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Gary

I can't speak to 4K TVs (which, btw, are often referred to as "UHD", not just "HD"). But most HD TVs available for the past 8-10 years or so have not done a good job displaying non-HD (or SD [Standard Definition]) content. The aspect (high-to-wide) ratio is different so the TV typically ends up remapping the display some to fill the screen as much as it can. This is very similar to changing the definition on a computer screen (essentially the same reasons).

It's not clear to me whether you're discussing TV stations or all non-HD content. If you're trying to play videocassettes or MP4s through the TV, it won't look good because these sources don't have enough display information to look high-definition.

You might try changing some features on the TV. Try changing the aspect ratio, perhaps accepting black borders on the screen rather than trying to fill the screen with the lower-definition signal. If you're talking about one particular input (say, VCR), you may be able to change that particular input and leave the rest optimal.

The simplest explanation is this... Imagine that you are now looking at the Standard Def channels through a microscope. For the first time, you can really see all the "junk" that you never noticed before. Also, the fact that your eyes are going immediately form HD to SD, you now have a new baseline of comparison. Makes the difference really stand out.

Manufacturers don't invest much in standard definition processing. Similar to basic 1080P or 720P sets a few years ago. It doesn't make sense to design a tuner section to support antiquated standard def. The same reason why there is only 1 standard def input (and usually combined with the only component red, green, blue input).