Sorry in my long post I did not address the sources of lip-sync error - where the error actually originates. I focused mostly on how it causes us to look away from the offending lip movement and how that negatively impacts our impression of the characters and story.
Most viewers seem to believe the audio and video in the signal they receive are in sync and that their equipment causes them to get out of sync. In fact TV manufacturers often assume the same thing which is quite often not the case.
While it is certainly true that the end user's equipment "can" cause a delay in video and/or audio creating the lip-sync error such an error will be static or fixed and won't change from programme to programme. Generally its a video delay introduced by the equipment because video data (especially 4K) is enormous compared to audio data so left uncorrected the audio will arrive first. Most Sony TV's (if not all?) add a fixed audio delay to match their video delay. If the source signals are in sync that will keep them in sync.
The problem, however, is that source signals are rarely if ever in perfect sync.
Even BluRay and DVD's which are normally very close to being in sync can have an offset between video and sound when encoded. It can be in either direction. I recall watching Denzel Washington's :inside Man" DVD a few years ago and it took 41 ms of video delay to correct for its excess audio delay.
Excess video delay is the opposite of the normal problem so you might ask how I could use an audio delay adjustment to add video delay to correct an arriving audio delay: The answer is the inherent video delay of your TV becomes an adjustable video delay when used in conjunction with a variable video delay.
But you must be able to access that video delay. To do that you will need to take the sound signals (usually optical or coax digital) directly from your sources to a soundbar or AV receiver bypassing your TV and the audio delay it would have automatically added.
For example I take optical audio directly to an input on the DD740 bypassing the TV and then to the soundbar. I still use HDMI for video input to the TV. I then add the audio delay the TV would have added (60 to 100 ms for most TV's today but it can be over 300 ms for some TV projectors and wireless TV transmitters I am told.
Once I have offset the TV video delay - 88 ms in my case - any signals arriving in sync will remain in sync but that is rare indeed. I normally have to add between 20 and 100 ms audio delay as sync changes from programme toi programme.
When signals arrive with audio already delayed I simply lower the delay setting a ms at a time until it is synchronized using the inherent video delay of the TV: My 88 ms starting delay becomes a 10 ms video delay when the audio delay is lowered from 88 to 78.
Of course in my case I can't correct for more than an 88 ms arriving audio delay but a video projector owner with a 300 ms video delay would start out with 300 ms audio delay setting and have a range of -300 to + 380 ms adjustment since the DD740 offers a 680 ms delay.
Until recently there was no SMPTE standard to define when audio and video were ever in sync and the new standard ( a fingerprinting scheme to store digital signatures of each video and audio frame in metadata) has not been implemented to my knowledge.
There are standards for measuring video and audio delay at every point along the broadcast chain and broadcasters were supposed to "measure and correct" AV sync at any points it could change (basically at every digital video effect- DVE). Since this is an open loop control scheme any error that crept in will not be detected downstream and is therefore cumulative.
Since older analog TV technology didn't store video or audio there was little that could change the timing and CRT's in the home didn't either. The advent of widespread digital processing - everything from green screens to the bugs that crawl at the bottom of your screen to inform you of what's coming next - can upset lip-sync.
Compounding the problem are the multiple formats - especially with 4K images - confronting broadcasters. A 4k image is 4 times the data of a 1080 image and 8 times as much data as a 720 image. Often when you see audio arrive delayed it's an error caused by an attempt to correct for excess video delay of these larger formats.
