Hi Infodrive!
Thank you for your comments! 🙂
I have never seen a sound card where one of the outputs affects the other in that manner. Using one and leaving the other hanging should not make a difference at all.
On your transmitter, make sure pre-emphasis is disabled. It must be set to flat frequency response to accept an MPX signal.
After that, check the level — if it’s distorted, turn it down either the output level on sound card’s mixer, the input gain on the exciter, or the MPX ref level in BBP.
Do you have the make and model of the transmitter? Are you setting up a station or an in-house test setup?
Best regards,
Leif Claesson