Progressive scan
I suspect the 80 percent is due to using "progressive scan" output from the DVD player.
I have an old HDTV (picture tube type) upstairs. My DVD player does not play HD of any kind, but it has a progressive scan output, feeding through four color cables into the TV. What this does is eliminates "interlacing" common to normal TVs, and creates a better picture even from normal DVDs.
Only HDTVs have the chops to accept this kind of output, so you're still stuck buying one. But at least most of your old DVDs will look better if you have a player with this kind of output.