I should be able to receive by piping the audio from my radio to the
audio input of the soundcard, right? I wouldn't need a PPT circuit or
anything else unless I wanted to transmit, correct?

You might be able to manage reception with a simple audio interconnect
cable. The problem with this sort of arrangement is that it's fairly
common for ground loops to exist between the computer and the radio
(one ground through the audio cable shield, another through the
power-line grounding). Ground-voltage differentials can create a
current flow in the loop, which introduces hum on the audio signal,
which could mess up the soundmodem's ability to decode the signal
properly.

A good solution for this is to place a 1:1 audio transformer between
the radio and the computer's sound-card line input. One of the
miniatur "audio coupling" or "audio isolation" transformers will do
the job... it looks as if you can still get these at Radio Shack (part
#273-1374, $3.99) or through many electronics-surplus dealers.

Just cut the audio cable in the middle, solder one half of the cable
to the "primary" winding, the other to the "secondary" winding, and
box it up or pot it in epoxy or wrap it with tape or whatever you
prefer. The transformer will couple the audio frequencies through,
but will not allow ground-loop currents to flow.

The commercial (and better-quality homebrew) PC-to-radio interfaces
usually have two of these transformers (one for receive audio, one for
transmit audio) and an optoisolator for the PTT circuit.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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