SoundModem

[last updated: 2022-05-04]
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• SoundModem is a software driver for Linux (and perhaps others) which turns soundcard hardware (like SignaLink) into a software TNC.
  Is this correct?? saw a forum post that SoundModem was not an option under Linux (except perhaps with wine?) but that SM is "preferred" for Windows...
• I installed it on tower with $ sudo apt-get install soundmodem ... on 1/16 ... version 0.20-5
• Per notes in (Link 28), there is indeed a file:
  /usr/bin/soundmodemconfig ... used to configure it
  but $ sudo soundmodemconfig works too
  It also says that soundmodemconfig will write your chosen config parameters to:
  /etc/ax25/soundmodem.conf
• Other info from the link:
• "This section derived from a posting by Dave Platt, AE6EO, with his permission:
• I've been able to make a soundmodem 1200-baud connection using a SignaLink USB
  (setting the soundmodem PTT to /dev/null, since the SignaLink USB has its own vox-driven PTT circuit).
• Some tricks:
• If possible, use the direct ALSA "hw:" interface, and set the soundmodem's sample rate equal to one of the "native" rates supported by your USB sound card.
  This ensures that no DSP- or CPU-based sample rate conversion needs to be done.
  With some USB sound cards this may not be possible... there are those which have *different* sample rate sets for input and output, and might not even have a single rate in common which works in both directions. In this case, specify the ALSA "plughw:" interface, so that ALSA's sample-rate conversion plugin does the necessary rate conversions... it seems to work quite well.
• Do *not* use the OSS compatibility driver for ALSA (e.g. the /dev/dsp interface). Although this driver does attempt to perform sample-rate conversion, it doesn't do it well... I've seen periodic dropouts in the incoming data (sequences of sample values of zero) whenever the buffer wraps around. This really mucks up digital modes!
• Make sure you've got the most recent version of the soundmodem code (it looks as if it's up to version 0.13). There were some important patches to ALSA support a few months ago, which eliminate a bunch of errors and freezes during TX/RX switching... versions 0.10 and earlier did not work well with ALSA USB.
• from 2015 forum post:
  A few guys have done side by side tests with soundmodem and Direwolf(and a few others)...
  Direwolf decodes ~10 to 30% better.... Why waste time with soundmodem??

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