In late December 2007, after returning from holiday in Upstate NY with a RadioShack PRO-94 scanner, I ended up deciding to get back into the hobby with renewed interest. One of my first projects was a simple APRS viewer.
In my case, I run Kubuntu Linux. I have a laptop running Windows XP, so my first hunch was to set up AGWPE on it. Turns out my laptop's audio interface apparently artifacts the audio sufficiently as to corrupt packets.
So I went to my Linux desktop (my preference anywho). First, I found the Multimon application. Multimon is a handy CLI (Command-Line Interface) utility to decode AX.25 (packet radio), POCSAG, DTMF, and ZVEI. It can use a SoundCard live, or can use other inputs (.WAV file comes to mind). I now know that my sound card is sufficient to decode 1200baud AFSK, and the audio is tuned (using Multimon's built-in "scope").
My next step was to find a simple way to turn my SoundCard into a
packet modem. After a brief search, I found the Multiplatform Soundcard Packet Radio Modem application. After a brief configuration with soundmodemconfig, I now have it set up so I have a serial modem interface at /dev/soundmodem0. Of course, I can only receive on this device because of my hardware configuration, but it can just as trivially support full duplex.
With the soundmodem configured and running (by executing the `soundmodem` application), I now need an APRS application. The Kubuntu repositories yielded Xastir. After configuring it to use my SoundModem (I used 1200/8/n/1 on /dev/soundmodem0), it started to receive incoming data! This program seems to be quite fully featured, showing iconically the station type and decoding Wx data automatically. You just need to try it!
Note that I used a very simple hardware setup... That is, a standard audio patch cable from the headphone jack of my scanner to the Line In jack of my sound card. For transmit capabilities, the radio should be isolated from the sound card (probably should be anyway, but whatever), and a PTT circuit would need to me made. This page seems to illustrate that well.
For digital operations, it is common to create an interface on your radio that grabs the audio before any filtering and amplification (/me forgets the name of the stage). Note that many radios have this built in (usually via a DIN [round] or DB9 port). The amplification and filtering stages can distort the digital tones. However, for 1200 baud, the microphone and speaker connections are quite sufficient.
Hope this helps! Feel free to contact me with any questions.
