New Nielsen data shows that more than three-quarters of all U.S. homes have HD TVs, up 14% from last year, and that nearly 40% of homes have multiple HD sets. Yet the same data shows that most viewing is still taking place in standard definition.

In May 2012, 61% of all primetime viewing was done on an HD set, but much of that viewing was of standard definition feeds, according to Nielsen. During that month about 29% of English-language broadcast primetime viewing and 25% of cable prime viewing was in "true HD," the study found.

For "true HD" viewing to occur, the HD set must be connected to an HD signal and the set needs to be tuned to a HD channels.

As expected sports and entertainment genres are more likely to be viewed in HD than news and kids programming.

The Nielsen study was based on usage of 17 networks -- five English-language broadcast networks and 12 ad supported cable networks -- in May 2012.