I seem to remember two or three years ago reading about PBS reaching an agreement with some company, where customers who subscribed to the service would be given some kind of hard drive for their TV. The hard drive would continuously be downloading movies over the otherwise unused portion of the PBS station’s broadcast spectrum. At any given time, a customer could fire up the hard drive and watch one of the 50 or so movies stored on it.
Unfortunately, I haven’t had any luck in searching for it, since all the relevant words are so commonly used.
It died, and good riddance to it. The idea was not to use “…an otherwise unused portion of the PBS station’s broadcast spectrum” but to compress the HD signal that PBS and other broadcasters transmitted more, fitting it into a smaller portion of their broadcast spectrum. The resulting “freed up” space could be used to transmit movies.
Either the broadcasters discovered that viewers complained when the quality was reduced, or the investment money dried up, or broadband movie download services like NetFlix made this concept irrelevant, or some combination of the three.
digital over the air terrestrial broadcast allows subchannels. you can have HD and subchannels, there is discussion on how this can be split and the resulting quality. some stations will alter the split based on the content they want to show. PBS has HD and 3 streams (kids, create, world) which a local station can mix into the subchannels it runs as well as their local and purchased content.