Here is a presentation that I did on Amateur Radio Sound Card Digital Communications Modes at London Amateur Radio Club October 10, 2002.
Building an interface
http://www.qsl.net/wm2u/interface.html (this link is slow, but has a good collection of schematics and links )http://www.waypoint.com/users/~discobay/Sound_Card_Presentation.htm (another presentation with schematics)http://www.arrl.org/news/features/1999/0701/2/?nc=1 (overview article with scheamtics)
As mentioned in the presentation, I prefer an interface with FULL isolation via audio transformers in RX and TX and an opto isolator in the PTT circuit, as it eliminated all chance of audio or rf ground loops.
Buying an interface
Note that there are others appearing all the time
Sound Card Mode Overview
http://www.arrl.org/tis/info/digital.html (good collection of articles)
SW Collection Downloads
http://www.muenster.de/~welp/sb.htm (this is a GREAT master list of all kinds of sound card sw with download links)
Sound samples, Good for figuring out just WHAT you have tuned to!
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