"Paul L" wrote ...
Just for curiosity, I used a standard RCA video cable and plugged it
into the digial coaxial jacks of my DVD player and Yamaha reciever. I
get very good audio from the connection. Is digital coaxial just
another fancy name for RCA?

For all practical purposes, Yes.

Does using a more expensive digital audio coaxial cable really improve
the sound over and RCA cable?

Only if you are a true believer and have money to burn.