I have an audio signal made by PIC microcontroller using 20KHz PWM (It passes through a low-pass filter). It is very clear on an 8ohm speaker. But when I connect the output to the MIC inputs of SIM800 module, It has a very bad quality on a phone call and can not understood at all. I connected audio output directly and through a 100nF ceramic capacitor, but in both cases the audio was very bad. How should I connect audio output to MIC input of SIM800.
simulate this circuit – Schematic created using CircuitLab
I already have seen that output of ISDxxxx (audio recorder and playback IC) is connected to MICP pin of a SIM900 just through a capacitor in a firewall system and the audio quality was very good.