Impedance Matching

[Greg Esres]

I have a low impedance output (aircraft intercom system) that needs to
go into a medium impedance input (digital voice recorder microphone
jack). Right now, the recording is distorted and low volume.

The only impedance matchers I can find work in reverse....high
impedance output, low impedance input.

What do I need to do what I want?

Thank you.

 

[Bob Masta]

On Mon, 16 Feb 2004 21:09:43 GMT, Greg Esres <nospam@spamproof.com>
wrote:

I have a low impedance output (aircraft intercom system) that needs to
go into a medium impedance input (digital voice recorder microphone
jack). Right now, the recording is distorted and low volume.

The only impedance matchers I can find work in reverse....high
impedance output, low impedance input.

What do I need to do what I want?

Thank you.

You don't specify what "low" and "medium" impedance
values are, but in general this is not the sort of area
where impedance matching is done anymore. Modern
designs usually have low impedance outputs similar
to an ideal voltage source, and they feed high impedance
inputs that don't load it enough to matter. Impedance
matching for best power transfer was done in the good
old days, and still in certain PA *speaker* applications, but it isn't
an issue with mic recording.

So the real question is what is causing the distortion
and low output levels? I'm assuming the intercom
output can easily drive any mic input, and since most
mic inputs are *much* more sensitive than typical line
outputs, 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 Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

 

[Greg Esres]

<<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!

 

[Bob Masta]

On Tue, 17 Feb 2004 16:51:39 GMT, Greg Esres <nospam@spamproof.com>
wrote:

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

 

[Greg Esres]

<<Hope this helps!>>

That helps enormously, Bob. Understanding the problem makes it half
solved. You're the first one I've communicated with on this subject
that had enough understanding to tell me that my original analysis of
the problem was incorrect.

Thank you!

 

[Dimitrij Klingbeil]

"Bob Masta" <NoSpam@daqarta.com> wrote in message
news:4033749d.4797010@news.itd.umich.edu...
<snip>

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!)

That's an idea, but please note that the mic input preamp can get messed up
by a decoupling capacitor unless a resistor is added to ensure that it has a
(minimal) load attached. Since some mic preamps require a current to flow
through the electret mic's FET, providing an 'infinite resistance' may cause
problems with the preamp's operation. A 1K resistor will probably be fine.
Than a decoupling cap can be connected to get rid of the DC and a resistive
voltage divider to attenuate the signal as you suggested. Impedance matching
will still be necessary, but only to a limited extent (just to make sure the
mic input is loaded and the whole thing doesn't become a high pass filer).

Hope, this wasn't too wrong.

Dimitrij