Advocates for HDTV systems fall into two major categories. There are those that feel that these systems will ultimately be successful outside the conventional channels of terrestrial broadcasting. Equally vehemently, are those that think HDTV can and must use existing terrestrial broadcast channels.
NTSC terrestrial broadcast channels are essentially 6 MHz wide. Service in a given area (roughly a 50 mile circle around the broadcast station) is typically offered on every other channel in order to avoid interference effects. A relatively small range of channels are available (channels 2-69, 55-88, 174-216, 470-806 MHz).
In 1987, the FCC issued a ruling indicating that the HDTV standards to be issued would be compatible with existing NTSC service, and would be confined to the existing VHF and UHF frequency bands.
In 1990, the FCC announced that HDTV would be simultaneously broadcast (rather than augmented) and that its preference would be for a full HDTV standard (rather than the reduced resolution EDTV).
These two decisions are very interesting, as they are almost contradictory. The 1987 decision is clearly leaning toward a augmentation type format - where the NTSC service continues intact and new channels provide HDTV augmentation to the existing. The 1990 decision is a radical and non-conservative approach - one which basically removes the requirement for compatibility by allowing different HDTV and NTSC standards to exist simultaneously for a period of years. Then the NTSC is gradually faded out as the HDTV takes over.
Now, the FCC does not have jurisdiction over channel allocation in cable networks. Thus, there is the rather interesting question of what the cable TV companies will do. They have a number of interesting options. They can continue to broadcast conventional NTSC, they can install 20 MHz MUSE-type HDTV systems (or other types of HDTV systems), or they can go with the digital Grand Alliance systems. This presents the interesting possibility of two different HDTV standards, one for terrestrial broadcast and one for cable broadcast.