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Soundmodem is a technique to use an ordinary sound card as an analogue-to-digital converter like a TNC does.

<?xml version="1.0"?>
  <configuration name="AX25">
    <chaccess txdelay="1000" slottime="100" ppersist="40" fulldup="0" txtail="100"/>
    <audio type="alsa" device="plughw:0,0" halfdup="1" capturechannelmode="Mono"/>
    <ptt file="none" gpio="0" hamlib_model="" hamlib_params=""/>
    <channel name="sm0">
      <mod mode="afsk" bps="1200" f0="1200" f1="2200" diffenc="1" inlv="8" fec="1" tunelen="32" synclen="32"/>
      <demod mode="afsk" bps="1200" f0="1200" f1="2200" diffdec="1" inlv="8" fec="3" mintune="16" minsync="16"/>
      <pkt mode="MKISS" ifname="sm0" hwaddr="VK2HAQ-1" ip="" netmask="" broadcast="" file="/dev/soundmodem0" unlink="1"/>

Noteworthy in this file are the following parameters:

  • txdelay -- set this to be long enough for the radio to open on transmit (default is 250ms)
  • plughw:0,0 -- points to the first sound card (1,0 would point to the second), make sure the order is configured properly (see here (page missing))
  • ifname -- this needs to match the network interface and is also referenced in Rpi-aprs-axports
  • file -- points to the /dev/soundmodem character device, even though the system will not show it in /dev (do not use the sm0 interface)