From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Platt Subject: Re: Xastir Development Snapshot + Soundmodem/AX.25 HOWTO's Date: Thu, 21 May 2009 10:47:13 -0700 Message-ID: <4A1593A1.4000806@radagast.org> References: <4A1585AA.5000300@columbia.edu> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4A1585AA.5000300@columbia.edu> Sender: linux-hams-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Alan Crosswell Cc: "Curt, WE7U" , linux-hams@vger.kernel.org, linux-hams-using-ax25@yahoogroups.com > Somewhat OT: but has anyone successfully used a USB sound "card" with > soundmodem? 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 so at all 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.