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?

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    \$\begingroup\$ Is there any method to access the voice received in GSM modem using the AT command? It is very easy to access the SMS from the GSM modem like Wavecomm .. \$\endgroup\$ – Bibek Apr 23 '12 at 5:45

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.)

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    \$\begingroup\$ Thank you for sharing your Opinion. I also want to ask that is there any AT command to access the Voice received in GSM Modem ,like the SMS ? \$\endgroup\$ – Bibek Apr 23 '12 at 5:42
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    \$\begingroup\$ @Bibek I don't have the manuals at home, so I'll have to check at work tomorrow. There would be no way to actually retrieve the voice over the channel used for AT commands (e.g. as data), but there would be commands to route it to the PCM port I mentioned. \$\endgroup\$ – tcrosley Apr 23 '12 at 6:25
  • \$\begingroup\$ @ Thomas ..Thank you !! surely i will wait for your reply. So from the GSM modem we can route the voice to PCM port and then from the PCM Port we can access it ? \$\endgroup\$ – Bibek Apr 23 '12 at 6:56
  • \$\begingroup\$ I checked the manuals for a couple of chipsets, and updated my answer with some additional details re the PCM. \$\endgroup\$ – tcrosley Apr 23 '12 at 17:57
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    \$\begingroup\$ @Bibek The problem with most of these modules is that you can only get them through OEM channels. The Telit GE865-QUAD is an exception. You can get one at the SemiconductorStore.com -- search for GE865-QUAD. The only gotcha is that it is a BGA. They sell an interface board, which I believe is supposed to be used with their pricey EV2 evaluation kit, but it might be possible to use it standalone. \$\endgroup\$ – tcrosley Apr 24 '12 at 18:51

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