OK, I take issue with the comment that some ballasts that can run MH and HPS are "usually called a digital ballast".This can cause confusion to new growers. A magnetic or transformer ballast is an analog device and could be called a boat anchor due to the size and weight.
A digital ballast is electronic, thus being digital, and a lot smaller in size and weight.
Both can run both bulbs. The transformer ballast only needs the switch circuit to make the difference between MH and HPS and that does not make it a digital ballast.
Digital ballasts run the bulb voltage at higher frequencies (mostly) and that is why when they first came out some bulbs would not work. The digital manufactures have mostly eliminated that problem, but some report of TV and radio interference and they are working on that problem also.