HD Radio

HD Radio is the US digital-radio standard, technically known as In-Band On-Channel (IBOC). Unlike DAB, which occupies entirely new spectrum, HD Radio broadcasts digital data streams in sidebands immediately adjacent to the existing analog AM or FM signal. Receivers that support HD Radio decode the digital stream; legacy receivers continue to receive the analog signal unchanged.

The IBOC approach was a strategic choice: it preserved investment in existing AM/FM infrastructure and avoided the spectrum-reallocation politics that slowed DAB adoption in some markets. The trade-off is lower multiplex efficiency: HD Radio typically fits two or three digital channels per station rather than the 8–18 a DAB multiplex carries. Penetration in US vehicles has grown steadily, though not as rapidly as DAB penetration did in Europe.