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Use iPhone as a speaker phone in the car
Authored by: u2mr2os2 on Jul 28, '08 05:27:38PM

It would work for me. The thing is to understand what the iPhone does so that you can debug what's going on.

If you have a Bluetooth headset connected to the iPhone, the iPhone will route the call to that first. So, if you want to use the FM transmitter for call audio, you should turn off the headset. This is also why I've been put into the FM transmitter speakerphone mode inadvertently, because my BT headset was off or had become unconnected.

The iPhone will route call audio through the headphone jack if connected rather than the built-in earpiece. This is what happens with the headset that comes with it. However, that headset also has a mic and a 4 conductor plug to go with it. When a 4 conductor connector is plugged into the headphone port, the iPhone routes both the speaker and mic audio to the headset. Your FM transmitter and ordinary headphones without a mic only have a 3 conductor plug. If a 3 conductor plug is in the headphone jack, the iPhone will only route the call speaker through the connector and use the iPhone's built-in mic to pick up your voice.

Another problem that can cause you to not hear the caller's audio even if your FM transmitter setup is working is the half-duplex speakerphone effect. If your iPhone's mic picks up enough sound, it will send that audio, but mute the caller's audio to avoid picking that up and resending it as well, creating feedback. The problem is that as long as your mic is picking up sound, it will keep the caller's sound muted. You may have been on the other end of this sometime where you couldn't seem to be heard by the other person who had you on speakerphone, but had noise on their end, keeping your audio muted.

I think the iPhone goes into a speakerphone mode when the earpiece audio is going out the headphone jack and it is using the built-in mic. It doesn't when both go out the headphone jack as with the headset it comes with. Of course, this situation is easy to come by in a noisy car cabin, but could also explain why not everyone has the problem (they might have a quiet car).



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