I've been tearing my hair out over this one: when recording audio or video with my iPhone, I can't figure out how to use the lightning port for an audio-in signal.

For example, I'm filming someone playing an instrument, and I want to use the audio-out of that instrument as the direct sound source for the video. (without holding a microphone near the speakers...)

I have tried a 3.5mm 4-pin minijack cable from the instrument to the iPhone (with the minijack to lightning converter) but this 4-pin audio cable is not recognised as a microphone/sound input. I have also read an article about soldering your own cable with the right resistor values, but that can't be the only way to record audio on an iPhone???

The only microphone that gets recognised is the one that is built in the official apple EarPods.

How can I record an audio source directly into my iPhone?