Chapter 5. Using Packet Modes

5.1. Soundmodems for AX.25

A soundmodem allows a standard PC sound-card to be used as a packet radio "modem". By connecting the speaker and line-in (or microphone) ports to the radio, AFSK signals can be generated and decoded by soundmodem software. Typically, interface cables for radios will also have PTT control via a serial port, or an emulated serial port by USB. PTT control is not required if your radio has VOX functionality, but there may be issues arising with the PTT being held for too long after a packet is transmitted or having delays in starting to transmit and missing the start of a packet.

This handbook will introduce two popular soundmodems: direwolf and soundmodem.

5.1.1. Dire Wolf

Dire Wolf is a software "soundcard" modem/TNC and APRS encoder/decoder. It can be used stand-alone to receive AX.25 or APRS messages, as a digipeater, APRStt gateway, or Internet Gateway (IGate). It can also be used as a virtual TNC for other applications such as APRSIS32, UI-View32, Xastir, APRS-TW, YAAC, UISS, Linux AX25, SARTrack, RMS Express, and many others.

Dire Wolf is packaged in Debian as direwolf with the documentation packaged as direwolf-docs. Both of these packages can be installed with APT and will be installed if you install the hamradio-packetmodes blend metapackage.

You can get started with Dire Wolf very easily if you only have one sound card in your machine:

$ direwolf
Dire Wolf version 1.2
ERROR - Could not open config file direwolf.conf
Try using -c command line option for alternate location.
Audio device for both receive and transmit: default (channel 0)
Channel 0: 1200 baud, AFSK 1200 & 2200 Hz, E+, 44100 sample rate.
Note: PTT not configured for channel 0. (Ignore this if using VOX.)
Ready to accept AGW client application 0 on port 8000 ...
Use -p command line option to enable KISS pseudo terminal.
Ready to accept KISS client application on port 8001 ...

By default, Dire Wolf uses the first sound card available and does not configure computer controlled PTT. It listens for AGW applications on TCP port 8000 and KISS applications on TCP port 8001.

You can configure the behaviour of Dire Wolf to use a different soundcard or to use computer controlled PTT by creating a direwolf.conf file. For more information on this, and on configuring Dire Wolf to act as a digipeater or APRS IGate, see the main Dire Wolf documentation. If you have installed the direwolf-docs package, this will be available here.