# Why do ATSC channels need 6MHz of bandwidth when they're digital?

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?

• So, what do you think is the bandwidth of something that changes 6.46 million times a second? Mar 14, 2022 at 16:33
• You should read the first few chapters of any digital communications textbook (I recommend Lapidoth -- it's free).
– MBaz
Mar 14, 2022 at 17:48
• @MarcusMüller I'd hazard a guess at ~6MHz, but I didn't want to assume that, since I don't know if a conversion from bits-per-second to cycles-per-second is valid (kinda like how liters and grams aren't compatible). Mar 14, 2022 at 23:05
• So you're proposing a means of transmission that has a bandwidth more than 6MHz because you want a bandwidth less than 6MHz? Mar 15, 2022 at 5:43
• My proposal is obviously impossible, I'm just curious as to why. Bandwidth and frequency don't seem related at a casual glance - one is a single point on the frequency line, the other is a span on the frequency line. I'm just trying to resolve that confusion. Mar 15, 2022 at 14:03