What quality digital cable? Is it BNC-BNC with RCA adapters? If not, then it's not "quality", sorry. The only way to get a good termination to a coax is using BNC, not RCA.
I make the best $275 coax cable. If you do use a USB converter, this cable should be on the output. Your USB cable is at least as important. Silver is best. I use a WW cable.