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There is a lot of information out on the web about soundcard/radio interface hardware. If you decide to use any kind of software that employs your computer's soundcard as a TNC, you will need to buy or build such an interface. (If you use a serial or USB connected external TNC then you don't need a soundcard interface.)

This device is not mode or band specific. You can use the same device that you employ for other digital modes (PSK, slowscan, RTTY, etc.), as long as it will connect to the radio and the computer combination that you are going to use for APRS. For example: if you use your Icom 706 mkIIG (an all-mode HF through 70cm rig) for PSK-31 on HF, you probably can use the same soundcard/radio interface device for APRS. Just set up the radio for the mode, band and frequency used in your area for APRS operation, and run your APRS software instead of your PSK software.

The task is to connect the audio inputs and outputs between the radio and soundcard. Solutions range from direct connection to electrically isolated systems using audio transformers or optical coupling. Some provision for keying the radio is also useful. Usually this involves a connection to the computer via serial port (USB or rs232) or parallel port.

This is a good opportunity for doing a little junkbox homebrewing. The guts of old computer modems usually contain audio isolation transformers, rs-232 ports and other hardware bits.

For an off-the-shelf solution, West Mountain Radio's RigBlaster units are popular.

As to software for doing APRS over a soundcard, investigate LinuxSoundmodem for Linux or Solaris, and AGWPE for Windows. Dire Wolf works on both Windows and Linux. All of these will do the tone encoding/decoding, then you need a compatible APRS program to connect to them.