Apple has consistently worked at making the iPhone ever slimmer, and has been willing to make compromises to achieve that, most notably in battery-life. But with the iPhone 6 and 6s, it is close to the limit on how slim an iPhone can be – and the reason for that is the oldest piece of tech in the phone. The iPhone 6/6s is not very much thicker than the diameter of the 3.5mm headphone jack.

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The latest iPod touch shows that Apple has a little more room for manoeuvre (above photo Anandtech, below iFixit).

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But really not much. If Apple wants to continue the iPhone’s diet, at some point very soon it’s going to have to ditch the 3.5mm headphone socket in favor of an alternative. There are four possible options open to it … 

First, it could replace the standard 3.5mm audio socket with a 2.5mm one. While 3.5mm has been the standard audio socket for mobile devices for a long time, it’s not the only option out there.

Audio kit originally used a quarter-inch socket (6.3mm) – and high-end home kit often still does. That size was too large for mobile devices, so the 3.5mm system was devised to solve the problem, but some manufacturers went further and opted for 2.5mm instead.

2-5mm

The 2.5mm system never really took off, as 3.5mm was small enough for all the mobile devices that existed at the time, but it’s still out there and Apple could adopt it, shaving off a millimeter by doing so.

There would be some advantages to doing this. You could snap a small 3.5mm to 2.5mm adapter onto any set of headphones without any compatibility issues, and Apple wouldn’t need to make any changes to the electronics inside the iPhone. Adapters are cheap, so Apple could even throw one in with the iPhone, so no-one could grumble about being forced to lay out cash on a new adapter.

One commentator reminded me that Apple has patented a half-height jack. This, too, would require nothing more than a low-cost adapter.

But all this is old tech. If Apple is going to do anything as radical as drop an industry-standard socket, it’s likely to replace it with something hi-tech.

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Which brings us to the second option: drop the audio socket altogether, and co-opt the Lightning socket into performing audio duties as well as power and data. A recent report claimed that Apple plans to do that for the iPhone 7.

That’s possible right now, of course. The Lightning socket can already deliver audio signals, and already does so with a range of audio docks. You can also already buy headphones with Lightning plugs instead of 3.5mm ones (though admittedly the selection available today is rather small).

There are both pros and cons to this approach. On the plus side, Lightning delivers a lossless digital audio signal. This means that headphone manufacturers can include their own digital-to-analog converters, some of which would be of a higher quality than the one built into the iPhone.

Apple’s Digital Audio Module standard also allows headphones to draw power from the iPhone, negating the need to charge the headphones – though at further cost to battery life. Conversely, though, the headphones could supply power to the iPhone, and the protocol supports pass-through power, allowing you to listen to music while the phone is charging.

The big downside, of course, is that you can’t use existing headphones. Because the signal provided by the Lightning port is digital, a simple converter won’t do the trick: the headphones need an on-board DAC to convert to analog sound.

This would normally be a killer argument against the change. Many of us have expensive headphones we love, and we wouldn’t be amused by a new iPhone effectively rendering them obsolete for mobile use.

But there is a potential get-out clause here. It should be technically possible for a DAC in the iPhone to handle the conversion to analog signals, at which point a simple snap-on 3.5mm to Lightning adapter would be all that would be needed. If Apple took that route, existing headphones would continue to work just as they do now.

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The third option, and one a sketchy rumor suggests Apple is playing with, is to replace the Lightning socket with a USB-C one.

You could see an argument for this. The USB-C is an extremely powerful standard that could do everything an iOS device needs and more. Audio is just one of its many capabilities, and it wouldn’t be a problem for manufacturers to create USB-C headphones.

Apple has already adopted USB-C for the 12-inch MacBook, and will almost certainly do so for the complete MacBook range next year. There is something to be said for Apple standardizing on a single port across all devices.

But there are three reasons I think it won’t. First, there was enormous controversy when Apple abandoned the old 30-pin connector in favor of Lightning. Many people were vocal in their objection to Apple making old docks and audio kit effectively obsolete – or at least both fiddly and ugly by the time you added an adapter. It was a change that had to be made eventually, but Apple certainly won’t be in a hurry to make a second change so relatively soon after the first.

Second, while USB-C is incredibly powerful, an iPhone doesn’t need most of those capabilities. No-one is going to be hooking up an iPhone to a Thunderbolt display, and an iPhone doesn’t have the computing power to do half of it anyway.

But the third reason is the real convincer. If creating ever slimmer iPhones is Apple’s reason to abandon the 3.5mm socket, switching to USB-C wouldn’t help much. Product designer Josh Flowers helpfully created this graphic to show the problem: the USB-C socket is pretty much the same height as the 3.5mm socket.

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So for all these reasons, I think we can rule this one out.

wireless

Which leaves one final possibility: simply remove the headphone socket and point people to wireless headphones as the future.

This would, of course, bring us right back to the ‘existing headphones’ objection. If someone has spent $400-500 and up on a pair of expensive headphones, they are not going to be impressed by Apple providing no means to connect them to an iPhone. You could continue to use them with a Bluetooth headphone adapter, but that’s a very clunky solution.

I’ve often observed that Apple has no problem making ruthless decisions when it comes to what it considers legacy technology. Floppy drives. Optical drives. Upgradable laptops. Most ports, in the case of the 12-inch MacBook. Apple consistently does this earlier than almost everyone else.

But while I’m sure wired headphones will, within a few years, seem as quaint as loading a DVD into a laptop to watch a movie, I don’t think we’re there yet – even for Apple.

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So, what will Apple do? My money’s on the Lightning route, with an on-board DAC to maintain compatibility with existing headphones. I think that could realistically happen by the iPhone 7, and I’d put good money on it happening no later than the iPhone 8.

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