What is HD Radio?
In its current hybrid implementation, HD Radio adds digital sidebands to an analog FM signal. The sidebands consist of hundreds of digital subcarriers that occupy spectrum 129 to 198 kHz from the channel center frequency. Total power for both sidebands originally was 20 dB below the analog carrier level, or −20 dBc. FCC rules now permit levels up to −14 dBc for most stations without special approval, and up to −10 dBc in certain circumstances. Some transmission modes use additional subcarriers that extend the sidebands as close as 102 kHz from channel center. These extended hybrid signals are becoming more common.
An HD Radio station's digital sidebands may interfere with reception of its own analog signal in two ways. First, FM detection of the sidebands may generate intermodulation products that overlap the analog signal. Interference also may occur when the stereo decoder demodulates the digital sidebands along with the stereo subchannel signal. Because the digital subcarriers are numerous and their data randomized, analog detection yields noise.
HD Radio technology allows broadcasters to transmit a high-quality digital signal. For listeners who have an HD Radio receiver, the benefits are:
• FM radio that sounds almost as good as a CD
• AM radio that sounds as good as traditional FM
• No more static, pops, crackles or fades...and no subscription fees
• Displayed information, including station ID info, song and artist titles, weather, traffic, and emergency alerts
• Increased listening options with station multicasting
FM radio with near CD-quality sound
Digital technology allows a radio station to transmit more information in the same radio wave. Primarily, this means higher quality sound. So much more so that FM transmissions can sound nearly as good as CDs, and definitely much better than the hollow sounds of old analog FM. How much better the music sounds will depend on your local stations and your radio gear.
AM radio uses smaller sections of bandwidth than FM. There is not enough bandwidth for HD Radio to give AM stations the same CD-quality signal as FM stations. But there is enough room to give AM stations clarity equivalent to current analog FM stereo radio. This boost in sound quality makes AM radio a viable alternative to FM, which means more options for listeners.
No more static, pops, crackles, fades...
The digital signal is less vulnerable to reception problems. The radio tuner's digital processors eliminate the static, pops, hisses, and fades caused by interference. You hear only clear, clean, and rich sound. Should you lose the digital signal for some reason (obstructing terrain, nearing the edge of the broadcast area, etc.), HD Radio technology defaults back to analog mode, similar to the way non HD Radio receivers switch from stereo to mono mode when receiving a weak signal. And like analog radio, HD Radio is subscription-free.
Transmission of extra bits of information
Another benefit of digital radio is the radio station's ability to transmit additional information along with the music signal. This can take the form of scrolling text on your receiver's display, such as a song's artist and title and station call letters. HD Radio also supports Artist Experience — with compatible receivers you can view album art, logos, and more, when provided by the station. Stations can also include local and regional information, such as weather updates, emergency alerts, or even traffic jams and road construction. Traffic data is delivered up to 10 times faster than other broadcast methods.
Multicasting
In addition to duplicating their analog programming with an HD Radio broadcast, stations can subdivide the digital portion of their signal. This allows a station to "multicast" — that is, broadcast two or more programs simultaneously. Listeners might have a choice of, say, a sports game or music. These additional channels can only be received on an HD Radio tuner. But just as cable TV allowed specialized networks to flourish, multicasting provides the potential for stations to offer more niche programming - ultimately giving the listener a greater variety of formats to choose from.
How does it work?
HD Radio technology works pretty much just like traditional analog radio transmission:
1. The radio station sends out the analog and digital radio signals, along with a third signal for text data.
2. The digital signal is compressed before being transmitted.
3. The three-layered signal is transmitted from the radio station's upgraded digital transmitter.
4. Multipath interference, caused by the signal reflecting off of buildings, is ignored by the digital radio, which is able to discern the true signal and ignore interference.
5. Your radio receives the signal and, depending on your equipment, you hear either the digital or analog feed.
HD Radio technology is not a subscription service, like satellite radio. It is the same free, over-the-air broadcast radio that we've always known. Only better. You just need gear that includes a built-in HD Radio tuner to enjoy it. Everyone can choose to continue listening to their current radios, but eventually all AM/FM radios will incorporate digital technology. It is a natural evolution of the medium.
HD Radio is not the same as satellite radio. Rather, it's an improvement to terrestrial AM and FM radio. Satellite radio, on the other hand, is an alternative to broadcast radio, in the same way that cable or satellite TV are alternatives to broadcast television. Even if you do have satellite radio, there are often times when you want to listen to your local station - and that's where HD Radio comes in.
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