About eighteen months ago I did a substantial piece on lip sync which was published in Sound and Image. I’ve just uploaded it, and here it is. It deals with what lip sync problems are, the processing in our brains that generate different results, and what can be done about it.
Let’s say that I am standing next to you. If you speak to me, the sound of your voice, relative to my view of your lips, will be delayed by around one millisecond (one thousandth of a second). If I am standing in one corner of my office and you are standing in the other corner, about seven metres away, your voice will be delayed by about twenty milliseconds. Now twenty thousandths of a second may seem like too short a time for a person to notice any difference, but I have just this moment made an audio file with two clicks separated by twenty milliseconds, and they are clearly distinguishable to any ear.
Yet here’s the odd thing: my brain will delay my vision of the movement of your lips so that it matches the sound of your voice. Likewise, your brain will do the same to your vision of my lips when I reply.