I am trying to use the sound card in PC 1 as a synthesizer from PC 2, which sends MIDI commands to PC 1 through Ethernet, which is not a problem. The problem appears when I connect line out to the analogue input, for audio capture.
Here, PC1 is a Dell Dimension XPS d333 machine, and the device I am using is TurtleBeach montego (vintage gaming sound card).
Doing so, results in a bee swarm sound (not 50 Hz mains hum, but a tone with a fundamental around 160 Hz), with or without the shield connected.
What I have tried or thought of:
Record "What you hear" on PC 1. This would add to much latency. I need Windows 98 to host the card, and thus, I do not have access to a low-latency API. Also, even if the synthesizer worked on ALSA, streaming data back through TCP would also add too much latency (MIDI commands uses a custom UDP based protocol with fixed sized packets).
Use the soundfont from PC 1 in a soundfont player. It should work, but the file is in a proprietary format that needs to be reverse engineered.
Separating the two grounds solves the problem, but introduces other strange artefacts like unexpected voltage on chassis.
From the schematics, you can see that there is no true ground, since the setup is in a room without grounded sockets. I temporarily (and probably illegally) tried to power the system from from a socket in another room with ground, to see if that would help. Doing so did not help, so paying an electrician for adding grounded sockets to the room would not give any payback.
Another solution could be an audio transformer. I tried that, but the problem remains + it adds 50 Hz mains hum to the signal.
Use some RF modulation and a pair of antenna. This requires a good choice of modulation frequency, or some kind of isolation box (for 2.4 GHz, a cavity surrounded by water would do the trick.
Add a linebox. The USB audio device can drive 48 V phantom, if required by the linebox. Maybe http://artproaudio.com/isolators/product/cleanbox_pro/, however, the manual says that I need properly grounded mains power, which I do not have.
Replace the power supply in PC 1