RPi headless aprx server using soundmodem

I have posted several articles on headless APRS servers based on RPi. This article describes one based on RPi, AX25 soundmodem using a $2 USB sound card, and aprx v2.09.


Above is the server hardware. It uses the Sailer sound modem and kernel AX.25 support. This is currently configured as a RX only iGate for 30m, hence no tx audio path and no PTT (though on HF, PTT can be done simply using transceiver VOX). The small black USB module is a Belkin WLAN adapter.

I am not sure there is a lot of value in transmitting, as it would be beacons only (surely no one iGates to HF)… but plenty of other hams transmit frequent beacons, they are innate propagators… use it or lose it is their mantra… but I think they lost it a long time ago!

The node draws about 0.1A at 12V, and combined with a low current receiver (eg Codan 7004), total current is less than 0.2A @ 12V, <2.5W.



Additional to installing aprx v2.09, the kernel AX.25 utilies and user space soundmodem app need installing:

apt-get install libax25 libax25-dev ax25-apps ax25-tools soundmodem screen

The file soundmodem.conf contains the configuration details for soundmodem:

xml version=”1.0″?>
<configuration name=”HF”>
<chaccess txdelay=”150″ slottime=”100″ ppersist=”40″ fulldup=”0″ txtail=”10″/>
<audio type=”alsa” device=”plughw:1,0″ halfdup=”1″ capturechannelmode=”Mono”/>
<ptt file=”none” hamlib_model=”” hamlib_params=”” gpio=”0″/>
<mod mode=”afsk” bps=”300″ f0=”900″ f1=”1100″ diffenc=”1″/>
<demod mode=”afsk” bps=”300″ f0=”900″ f1=”1100″ diffenc=”1″/>
<pkt mode=”MKISS” ifname=”sm0″ hwaddr=”MYCALL-4″ file=”/dev/soundmodem0″ unlink=”1″/>

The port needs to be added to axports:

sm0     MYCALL-4        300    255     2       HF APRS (300bps)

The AX.25 kernel port is specified in aprx.conf:

ax25-device   $mycall