I have an audio signal made by a 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 be understood at all. I connected the 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 the MIC input of a SIM800?
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.