iPhone headphone audio jack re-routing

Question

We created an external iOS notification light that uses the device’s audio for power. When you get a phone call on iPhone and the light is plugged in, you still get the ringtone but when you pick up, the audio is rerouted to the headphones (the iPhone thinks our light/device is a headphones set) and the user has to extract myLED for at least 2mm to get the audio from the front receiver of the phone.

We have been exploring alternative solutions to this challange - recently we made a prototype with a particular jack shape so that it could be rotated by the user when getting a call to "reroute" the audio to the iPhone speaker/mic. Although it may sound a clever option, this hardware solution is far from being neat - this leads to having positions where the myLED does not work/ it is not reliable, plus other complications.

I know of the existence of kAudioSessionOverrideAudioRoute_Speaker however I suspect that this will only direct the app audio to the rear speaker (the “loud” one) and not to the front receiver (because the “receiver” for the iphone is the headphones set if they are detected).

What would you suggest? Super appreciated!

Answer 1

I think you're in a tough spot:

1. It's highly unlikely Apple will ever release the option to override audio routing for phone calls. As a key functionality of the phone, they tend to keep the call aspect under lock and key.

2. The headphone jack (probably - this is how most of them do it) uses the impedance between ground and one or both speakers or the remote control to determine if the plug is in. Other than breaking the circuit, there is no good way to simulate this.

The only options I think you have are these:

1. Require the user to remove the device when a call comes in.

2. Provide a microcontroller on the jack to drive a transistor; this transistor can electronically break the circuit to provide the same sort of impedance signature as an unplugged jack.

How, when, and if you can provide the information to the jack that a phone call is in progress is beyond my knowledge: is there an API for "incoming but not yet answered call" you can hook to? Will you have to do a watchdog thing to ensure communication with your app? Would it be possible for you to use the dock connector instead? I think these are really your options. Not a complete answer, but those are my thoughts.