A best, one cycle of an analog video frequency can provide information to two pixels. (NOTE: This is AT BEST - it can easily be argued that one cycle only provides full video information to one pixel!)
A conventional NTSC image has 525 lines scanned at 29.97 Hz with a horizontal resolution of 427 pixels. This gives 3.35 MHz (assuming 2 pixels per video cycle) as a minimum bandwidth to carry the video information without compression.
If one decides to move to an HDTV image that is 1050 lines by 600 pixels (keeping the same frame rate), then this means a bandwidth of 18 MHz. Clearly we have a problem here - as the current terrestrial channel allocations are limited to 6 MHz!
(As an aside, the word "terrestrial" as used by TV people means conventional wireless TV transmission. This is to differentiate it from satellite or cable.)
The options for terrestrial broadcast (assuming a 20 MHz bandwidth) are roughly as follows:
Options 1 and 2 are virtually incompatible with current NTSC service. About the only possibility for maintaining compatibility is simultaneous broadcast of NTSC information over certain channels and HDTV information over other channels.
Option 3 does allow compatibility - as the first 6 MHz of the signal could keep to the standard NTSC broadcasting and the remaining be additional augmentation signal for HDTV. Typically, in this type of augmentation system, an existing VHF channel would be tied to one (or two) UHF channels. The VHF channel would carry information similar to the current NTSC signal and the UHF channel (or channels would carry augmented high resolution information).