[quink]

What? No.

Comcast has their network reliably built out to 750 MHz. If you think there are 600 MHz unused on that, you're left with 150 MHz. Which is about enough space for about 20 NTSC channels and the FM band. I can assure you that there's basically no HFC/CATV provider in the world, never mind the US, with 600 MHz unused just sitting there.

[sitkack]

I use the term "sitting there" very loosely, meaning TV channels that no-one in my neighborhood is watching. From my completely unscientific poll of people on my block that I interact with (selection bias) most do not use the channels, Hulu, Netflix, Amazon Prime, torrents, etc. The only person I know that actually uses a TV does so with terrestrial signals, but she is 5 std dev away from normal.