I'm transmitting data from a microcontroller to a smarphone mic input and I was suggested to use this circuit to adapt the 5v digital signal:
Taken from this tutorial
The way I see it R2 and R3 have effect on the filter but their main function is to scale down the signal to aproppiate levels. The circuit is clearly a high-pass filter and I think its only function is to eliminate the DC offset. But why would they do it if the audio input does the AC coupling itself?