I recently bought a Panasonic TH50PX75 and I'm having a issue.
It seems that HD channels are being stretched way out, even after I've set Aspect option to Full, or even 4:3.
I'm using Cogeco as my HD provider and I have the HD box (not the DVR).
I just don't know what to do anymore. I've looked high and low for answers and Panasonic and Cogeco keep blaming each other when I ask them.
Once the STB is set up properly (16:9 TV, 480i for the 4:3 override, 1080i for the component video and no "wide"), Make sure that your TV is also not stretching/zooming the image, as discussed above. I believe the proper mode is "full" for the HD channels. Also remove any other cables other than component video, sometimes people connect other cables and tune to the incorrect input on the TV by mistake.
On the STB, you should set the 4:3 override to 480i, so that the TV can be set to "just" for the SD channels and it will automatically stretch them properly and it'll be forced into "full" for the HD channels - so no input is required on your part for stretching SD or watching HD, once this setup has been done properly.
As mentioned in the posts above, not all programming on the HD channels is true HD, but upconverted SD will have black bars on the sides, which doesn't sound like your problem. See: