dta's are the digital transition boxes that are given out for older tv's to downgrade the new digital signal to analog. They are not good for a good picture. If you have a non hd tv the hd box is doing nothing for you so you should not be paying for it.
I have a regular set-top box (has a program guide and Ondemand) and two DTAs (the little cheap boxes) that handle my digital starter tier of service quite nicely on a smaller TV. You shouldn't have any difficulties.
I have a hd cable box that I have had for 2 years and have not had to pay any xtra fee for it. I have a 13" 10 year old tv that is an analog tv ,it works well with the Motorola HD box. Does anyone one have one of the non-HD cable boxs? I am afraid that I will get a cheap box and not have the same great picture I have been enjoying.