When Apple recently launched its new MacBook Pro, many commented that it lacked the “courage” to remove the headphone socket from that new model, whereas it had the “courage” to take it from the iPhone 7 and 7 Plus. In fact, there is probably a more fundamental reason that Apple is likely to retain hardware support for wired headphones in future Mac and iPad models: lip synchronisation (lip sync).

Human brains are very sensitive to sensory mismatch. Although we’re used to seeing the puff of smoke from a distant gun firing, and hearing the report significantly later, when it comes to speech we expect visible articulation of the mouth to match the sounds that we hear it enunciate. If you have ever watched a video clip in which lip sync has been lost, it is first curiously amusing, but can rapidly become very irritating.

Expert viewer tests have shown that audio which precedes the corresponding video by more than 25 milliseconds, or which lags the video by more than 95 ms, can be detected by a significant number of viewers as being mismatched: lip sync error. A small number of individuals may have significantly lower thresholds of detection, and the film standard for the maximum acceptable difference between audio and video timing has been set at ±22 ms.

Decoding and delivering digital movies is a complex process, and handles the streamed audio and video separately, aiming to serve them to the viewer’s eyes and ears roughly simultaneously, using direct audio output and a built-in display.

Switch the audio output to Bluetooth headphones or earphones, and that adds significant delay: instead of converting it to analogue audio and passing that along wires to headphones, a sound system, or speakers, the audio now has to be recompressed (using a low-complexity codec), transmitted over Bluetooth to a connected device, decompressed, and then converted into analogue audio for output.

Bluetooth audio output is thus inherently slightly slower than regular wired audio output. Whether this delay crosses the threshold to result in noticeable lip sync error depends on a lot of factors, and is therefore prone to variation. Once that delay amounts to more than about 100 ms, then some users at least will notice the lip sync problem, and complain about it.

Already, Bluetooth headphones and earphones have caused many users to report lip sync error. Usually this can be brought back within the threshold of detection by bringing the player’s operating system up to date, updating any firmware in the headphones or earphones, or switching to better-performing wireless phones. Some users, though, continue to notice it, and are driven to return to using wired phones instead.

The worst thing that you could possibly do in these circumstances is to attempt to listen simultaneously to both Bluetooth and direct audio. This will only accentuate difficulties, as you will then be able to hear the slight lag. This is not quite as disruptive as playing back slightly delayed audio through closed headphones to someone speaking into a mike: that quickly reduces them to slurred speech as if profoundly drunk!

Our brains also tend to accommodate to small amounts of lip sync error, so if it is not too noticeable, persevere and you may find that the error becomes imperceptible after a while.

Another important factor is display size. The larger the image, the more perceptible lip sync error becomes. What may be unnoticeable or at least untroubling on the small display of an iPhone, may become very irritating on the larger display of a laptop like the MacBook Pro, and even worse on a big 5K display.

Thus the chances of lip sync error, resulting from Bluetooth audio output, causing distress on an iPhone 7 or 7 Plus are significantly smaller than on a MacBook Pro, and are greatest when using a large external display.

It may be that one day, iOS and macOS will introduce variable video delay to reduce the chances of detectable lip sync error when using Bluetooth audio output. Until then, Apple (and other wiser manufacturers) are likely to preserve the option of wired analogue audio output on Macs and iPads. That way, if you do find lip sync error is troubling, you can always use conventional wired headphones.

If you observe lip sync error sufficient to trouble you, minimise other Bluetooth use, clean up your network options, and ensure that your headphones and the player system have fully up-to-date firmware and software. You may find that different headphones make a significant difference, but you’ll need to test them out carefully before committing to purchase: some might actually make the error worse.

So for the time being, Bluetooth headphones and earphones may be excellent for listening to music, but they can still have snags when you watch a movie instead.

Reference

ITU-R BT.1359-1 (1998) contains explanation of the recommended maximum delay between audio and video.