samsung, Vizio and some others allow setting any audio delay to zero but they don't accept Dolby Digital on their HDMI inputs. (They do support Dolby on their internal tuner and streaming inputs but NOT via their HDMI input where we can delay it.) Consequently we stopped production after 300 HD940's were built and never released it. Our DD740 does a better job correcting lip-sync because it intercepts audio before a TV can touch it!


We thought perhaps manufacturers would realize what they have done (delaying audio without allowing a user to defeat it) deprives users of the ability to synchronize audio with video but even if they ever do the market has moved on and expects HDMI 2.0.  ADI whose ADV7623 chip we used in the HD940 never produced a 2.0 version and their new chips gained in their recent Invecas acquisition are not similar and don't have video overlay which the HD940 requires.

If you are interested in viewing the preliminary manual for the HD940 it can be viewed by clicking   HERE. 

When we developed the HD940 we thought SMPTE’s new standard which would allow meta data to carry “signatures” of audio and video frames was likely to finally solve the lip-sync error introduced by broadcasters. If that had happened lip-sync error would have been reduced to the different video delays caused by different video formats within the user’s equipment.  We thought we could finally make lip-sync correction automatic! The blue AUTO button on the HD940’s remote invokes a mode allowing the video delay for every supported VIC code in the HDMI spec to be automatically switched in every time the video format changes. The SMPTE standard has never gained acceptance making that feature of the HD940 worthless. There remains no way to “automatically” correct lip-sync!


An HDMI cable carries both video and audio but most sources also have separate digital audio outputs (coax and optical) the DD740 can intercept before they go to your TV and on to your AV receiver or soundbar.  If you are using your TV's internal speakers a few TV's have digital audio inputs but not very many so to use the DD740 you may need to add a soundbar or a DAC (digital to analog converter) to change the D740's digital audio output to analog stereo since most TV's have stereo inputs.  A DAC will work but restricts you to stereo sound from your TV speakers whereas a soundbar will provide much better sound and may offer Dolby Digital surround sound too.

To buy the DD740 for $249 plus $9.95 shipping click the link to PayPal below. Our products are also on Amazon and eBay at slightly higher prices.

If you have a special application requiring an HDMI audio delay and HDMI 1.3 is acceptable we will consider releasing an HD940. Disclaimers will be required so let us know your application. Email:


Click HERE to buy the DD740


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