Tax for the TV (Duesseldorf)

Okay, so I have a question:

At home, we pay this tax for the TV which is approx. 18 euros per month. I was at a shop today and the sales force told me that if I have a digital TV at home, I will not have to pay this tax no more because it is for ground TV.
Anyone could provide a confirmation this is correct or not and how to proceed to be exempted if this is relevant.
Thanks
A