No problem , already knew that and i will give you a short explanation of why this is a normal situation that cannot be improved
O.k then through the hdmi cable i`m sure you are getting a perfect picture when playing a pre-recorded commercially made dvd movie for example as the hdmi connection is high definition.
On the other hand the connection you have to the cable box is the lowest resolution called SD (standard definition ) but could also be called BD for bad definition when played through HD equipment like your tv or even your computer screen , you would be better off with an old tube tv as it does not show clearly the fuzinesss that we were used to in the days of SD as it shows now when viewed in HD.
Sorry to say but this is it , there are absolutely no recording devices with removeable media like DVD that has a HD input as in component or HDMI input .
The entertainment industry made sure of that and those devices that do have HDMI inputs like receivers for example have to respect a protocol called HDCP which is basically a system the verifies that there are no HD recording devices connected along the sytem , this hdcp is an industry standards so as i said everything has been thought of here so that you cannot record anything in HHD unless you use a device like a DVR /PVR unit and all cable companies will supply that as part of their equipment and you can also buy the popular TIVO which is recognized by cable companies and can be activated through them .
But again those are recording made on a hard drive with the HDCP scheme active and no way to tranfer the recording to anything else in HD and especially not a dvd.
Only way you can burn HD dvd is through your computer and that is by illegally downloading a hacked movie.
So to answer your question ;
The reason you are not getting a good picture is because you are viewing the lowest quality picture available through HD equipment .