If I'm understanding it correctly, an ATSC channel is a digital channel that uses 6 megahertz of bandwidth to send 19.38Mbps using 8VSB modulation. For analog broadcasts, that makes sense, but I'm struggling to understand why digital data needs such a wide bandwidth and have assumptions that are probably wrong.
For instance, why couldn't we just have a single carrier wave that changes amplitude between one of eight levels at a rate of 6,460,000 times per second? The lowest ATSC channel, channel 2, has a carrier frequency of 54MHz, so each change in amplitude would persist for about 8.4 cycles of the carrier (up to 107 cycles for channel 51 at 692MHz), which seems like it should be enough for receivers to recognize the level and greatly reduce the bandwidth needed.
Clearly the above is an incorrect, unworkable idea, but I'm curious as to why it wouldn't work. Is it because changing the amplitude that quickly takes up about the same amount of bandwidth?