What model TV and how is it connected? Where the bars are introduced may depend upon how the signal is being transmitted from the source. For example, there is SD material occasionally transmitted on HD channels, where the black bars are "supplied" by the broadcaster. For some formats, I believe it may be generated by the CATV box. In other cases, it is generated by the TV...
But, if you can't find a control for changing the bars on either your TV or the CATV box, sounds like it is pointless anyway...
But, if you can't find a control for changing the bars on either your TV or the CATV box, sounds like it is pointless anyway...