A High-Performance Sound-Card AX.25 Modem

My APRS RF-to-internet gateway (iGate) had been running  a sound-card modem called soundmodem by Thomas Sailer. The software modem caused various problems and I have not found a suitable replacement. Eventually, I decided to try to implement my own sound-card-based software modem. The results have been very good, in spite of the fact that I do not have much background in digital signal processing.

I’ve been using the modem for many months now in my iGate. The modem is run is a component of javAPRSsrvr, not as a separate program. It’s working reliable non-stop.

An article that I wrote for QEX explains how the modem works and how I designed and optimized it. It is also available on my university web site.

5 Responses to A High-Performance Sound-Card AX.25 Modem

  1. Thanks for making this available. Very informative article for those of us who use APRS but weren’t around for packet radio the first time around when the protocols were written

  2. Is there any further documentation on compiling this? I am not familiar with Java, but it seems when I use `ant compile` it is looking for other libraries or source files that I find no reference for online: “com.ae5pl.nsutil” is the first.


    • Hello again,

      I managed to get it working with a friend who does Java development. You might want to make note in the README that you need two jar files to make it work: RXTXcomm.jar and javAPRSSrvr.jar

      I’m successfully decoding packets off the air as I type! I also forked your code on github and look forward to rounding it out to meet my requirements.

      Thanks for sharing the code and for the awesome write-up!

  3. Steve G6UIM says:

    working ok on receive trying to get ptt operation going on Windows 7 x64, any help would be appreciated, keys the PTT the first time, the I get “get_java_var: invalid file descriptor” can anyone point me in the right direction

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