Questions on mic/line level and audio transformers

[Loren M. Lang]

Hello all,

I have a project that involves using audio signals to transmit digital
signals over a PC sound card. Ultimately, this will be used to transmit
and receive digital signals over a radio. However, at the moment I am
just trying to get a better understanding of line-level vs. mic-level in
regards to how a typical PC sound card implements it.

I remember back in the 90s, it was common for sound cards to have both a
line-in and a mic-in. In fact, I think the Sound Blaster had 4
connections that also included a speaker out as well as line-out. The
speaker-out disappeared when sound cards moved to the motherboard, but
it was still common to have both a mic and line input. Now, I can't find
any sound cards that have any besides one mic input and a line output.
From what I remember learning, mic level can be a few hundred millivolts
lower and presents a higher output impedance than a line-level device.
Both those differences can cause a line-out to easily overdrive a mic-in
and cause distortion which would be bad for trying to pass data through.
So, is my understanding correct for the state of PC sound cards
including even the cheap USB ones I've looked at?

I'm also planning on adding an audio transformer on both paths to keep
them electrically isolated. It may not matter as much when I am testing
between to PCs, but I expect it to be more important when I attach a
radio instead. I seem to recall that audio transformers can be
classified as an input or an output transformer depending on whether
it's used with a mic or a speaker for impedance matching, but it's been
ages since I looked at that.

-Loren
--
Loren M. Lang
lor...@north-winds.org
http://www.north-winds.org/


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