My understanding was that the implementation was protected by patents.
And can one call something a standard if the codec is not documented and needs to be reverse engineered?
Like I said, it's the protocol that is standardized. HD Radio as a whole is not, because of the codec. I think it's absolute BS that the FCC went down the path of a proprietary format, but it's the way things are. And the fact is, we do have at least one FOSS implementation. Since it's not DRM, I believe (but IANAL) that it's perfectly legal to reverse-engineer it.
I doubt Xperi really cares, since HD Radio is a trademark so it's not like you could build, sell and market an "HD Radio" device without licensing from them anyway.
The protocol itself is actually standardized: https://www.nrscstandards.org/standards-and-guidelines/docum...
It's just the audio codec that's proprietary, but it's basically just a slightly modified version of AAC so it's been decoded.