Can you clarify the performance of the GSM voice channel for custom DTMF signalling?
A – If I’ve understood well how the application works, the DTMF tone is sent through the voice channel from the microphone of the module and received on the speaker of the second module/phone like a voice signal. This could cause problems because the GSM vocoder cannot manage single or double tones (the voice is modulated) and sometime it can start modulating its amplitude with a variation from 0% to 100% depending on the frequency. This is not a specific problem of Telit or other manufacturers’ modules but it is related to the GSM technology. Only constructed tones like the one generated by AT+VTS can be received without any problem because the tone is not sent on the voice channel as an analogue signal but as a digital one and regenerated by the DSP receiver.
