5
Our goal here: We want the PC to call a designated phone number when something happens, and report it in easy-to-understand spoken audio snippets which are saved in the form of pre-recorded wave files.
PART I: Find out about your modem's capability.
Most modems can be classified with these characteristics:
Data/fax
Data/fax/voice
Data/fax/voice/speakerphone
In our case here, the modem at least has to support the voice feature. The easiest way to find out if your modem supports voice is to run the HyperTerminal to use AT commands.
Run HyperTerminal and use the following commands:
ATZ
The modem should respond with "OK"
AT+FCLASS=8
If your modem supports voice, it should come back with "OK". Otherwise, "ERROR".
If you are interested, you can type AT+FCLASS=?
The modem should respond with something like: 0,1, 2, 8.
0: Data
1,2: Fax
8: Voice
If it turns out you don't have a voice modem, then we're stuck. If you want to go further with this approach, you'll need to install a modem with voice capability.
Now, if you do have a voice modem, you need to find out about its capabilities regarding the voice data formats it can handle. Use the following AT command to find out:
AT+VSM=? (Remember that AT+FCLASS=8 must be sent prior to sending this command.)
Our modem here comes back with the following:
AT+VSM=?
128,"8-BIT LINEAR",(7200,8000,11025)
129,"16-BIT LINEAR",(7200,8000,11025)
130,"8-BIT ALAW",(8000)
131,"8-BIT ULAW",(8000)
132,"IMA ADPCM",(7200,8000,11025)
OK
We interpret this as: Our modem supports 5 different methods of encoding voice data. If you are interested in learning more about these methods, just Google them. In this example, we will focus on the first method (the simplest one). This method uses 8-bit linear (signed numbers) and supports the following sampling rates:
7200, 8000, or 11025 samples per second.
In theory, most human voice frequency falls between 1kHz to 4kHz. Sampling theory (Nyquist) indicates that you will have to take at least 8kHz in order to reconstruct voice content up to 4kHz. Higher than 8kHz would be better to avoid anti-aliasing. However, higher sampling rates come with the price of heavier data load.
Now, come back to our example, we will choose: 8000 samples per second which is 8kHz. We tell the modem our selection by typing the command:
AT+VSM=128, 8000 (Should get "OK" back.)
Following the above procedure, you should now know if your modem supports voice, and if so, what voice data format it uses. You have to choose the data format now in order to prepare the wave file which will be the topic in PART II .
