Anything broadcast over a subcarrier is, by FCC definition, not a public broadcast, but a subscription service. So, even if it works great, it is technically illegal to sell receivers for it to the public. The rules require that receivers for subcarriers be fix tuned to a specific station (though I've seen many tunable SCA receivers over the years). In the 1970s, by brother bought a tuner with an SCA decoder that was disabled by default. When permission to receive an SCA was received from a station, in writing, he mailed a copy to the manufacturer, which then sent instructions for enabling the SCA. I think even that was technically illegal since the radio was tunable to other stations. In fact, the SCA that my brother wanted was a subscription service coming from another market. He used that radio to listen to it at home which was exactly what the FCC was trying to avoid. The rules have not been changed and they would need to be changed before any subcarrier based system could be used as an alternative to IBOC or anything else.
