The crowded radio airwaves are crackling. Local AM and FM stations remain a staple among all age groups, whether you're tuned in while stuck in morning traffic or you've got the head-phones on during an evening jog. The Sirius and XM satellite radio business also continues to soar—it now totals just over 10 million subscribers. Internet radio plugs along, though few have tried it. And aren't podcasts just another form of radio?
Now you can add another radio technology to the mix. HD (high-definition) Radio, a digital terrestrial broadcast radio service available in the U.S., has been in development for years with the requisite growing pains. But with hundreds of HD Radio stations now broadcasting throughout the U.S., the usual chicken and egg problem didn't occur.
With its free high-clarity programming, you'd think consumers would be clamoring for HD Radio. But there's been a long wait for the radios themselves, which are finally arriving. Even still, few consumers even know it exists. Like the satellite radio industry, HD Radio will need time to get recognized and adopted.
SO, WHAT EXACTLY IS HD RADIO?
HD Radio is a digital radio technology, but it differs from satellite radio. While satellite radio is received directly from satellites operating in the 2.3-GHz range, HD Radio operates on the same frequencies currently assigned to existing AM and FM stations.
Using orthogonal frequency-division multiplexing (OFDM) digital technology, HD Radio puts the new digital signals on either side of the existing AM and FM sidebands. On the AM bands (530 to 1705 kHz), the usual mode is to simulcast—that is, transmit the same programming in both analog and digital formats. Older radios ignore the digital content, while digital radios receive the digital signals.
Simulcast is the initial mode of operation on the FM bands (88 to 108 MHz) as well. An HD Radio will pick up the regular analog signal and the separate digital signals. FM HD Radio stations also have multicast capability. They can divide up their digital OFDM carriers and create as many as eight additional channels of broadcasting.
In essence, you get multiple additional stations at minimum cost. Moreover, multiple programs can be provided, just like satellite radio. Amazingly, no new spectrum is needed. This could potentially be a new source of ad revenue for a station, though initial multicasts will be ad-free. The additional content should attract new listeners with more focused music and talk venues.
HD Radio brings digital's benefits to broadcast radio. The high-definition nomenclature means that the audio frequency response is far greater than the 3.5- to 5-kHz bandwidth of typical AM radio and the 15-kHz bandwidth of FM. With improved frequency response, AM HD Radio now sounds more like FM, and FM radio has nearly CD quality.
Digital techniques significantly reduce noise and mitigate fading from multipath, as well as other effects usually experienced in a car radio. Overall quality is far better than that provided by current stations. And the cherry on the top is that all of these features are free. You just have to buy a radio.
HOW IT WORKS
The real innovation behind HD Radio is its ability to transmit the additional digital signals in the same spectrum now allocated to analog signals. This is unlike the digital radio that has been available for many years in Europe, Canada, Asia, and most other parts of the world.
Known as Digital Audio Broadcasting (DAB), this system uses separate spectrum in the 174- to 240-MHz VHF range and in the 1450- to 1490-MHz range. Such spectrum isn't available in the U.S. One company, iBiquity Digital Corp., developed HD Radio to combat the U.S. spectrum problem, though.
Originally known as in-band on-channel (IBOC), iBiquity's system was developed in 1991 and was officially blessed by the Federal Communications Commission in 2002. It's taken years for the company to create—in conjunction with broadcast equipment manufacturers—the transmitters and encourage the development of receivers.
We've now reached critical mass. Over 600 U.S. stations broadcast in HD format. Aftermarket car radios are available from Kenwood, Panasonic, Sharp, and a few other companies, and tabletop radios for home use are emerging. Some high-end stereo receivers come with HD Radio capability. Not to be outdone, car manufacturers—led by BMW— are beginning to incorporate HD Radio in their standard offerings.
Figure 1 illustrates the HD Radio concept. There are two basic modes of operation: hybrid and full digital. Hybrid operation is when both analog and digital information are transmitted simultaneously. Most broadcasts, at least initially, will be simulcast with the same content going out over both the analog and digital parts of the signal. This ensures full compatibility with older analog radios and the newer HD models. Eventually, HD Radio will go fully digital.
Digital content primarily consists of music or talk programming. But the system can also transmit other digital info, such as station identification, the song and artist being played, and the name of the program.
Most stations will offer a digital programming guide. In addition, each station can transmit other digital data that may be helpful to the local community, such as weather or traffic info, and potentially photos and video. This information is automatically displayed in a scrolling format on the receiver LCD display.
How does the system work? First, the audio content is digitized and then compressed, according to iBiquity's HDC codec, to reduce the overall bit rate and required transmission bandwidth. Next, the signal is multiplexed with the other digital data to be transmitted.
The composite signal goes through additional coding, including scrambling, forward-error-correction (FEC) coding, and interleaving. The scrambling randomizes or "whitens" the data to prevent long strings of 0s or 1s from occurring. The FEC coding, namely Viterbi punctured convolutional encoding, increases the robustness of the signal in the presence of noise and fading. The interleaving provides both time and frequency diversity that also help improve reception in loss-of-signal conditions.
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