The standard definition signals may not look ideal, and unfortunately, that's the 480 lines of resolution being upconverted to a 1080p signal. The result isn't always a clear picture, and that's just a SD-to-HD conversion. Not many tube TVs are 55", and as the screen gets larger, the artifacts can be more noticeable.

If you use a cable or satellite box, you might try upscaling the conversion or choosing a higher resolution in the box to see if that helps.

I don't see a firmware available on the site, although if there is one available, it's accessible through the TV's update system in the menu.