[Gogar]

+1 what John Cramer said. If you want to get a handle on the basic idea, Odb is how loud your receiver would be if it had no volume knob at all to get in the way of things(in theory). Some people call it "unity gain" and there's a mathy way to describe it, but that's how I wrap my pea brain around it. Maybe someone will describe it better or more correctly. Actually I'm sure someone will try.