If I subscribe to digital cable and am using a Tivo, do I have to use a set top box from my cable company along with the Tivo or does the Tivo do the up converting also??:confused:

Depends on the Tivo...

A Tivo series 1 or 2 only has an analog tuner so you will need to use a cable box to tune any digital cable channels that are not also simulcast in analog. (usually premiums and channels above CH-99)

A Tivo series 3 or HD-Tivo that have cable card slots will tune digital cable (QAM) and OTA-HD (ATSC) directly, but require cable cards to tune anything beyond clear QAM channels on cable.

I am not sure what you mean by up-converting.

Just a further note about the TiVo Series 3 and TiVo HD. Without CableCards, you can perhaps tune-in in-the-clear QAM channels, but you cannot get program guide data to match up with those channels.