<<you will probably need to attenuate the signal quite a bit before it
goes to the mic input. (I'm assuming there's no line input.) Also,
mic inputs often supply phantom power on the ring connector, so
depending on how you've wired things that could be causing a
level shift. You can probably check that with a DVM.
Also, are you sure the intercom output is good?>>
Bob, thanks for your excellent response. I think you're on target and
that makes a lot of things click. The lack of devices to adjust the
impedance in the way I described suggests that no one wants to do
this, because, I suppose, it doesn't need to be done.
I purchased a 50db attenuating cable at Radio Shack yesterday and am
looking forward to trying it out today. I did use it on my
transceiver, and the voice communication was very intelligible.
However, there was a strong background hiss, which I read is due to
recording at low volume levels. The transceiver probably doesn't put
out as much signal as the actual aircraft intercom system, so that may
improve when I use it in the aircraft. Regardless, I can filter that
out with my software.
Not exactly sure what you mean by phantom power, but the aircraft
intercom system does provide a DC bias in order to power the
microphone.
Thanks for your help!
Greg, you might want to jury-rig a simple pot or
2-resistor voltage divider to determine the best
attenuation to use. The resistance of the pot
or resistor chain should be around 10-50K,
but anything in the 1K to 100K range is fine.
You can easily rig up the pot by cutting a
cable in two and soldering it in-line. There
will most likely be some hum pickup since
the pot and your connections won't be
shielded, but unless it is swamping the
mic input you can probably get by without
a metal can around things. This is just a
test to determine the proper level. If it's
too high you get distortion, too low and
you get hiss, which might be what you are
getting now with your 50 dB cable.
Once you have the right range, you can
wire up a cable with fixed resistors under
the shield. (Use shrink tubing around the
resistors, or even electrical tape.)
As for phantom power, that's used to
supply power to electret mics. Your
sound card probably uses 5V or something
like that. This is typically on a separate
connector of the mic jack, which is the little
ring on the plug. Avoid that, since the added
DC bias onto the intercom output may
cause problems.
I think some sound cards may supply
the phantom power directly to the
mic signal connector (the tip), in which
case you may need a coupling capacitor
if the DC is messing up the intercom
output. (It shouldn't, but then again I don't
know what sorts of outputs aircraft
intercoms use!)
Hope this helps!
Bob Masta
dqatechATdaqartaDOTcom
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com