I would bet the under
I am unaware of HDCP ever being used in broadcast. It has been cracked virtually since it's creation, so the idea that a broadcaster today would purchase a license seems a little suspect, but I suppose it is possible. The purpose of HDCP is copy protection, not viewing protection. It is intended to prevent the recording of protected material, not the viewing of protected material.
It is still used for streamed content occasionally, and on DVDs, and I have heard of cable companies using it or something similar, but you said DTV. The more likely culprit is poor reception or a WM glitch.
BTW, I could never get WM to view both analog and digital channels simultaneously, so it was of no use to me. Did they ever change that?