I can't receive
HDTV signals because I don't have an HDTV. I bought a Samsung SIR-T 151 digital set top box at Sam's Club earlier this year. It was there last one and it was marked down to $179 so I bought it to play with.
Each station can broadcast up to four channels at the same time in SD.For
example
PBS Tucson broadcasts 6.1,6.2,6.3,and 6.4 all different shows from PBS during the day. In the evening they switch to HDTV. at that time they can broadcast one HDTV channel or one HDTV channel and one SD channel.
At present the only stations in my area broadcasting more than one channel are PBS and ABC.
Many digital stations are broadcasting at less than full power at the present time so some weaker stations will probably come in better in the future.
Tom