nrsc5 VS gr-nrsc5

That decision was made by the original author (Andrew Wesie), but I think it makes sense because nrsc5 only needs a tiny fraction of the functionality that GNU Radio provides. Implementing the functionality directly in nrsc5 avoids the need for a large and complex dependency.

One downside is that we don't get the broad hardware support that GNU Radio provides, but maybe we'll someday take another crack at integrating SoapySDR. (See https://github.com/theori-io/nrsc5/pull/131 for an earlier experiment with that.)

That's HD radio, the sidebands are digital audio. It can be decoded with nrsc5 and an rtl-sdr.

An article called "Decoding and Listening to HD Radio (NRSC-5) with an RTL-SDR" links to a security researcher's write-up called "Receiving NRSC-5" about how they wrote the software to do it. That links to the actual nrsc5 software on GitHub.

There is loads of software that will work such as SDR++, GQRX, CubicSDR, and SDRangel just to name a few. For HD radio reception, there is nrsc5. Nrsc5 only works with an rtl-sdr, so you will need one if you want to receive HD radio.

In theory could https://github.com/theori-io/nrsc5 be listening to all hd radio at once?

HD Radio is closed and proprietary, but there is at least one FOSS receiver out there: https://github.com/theori-io/nrsc5

NRSC5 is a decoder that I use. If you are on windows there are a couple of extra steps you have to take compile it, but it works well! They have a newer one with a GUI that can also display album art.

The code of nrsc5 (and my transmitter project, https://github.com/argilo/gr-nrsc5) would be the best source of information for the undocumented parts. I do have some private notes, but the useful information has already gone into the code.