MythTV Backend Setup

I am going to be running MythTV Backend server from 3.4GHz, i7, 8GB 1600MHz Mac OSX. MySQL is setup and functioning correctly, and at this point I intend on running MythTV Frontend on the Apple TV XBMC.

Question > If I need to buy a digital TV Tuner, can it be done when my digital TV based through a DSL modem?

Where can I get the tuned signal from to run on the MythTV Backend server? Thanks