What causes lip-sync issues?

Modern video processing systems in TV’s and Projectors, whether LCD, DLP or Plasma, take a finite amount of time to process the incoming video data before displaying it.  This processing time can vary depending on the input port of the device, the resolution of supplied video and panel manufacturing technology. The delay can be constant or variable, as majority of LCD monitors has a refresh rate of 60Hz, which is nowhere near 24 or 48 cinema film speed. This will always produce video judder and hence sync issues. Because the Audio more or less passes straight through the system without any delay at all, AV amplifiers have an audio delay setting, to permit the user to delay the audio by the amount required to bring things back into sync.  There are also in-line audio delay units available from various suppliers, simply to introduce an audio delay to bring things back into sync. But remember - this will not correct variable sync issues if they appear.

In Post Production where the audio and video are separate elements on a timeline, there is the possibility for one to slip with reference to the other.  Some codecs, software, hardware and many other toolsets can also introduce an error, so it becomes necessary to check the final rendered output to ensure things have remained in sync.  As a minimum checks should be made when any component, hardware or software, is altered or updated.

How ‘bad’ does it need to be for me someone to notice?

People can generally start to perceive a lip-sync error when the audio leads the video by 15ms - 35ms.  Oddly if the audio lags the video this goes up to around 80ms, probably because it’s more natural to have images arrive before the sound, for example speaking to someone across a room means you see them talking before you hear them. Many broadcast station do not accept audio material if it is out of sync more than +20 to -40 ms. Given different sources also may have differing levels of error, the sum of the errors can lead to things looking wrong, so it is essential to have your system stable as rock.

Can I check sync only using my eyes and ears?

Trying to adjust or check any system manually is always prone to error, as it relies on the subjective view of the person doing the testing.  Everyone will perceive the amount of the error differently, meaning that any correction made will not be a true reflection of the system performance.

Hang on, HDMI 1.3+ supports auto-lip sync, so why do I need to do anything?

There is a feature within HDMI 1.3 and above where the display can send an audio delay figure back to it’s source.  This is a generic delay, possibly one of two, depending whether the source is an interlaced signal or not.  This is an average delay coded by the display manufacturer and only covers the average processing delay of the display unit itself.  There is also a reliance that the source feeding the display actually knows what to do with this delay when it receives it. In real terms it’s a start, but it doesn’t take into account the entire system, just the display.  In addition there have been reported issues with the feature if the HDMI Consumer Electronics Control (CEC) is also used.  You may see this described as features such like Samsung Anynet+, SimpLink from LG, Philips Easylink, BRAVIA Link and BRAVIA Sync from Sony etc.