Why does the iPhone 4S treat a generic 30 pin cable as a line out source?

Question

I have an iPhone 4S, the original iPhone 4S AC to USB adapter, and a generic Dynex brand 30 pin cable. When ever this cable is connected to the iPhone sound will not play over the built in iPhone speakers. If headphones are plugged in they work fine, but when they are removed sound will not play on the iPhone's built in speaker. If I swap in the original cable that came with the iPhone it works as expected. Any ideas why the iPhone 4S thinks this cable and configuration is a line out source?

Answer 1

That's a third-party cable, not an official Apple cable, so what's probably happening is that for some reason that cable is bridging the 2nd and 11th pins, telling the device to send the audio out the 3rd and 4th pins:

From http://pinouts.ru/PortableDevices/ipod_pinout.shtml:

2 GND Audio & Video ground (-), internally connected with Pin 1 on iPod motherboard

3 Right Line Out - R (+) (Audio output, right channel)

4 Left Line Out - L(+) (Audio output, left channel)

11 AUDIO_SW If connected to GND the iPhone sends audio signals through pin 3-4, otherwise it uses onboard speaker.