General Audio Visual sync questions

Q. What causes lip-sync issues?

A. Modern video processing systems in TV’s and Projectors 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 the settings on the display itself.  By comparison, the Audio more or less passes straight through the processing without any delay at all.  As video resolution and colour depth increase with HD, 4K, UHD, and HDR the processing overhead is only set to increase and make the problems worse.

If you have a single TV and use the inbuilt speakers then everything should be in sync as the TV will delay the audio automatically to correct for the video processing delay.   If, however, at any point the audio and video are separated and processed by different equipment problems can occur.  This is why the likes of 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.  You will also see some TV’s have an Audio delay feature for use when connecting external speakers or sound bars.  There are also in-line audio delay units available from various suppliers simply to introduce an audio delay to bring things back into sync.

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, hardware, and 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 general system checks should be made when any component, hardware or software, is altered or updated.

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

A. 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.  

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.  Given different sources also may have differing levels of error, the sum of the errors can lead to things looking wrong.  

The ideal is to have the base system setup as accurately as possible so if a source, such as a particular Blu-Ray film, has a slight error it may well go unnoticed.

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

A. 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.  

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