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Comment by teraflop | Hacker News Reader

On a tangentially related topic:

When I used to listen to music on my previous phone (a Nexus 4) I would have to constantly adjust the phone's volume: all the way to maximum when using an aux cord in my car, and way back down when using headphones. Recently I upgraded to a Nexus 5X, and noticed a surprising new feature. Somehow, the phone detects which device it's plugged into, and remembers a separate, previously-set volume level for each device.

Does anyone know how this works? It's really convenient, which makes it surprising that I can't find it documented or even mentioned anywhere online. I'm assuming the audio hardware is somehow sensing the impedance of whatever it's connected to, but is that really a reliable indicator of headphones vs. line-level inputs?

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In my opinion as an EE there would be some reasonably reliable ways to detect either a line or headphone connection. Headphone impedances are typically between 16 and 600 Ohms, and line impedance is typically in the 10's of kOhm range. That's one way to differentiate. There has to be an output signal (presumably audio) first, however. Another way is to apply a very small DC current (that would certainly not damage headphones, so I would guess in the micro-ampere range) and check if there is continuity or measure the DC resistance of the load outright. For line inputs, as they're usually AC-coupled, DC resistance would be infinite, and for (DC coupled) headphones it is very low. It's an interesting subject and I wouldn't be surprised if there are some patents involved.

Detecting AC vs. DC coupling would be obvious to a practicing EE, if presented with the problem, yes? I'm just a software guy with an electronics hobby and I think I could have figured that one out.

That said, DC vs. AC coupling detection could probably be granted a patent if nobody had "disclosed" the idea prior to filing of such a patent, based on my experience with software patents.

Sorry to vent, but patents on this sort of thing drive me crazy. I don't have any gripe with you or your post.

I did a quick search on google patents: https://www.google.com/patents/US9084035 - it uses a combination of AC and DC methods.

> For line inputs, as they're usually AC-coupled

Headphone amplifiers often have DC-coupled 10k potentiometer from input to ground with washer feeding a DC- or AC-coupled gain stage. So much for AC/DC detection.

I think impedance measurement is used in practice. I can hear a quiet beep when I connect headphones to one of my laptops, for example.

A uA DC current can leak through a shitty AC-coupling capacitor, actually measuring the load is probably best.

Most devices just distinguish between TRS (Tip-Ring-Sleeve, or regular stereo mini jack) and TRRS (with an extra ring) connectors, with a separate volume level for each.

Line-level inputs will be in excess of 1 KOhm (10KOhm if properly designed) Headphones will be up to 600 Ohm (I guess some boutique brands could have examples higher but I digress, the grand majority is between 16 and 250 Ohm).

At least some MBP's and iPhones do this too (e.g. mine). In my opinion (though less reliable than the sibling's because I'm an ME not a EE) impedance detection is probably the easiest way to do this.

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