I have a comcast basic plan. My new TV (Sharp, LT-42SB45UT) gets all TV channels without connecting to a cable box. However, if I connect my old TVs (several years old) to the cable directly, I got only a few channels, #77-79, which are not really public channels such as ABC, NBC or PBS. I tried connecting the cable to a DTA box then to my old TV, and this did not help my old TVs to get more channels. the DTA box seems only working for antenna.
I understand that with a comcast cable box my old TVs will work. My question is why my new TV can get all channels without the cable box? What device I can purchase except comcast cable box, so I can make use my old TVs? Thanks in advance!