It was selected by the U. S. It is officially known as NRSC-5, with the latest version being NRSC-5-C, other digital radio systems include FMeXtra, Digital Audio Broadcasting, Digital Radio Mondiale, and Compatible AM-Digital. Although HD Radio broadcastings content is currently subscription-free, listeners must purchase new receivers in order to receive the portion of the signal. As of May 2009, there were stations in the world on the air with HD Radio technology than any other digital radio technology. More than 1,700 stations covering approximately 84% of the United States are broadcasting with this technology, according to iBiquitys website, the HD is simply a brand name and has no meaning. There is no connection with television, although like digital television the HD Radio specification provides enhanced capabilities over the analog format. Thus, there is no deadline by which consumers must buy an HD Radio receiver, in addition, there are many more analog AM/FM radio receivers than there were analog televisions, and many of these are car stereos or portable units that cannot be upgraded. Digital information is transmitted using OFDM with a compression algorithm called HDC. The cost of converting a radio station can run between $100,000 and $200,000, if the primary digital signal is lost the HD Radio receiver will revert to the analog signal, thereby providing seamless operation between the newer and older transmission methods. The extra HD-2 and HD-3 streams are not simulcast on analog, alternatively the HD Radio signal can revert to a more-robust 20 kilobit per second stream, though the sound is reduced to AM-like quality. Datacasting is also possible, with metadata providing song titles or artist information, by using spectral band replication the HDC+SBR codec is able to simulate the recreation of sounds up to 15,000 Hz, thus achieving moderate quality on the bandwidth-tight AM band. The HD Radio AM hybrid mode offers two options which can carry approximately 40 or 60 kbit/s of data, but most AM digital stations default to the more-robust 40 kbit/s mode which features redundancy. HD Radio also provides a digital mode, which lacks an analog signal for fallback. The pure digital mode transmissions will stay within the AM stations channel instead of spilling into the next to the station transmitting HD radio as the hybrid stations do. The AM version of HD Radio technology uses the 20 kHz channel, when operating in pure digital mode, the AM HD Radio signal fits inside a standard 20 kHz channel or an extended 30 kHz channel, at the discretion of the station manager. As AM radio stations are spaced at 9 kHz or 10 kHz intervals, some nighttime listeners have expressed concern this design harms reception of adjacent channels with one formal complaint filed regarding the matter, WYSL owner Bob Savage against WBZ in Boston. The HD Radio also provides several digital modes with up to 300 kbit/s bitrate. Like AM, pure digital FM provides a fallback condition where it reverts to a more robust 25 kbit/s signal, FM stations have the option to subdivide their datastream into sub-channels of varying audio quality
An example of information displayed by an AM HD station locking.
HD Radio Transmitter
HD-1 signal on KOST 103.5 in a Volkswagen RCD-510 receiver