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How do we reconcile the ground connection between audio eqp (TNCs, soundcards etc) and transceiver? Most of the audio eqp for digimodes has only one ground. I've seen that the common practice is to use only the PTT ground, either forgetting abt the MIC ground or soldering the MIC and PTT ground together. Obviously this would break the MIC ground isolation.
Related Q/A, Mic ground vs ground
An audio interface is a very different situation than a microphone, even though they both produce the same sort of signal at the same voltage.
The microphone, specifically, must be protected.
Assuming the microphone is of the electret type as is typical, the microphone has an internal amplifier circuit. This amplifier has to pick up the very weak signals from the electret microphone element itself and amplify them. Any interference that gets into this circuit could be amplified as well. The audio interface, on the other hand, probably has either a digital input or a line-level audio input. Both of these are much stronger signals which do not need amplifiers.
Source impedance differs.
Now considering the possibility of interference to the audio signal from the microphone or audio interface to the radio: the microphone will likely have a higher output impedance than the audio interface. Therefore, the effect of any other signal coupled in will be higher with the microphone than with the audio interface.
Miscellaneous:
The microphone is connected only to the radio and is used in close proximity to it. The audio interface, on the other hand, is connected to a computer, and the interface and computer are fully shielded boxes. There's more opportunity for any noise on shields to flow elsewhere.
The stock microphone and connector may be designed to handle worse RF-in-the-shack conditions than the audio interface will. People who are using computer interfaces are likely optimizing their setups more, and fixing RF-in-the-shack to avoid crashing their computer, and using lower power levels than the maximum the radio supports.
Finally, there's plenty of circuit designs out there that simply aren't as good as they could be — they work well enough for the designer.
From a practical standpoint, it seems to me that any variation on the classic fully isolated sound card interface should work.
http://www.qsl.net/sra/sound_card.html
Look at slide 9 or 12 and simply connect the computers sound card ports or a TNC to the sound card mic & speaker lines. The radios mic ground will then remain isolated.
I hope this is the correct interpretation of what the OP was asking.
