That was just a little before my time, but as a child growing up in Tuscaloosa I don't think 21 was carried on our cable system back then so I wouldn't have seen it unless someone hooked up an antenna.
Strangely, I don't remember WDBB's sign on, either, in 1984. I don't think it was on the cable, either. The cable was just 12 or 13 channels, and one of those was a local barker channel.
Two years after that, I do remember the debut of WCAJ 68, and receiving it off the air with just a cheap bowtie antenna in Tuscaloosa. That was around the time I started to marvel at off air reception of radio and TV, and the start of my life long love of all things broadcasting.