If it's flat and has a big-screen, it's high-def, right? Apparently, a lot of HDTV buyers think so.

A recent survey by the Leichtman Research Group (LRG) shows that 18 percent of HDTV owners think they're watching high-definition shows, when in fact they're viewing standard definition programming. The findings are based on a telephone survey of 1302 U.S. households. This is LRG's sixth annual study on the topic.

Given the general consumer confusion surrounding HDTV-all those mind-numbing specs like 1080p and HDMI aren't easy to grasp-it's no surprise that many buyers are still clueless. Standard content that's stretched to fill the entire screen may look funny, but at least the picture's big. Problem is, buyers may start to wonder why they abandoned their tube TV for a pricey set with a worse picture.

The halfhearted roll-out of HD service by cable providers isn't helping either. Optional high-definition service often includes only a dozen or so highly compressed HD channels, which look pretty crappy and don't do justice to HD's potential.

Retailers could do more to help too. Just 42 percent of HDTV owners say they were told to get high-def programming when they bought their sets.

And Blu-ray sales have been disappointing thus far, so most buyers aren't seeing HD in its full, 1080p glory. When you can count Johnny Depp's nose hairs in Pirates of the Caribbean, there's no turning back.

Despite the confusion, HD adoption is booming. A third of HDTV owners have more than one high-def set, and 25 percent plan to buy another one in the next year. Also, a third of U.S. households have at least one HDTV, roughly double the percentage that had an HD set two years ago, the survey says.

This story, "HD or Standard Def? One in Five HDTV Owners Don't Know the Difference" was originally published by PCWorld .