On 24 Jan 2018 23:07:23 +0000, you wrote:
HelloI am using a ROLAND EDIROL UA-25 ex USB audio interface, everything
directly connected, no optocouplers, no transformers, and a selfmade
USB to serial PTT with one open collector transistor,
also DC connected to a laptop PC with docking station.
It works and there are no line noise and other interferences,
audio clear down to -90dB.
But it took me quite a lot of work to get the noise down.
Shielding and grounding are my specialty :-)
I only want to say, optcouplers and transformers may help or not,
depending on one's electronics skills.