Noise in audio if I feed audio (PWM output from an audio module) to mic of my SIM900

[sherazi]

I am feeding audio from a Sound module wtv020sd-mini which is PWM , to the mIC PIns of My GSm MOdule Sim900, when I hear this sound ona a speaker although its not a high qulaity still its fair enough, but when I play the same voice and feed it to Sim900 MIc and listen to it over a call its quality is very poor, theer are alot of humming sounds and voice qulaity itself is also low.

I wana use the modulie for voice response to callers , Is it all because of PWM output ? I used another module from ISD17150 and its voice qulaity is very nice even on the call.

 

[FvM]

A possible problem is residual PWM frequency in the audio signal, that can be demodulated by the GSM module. You should provide sufficient low pass filtering. Speech isn't transmitted as analog signal in GSM, it's processed by a kind of Vocoder to reduce signal bandwidth. A worst case, the pwm generated signal, although accepted by your ears, is unsuitable for GSM transmission.

 

[sherazi]

A possible problem is residual PWM frequency in the audio signal, that can be demodulated by the GSM module. You should provide sufficient low pass filtering. Speech isn't transmitted as analog signal in GSM, it's processed by a kind of Vocoder to reduce signal bandwidth. A worst case, the pwm generated signal, although accepted by your ears, is unsuitable for GSM transmission.

so how should I deal with it ? would passing the PWM output through a lowpass filter work?? if so can u please guide how to acheieve that , I have no experience of fusing filters !