EMI (ie GSM) noise in speaker and DTMF module. How to get rid of this ?

Using Arduino Audio
1

Hello folks,

I got some noise in my circuit that puzzle decoding DTMF tones.

I am using a SIM900 shield (1), with a MT8870 module (2), even if really traditional I put some link below (1) and (2) so that it is possible to see what I am referring to exactly. I am interfacing all of this with an arduino uno.
Simply I plug the shield to the arduino, and I connect the module to the shield, also as expected.

All work fine, but the sim900 produce Electromagnetic Interference (EMI) noise, An example of such noise could heard at link (3).

The problem is that noise is sometimes troubling the DTMF tone, which prevent identifying the tone. there is no error but the tone in simply not identified. This is troubling when it comes to enter a code. Let say I want to enter the passcode "1234", maybe the decoder will get "124" because there were some noise during the "3".

Anybody can help please? Give some hint, how to deal with this?

Best regards,
Buenavista

Sim900
(1): Arduino GPRS (GSM) shield (SIM900) no long pins - Arduino Stack Exchange

Module MT8870
(2): MT8870 DTMF MODULE | Geek Electronics

Noise:
(3): Cingular GSM Speaker Interference - YouTube

2

Sounds like the DTMF module doesn't have proper RFI suppression on its input? Try 1nF ceramic cap between
its audio input and its ground right at the connector (on back of the PCB, very short leads or SMT capacitor).

3

Thank. Back on this topic after a bit of holidays,

I tried to filter the noise but that down-played the sound to a level where MT8870 cannot hear anymore sounds.
I circumvent the issue by decoding DMTF tone by the SIM900 directly rather than MT8870. It now works very nicely.

4

Try 100pF