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- General Modem discussion FAQ

Voice Modems

How can I tell if my modem is voice capable?
Posted: 28 Jul 03

Voice capable modems are easy to identify.  All that you need is a terminal emulation program, like Hyperterminal, and a basic familiarity with the AT command set.

Open the modem that you want to check in Hyperterminal and enter anything that you want in the number field.  While the modem is attempting to dial, press the ESC key or Cancel button.

You should have a blinking cursor in the terminal window.

Type AT+FCLASS=? and you should receive a response that contains a 0 and one or more of the following: 1, 2, 2.0, 8 or 8.0.

    0      Data
    1      Class 1 Fax
    2      Class 2 Fax
    2.0    Class 2.0 Fax
    8      Voice
    8.0    Voice View

There are two general command sets for voice modems.  One for Rockwell (Connexant) and another for Lucent (and others) chip set modems.  It is easy to tell the difference between the two, as most voice commands in the Rockwell modems begin with a # while most of the Lucent voice commands begin with a +.

You can issue the command AT#CLS=? and if you do not get an ERROR, then you have a Rockwell chip set.  If you get an error response, then it is a Lucent chip set.

There are also two categories of voice modems, full and half-duplex.  The terminology is the same as it was for data modems, back when modems were primarily half-duplex.

Half-duplex indicates that the modem can send audio, or receive audio, but not both at the same time.  Modems that are half-duplex in voice mode are generally referred to as answering machine or voice mail capable.

In contrast, full-duplex modems are capable of sending and receiving audio simultaneously.  Modems of this type are referred to as speakerphones.  Many full-duplex voice modems will have one or more jacks on the back that allow them to be connected directly to the sound card.  Ideally, these would be connected to the TAD (Telephone Answering Device) port on your sound card, but that is usually a different connector type, and is inside the computer case rather than external.

Half-duplex modems are useful for the following applications:
    IVR (Interactive Voice Response)
    Answering Machine
    Voice Mail

Full-duplex modems are useful for all of the above applications, plus:
    Speakerphone

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