The Tivo models are TCD746320, TCD746500 for the 2 tuner models. They record 2 channels at once and you can watch a recorded show at the same time. This can record from cable (analog or digital), as well as antenna. Of course, you can watch 1 channel and it record from the other tuner, but that is only if you are watching live TV.
The 4 tuner models are only for digital cable and no antenna, not even a digital converter box.
Most that have the 4 tuner model and want to cut the cost of cable, They would not be able to use this model any more without digital cable service.
Tivo is not meant to replace the cable service, it is just a Digital Recording device that goes in addition to the cable TV service or antenna, albeit an extra cost due to the Tivo box cost and Tivo subscription prices.
Series 2 (PLS, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "652")
Roamio! Oh, wait, Its Juliet!
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!