Do you need cable box for HD tv?

If you mean free Over the Air (OTA) TV using an antenna:

1) If you have an HD TV set, you don't need any kind of special box.

2) If you don't have an HD TV set, but your TV set has a digital tuner, you don't need a special box, but you will see the HD picture converted to a Standard Definition (SD) picture.

3) If you don't have an HD TV set and your TV set does not have a digital tuner, you will need a converter box. The converter box will convert the HD broadcasts to SD so your TV can display them.

If you mean cable TV, it depends on your cable provider and your TV.

If you don't have an HD TV, at best you will see an HD signal converted to SD.

If your TV has a digital tuner and it is also a QAM tuner, you might be able to receive some HD channels without a box if your cable company chooses to provide unscrambled HD channels. If your TV has an analog tuner or a digital tuner without QAM, you will need some sort of box.