Originally posted by Lanbrown
There are no HD format standards. They currently have over 17 different HDTV formats they can use.
If by "over 17" you mean 18.

There are 18 ATSC formats. All of them are standards. Six of them are HD. Two of those are used for HD broadcast.
The 18 ATSC formats are:
640x480 @ 30 Hz interlaced
640x480 @ 24, 30, 60 Hz progressive
704x480 @ 30 Hz interlaced
704x480 @ 24, 30, 60 Hz progressive
1280x720 @ 24, 30, 60 Hz progressive
1920x1080 @ 30 Hz interlaced
1920x1080 @ 24, 30 Hz progressive
Only 1280x720@60 progressive (720p) and 1920x1080@30 interlaced (1080i) are used for HD broadcast. 1080/24p is often used for HD production, but is converted to 1080/30i for broadcast. ABC currently broadcasts 720p, and Fox has committed to 720p by the fall. Fox currently broadcasts 480/30p widescreen, but will be phasing out those broadcasts in favor of 720p this year. Everybody else broadcasts 1080i.
Don't forget compression. In the digital world, not all of the complete frames need to be sent, only the changed portions.
Depends on what codec you use. For MPEG codecs, this is true. They use inter-frame compression. For Pixlet, DVCPRO/DVCPRO-HD, and some other codecs, they use only intraframe compression.
None of that matters, though, because we were talking about the ratio of NTSC to HD and half HD in terms of pixels per second. When you throw pixel packing (as I mentioned) and compression into the mix, there are too many variables to keep track of. It becomes impossible to meaningfully compare NTSC to HD.
For example, digital SD is often, but not always, broadcast at 8 Mbps, while digital HD is often, but not always, broadcast at 19 Mbps. But there are so many exceptions to this rule that the rule itself is meaningless.