ATSC HDTV is a set standard
In so much as it's a 19Mbps stream, yes. However, the broadcaster can do what ever they wish in that space... one high rate HD stream, one lower rate HD and two SD's, etc. FCC rules prohibit cable systems from modifying OTA broadcasts -- they have to broadcast what they're handed. (the broadcaster can hand them a different stream than their transmitter, and that's ok. PBS does that around here.) And ATSC (8VSB) is MPEG-2, and so's the US cable network.