By my understanding, if you are hooked up to cable, then you don't need a digital tuner....most cable signals are analog.....that allows me to still use my analog TVs at home. If you are hooking up to your antennae, that is when you will need your digital converter box.
Also make sure the antennae booster is off if you are hooked to cable......it will be a little button on the TV hookup plate. Green lite off for cable, on for antennae.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263