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[WriteLog] sound card SSB audio

To: Paul Hellenberg K4JA <k4ja@contesting.com>
Subject: [WriteLog] sound card SSB audio
From: Eric Scace K3NA <eric@k3na.org>
Reply-to: eric@k3na.org
Date: Mon, 27 Oct 2003 19:22:31 -0500
List-post: <mailto:writelog@contesting.com>
Hi --

   I've struggled a bit with this as well.  I have learned two things that 
might help others:

1)  The widely-quoted Radio Shack isolation transformer may be OK for RTTY high 
tones, but not very good for human speech.  It has
poor low frequency and dynamic response, saturates easily, and introduces a 
tinny characteristic to the audio.  See the 2002 July 17
posting by W5YR for details.

   K3MM has had good luck with Magnetek TY300P 600 ohm hybrid transformers.  
Another WriteLog reflector contributor found some less
expensive solutions from DigiKey, but I can't find his message.

2)  RF on the audio lines can do bad things when it gets into the audio 
amplifiers in sound cards.  Wrapping lines around ferrite
chokes sometimes makes a dramatic improvement.

   More generally, there are a lot of steps between microphone and radio 
transmitter input.  People tend to just throw together a
bunch of cabling and then expect all the levels and impedances to magically 
work out and sound good.  It's easy to get caught with
low level ground loops and other extraneous junk on the lines.

   I'm contemplating taking a more careful approach, using 600 ohm balanced 
lines and providing a more standardized signal level
(aka 0 dBm) to the transmitter line input/phone patch input jacks rather than 
the mic-in jack.  That, along with some little boxes
that provide isolation transformers and RF bypassing (as well as breaking out 
stereo soundcard line in/out into mono for SO2R should
help a lot.

   Receive audio is also important.  You may not run into this trouble with a 
multi-op station, but a SO2R station that's doing a
lot of headphone switching can also be exposed to this problem.

-- Eric


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