You can watch regular DVDs on a Blu-ray player, but you cannot watch Blu-ray discs on a regular DVD player. For Blu-ray to be worthwhile, your HDTV should have a resolution of 1080i or 1080p—that is, it should display (not just receive but also display) 1,080 lines of data. (1080i means TV's scanner reads every other line, the odd-numbered lines as it passes one way, the even-numbered lines as it passes the other; 1080p means it reads all the lines both ways. There are also HDTVs with 720p resolution, but they won't show Blu-ray to its best advantage.) Your HDTV should also have high-definition multimedia interface or digital visual interface video inputs; if your set is more than a few years old, it probably doesn't. When hooked up with special HDMI/DVI cable, these inputs allow digital video signals to pass between a DVD/Blu-ray player or cable box and your TV directly,and with no compression. Earlier HDTV connections, made through the three (Y, Pb, Pr) component inputs, shunted the signal from digital to analogue and back to digital again, losing a little bit of purity in each passing.

Blu-Ray players have a resolution of 1080p—that's 1,080 by 1,920 lines, or about 2 million pixels per frame. DVD players have a resolution of just 480p—480 by 768 lines, or about 400,000 pixels. Blu-ray discs can hold so much more data in part because the Blu-ray player's laser—literally a blue-ray laser—is much thinner than the red-ray laser on regular DVD players. Hence it can focus more precisely on the digital bits. Hence many more bits can be squeezed onto a Blu-ray disc.

DVDs look better on a high-definition TV, even though they don't have high-def resolution, because electronic processing gear inside an HDTV "up-converts" all non-HD to images to HD. However, some TVs have better processing gear than others. And "native 1080"—that is, an image that is naturally 1080i or 1080p—will always look better than an image that's processed to get there.

A clarification on 4K sampling: There are no televisions or disc players that can display 4K images (about 12 million pixels). When Robert Harris and his team were finished with their 4K restoration, they had to compress the data down to 2 million pixels per frame to make the Blu-ray discs—down to 400,000 pixels per frame to make the DVD. Compression is its own art and science; many of the very early DVDs looked bad, mainly because the technicians hadn't yet figured out how to do the compression.