In the United States, the original ATSC standards for HDTV supported 1080p video, but only at the frame rates of 23.976, 24, 25, 29.97 and 30 frames per second (colloquially known as 1080p24, 1080p25, and 1080p30).
In July 2008, the ATSC standards were amended to include H.264/MPEG-4 AVC compression and 1080p at 50, 59.94 and 60 frames per second (1080p50 and 1080p60). Such frame rates require H.264/AVC High Profile Level 4.2, while standard HDTV frame rates only require Level 4.0.
This update is not expected to result in widespread availability of 1080p60 programming, since most of the existing digital receivers in use would only be able to decode the older, less-efficient MPEG-2 codec, and because there is a limited amount of bandwidth for subchannels.

http://en.wikipedia.org/wiki/1080p