skipping of DTMF signals from GSM module

[piyushpandey]

Hi Guys


I am facing a very serious problem in case of GSM.

Actually I have DTMF receiver of HOLTEK named HT9170 which I have interfaced with the GSM module named AT139D from the ATWIN company.

The interfacing is as follows :

The SPK+ pin of the AT139 has been interfaced with the pin 2 junction point as shown in the schematic via a capacitor in series of 4.7 uF. The SPK- pin remain not connected.

The PNT line is also connected to the same junction and is input to the dtmf receiver.

Now the problem is that the dtmf signals are properly decoded by the ht9170 from the PNT line but in case of the GSM line these dtmf signals are skipped basically digits 4 and 5 are skipped by the dtmf receiver and 1,2,3 and # are properly decoded by the dtmf receiver.

I am attaching hereby the circuit connection of the DTMF receiver.

I want to know that is there any difference between the dtmf signals from the gsm line and from the normal telephone line.

Also is there any special schematic or circuitry for connecting the GSM modules with the dtmf receivers like ht9170.



Thanks and regards