When we’re talking about the HD Radio, we refer to the terrestrial digital broadcast signal that’s only present in the US. You can recognize it with ease by how it looks – the two rectangular blocks that appear on any side of the broadcasting FM station signal that you will see on the waterfall display or spectrum analyzer.
Seeing how this HD Radio is using its own protocols, decoding it has been a challenge for many listeners. This meant that this signal was unavailable to SDR users for quite some time now. There was a posting about Phil Burrs trying to decode it. He even went as far as partial implementation to the second layer of the standard, but he never made it into the third layer, to begin with.
Still, not that long ago, one of the cyber security researchers has been able to create a complete RTL-SDR based decoder meant for the HD Radio protocol. A post on the matter provides more info on the system and its 3-layer split. So the first layer identifies the signal to perform decoding as well as error correction. The second layer is multiplexing, allowing the different layer 3 applications that will therefore share the entire bandwidth. The final layer is basically the audio data layer.
The very initial finding was the audio compression codec. This codec was pretty much a basic HE-AAC that went through a series of quick modifications. Seeing how these modifications were not that significant, the user was able to adapt the FAAD2 library for HD Radio audio decoding.
The code used in the process is available to everyone on GitHub. It features the patch to modify FAAD2 for HD Radio. This patch is applied on itself throughout the build. Furthermore, there is a sample file for the decoder that can be used for testing. You can use the git readme in order to perform the decoding in real time.
In the event that you are looking for more information on the matter and need additional aid, you can easily use the source site to find out much more. Feel free to explore it in order to make the right call and you will definitely get all the aid necessary to make the most from it within the very least amount of time feasible on your own.