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


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 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.

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