Actually I am doing a Project. GSM Modem is interfaced with the PC. Remotely a mobile phone will dial to GSM Modem(SIM inserted). Is it possible to receive the voice signal trough the modem to PC? For SMS there is no any problem, we can easily receive using the AT commands.
I was wondering is there any AT commands to receive the Voice Signal and Transmit it to Computer?
Best Answer
On the cellular modems I have worked, the voice path was set up by default to interface to a microphone and speaker. However most had a separate PCM interface which allowed the cellular modem to be connected to a Bluetooth headset via a Bluetooth module, provided it had a compatible PCM interface.
According to this website, the format is typically a 64 kb/s log PCM format (A-law or μ-law) or a 64 kb/s CVSD (Continuous Variable Slope Delta Modulation). So it seems you should be able to digitally interface this with a computer, and decode the modulated audio.
On one of the chipsets I looked at (which is pretty typical), the PCM interface consists of 4 pins: PCM_SYNC, PCM_DOUT, PCM_DIN, and PCM_CLK. It supports linear (13-bit), A-Law (8-bit), and µ-Law (8-bit) companding algorithms. The AT commands to control the PCM interfaces all appear to be proprietary, unlike the standard (AT+) set of ITU-T V.250 commands supported across all GSM chipsets.
Besides using the PCM interface, the other way of course would be to feed the speaker output into an ADC of the micro. I have done the latter, when I needed to do echo cancelling on a speakerphone application (the cell module supposedly was going to have a function to do that, but it was not implemented in the firmware yet.)