OTHER TECH

Mixed SignalsSo, IBOC it is. And creating it took a lot of ingenuity. The digital content in an HD Radio broadcast shows up as low-power sidebands on each side of the station's main analog carrier.

On FM (Click for PDF), the two identical sidebands each contain digital stereo audio plus data. Because the sidebands are about 300 kHz apart, multipath and other problems that affect one sideband aren't likely to affect the other at the same time. The receiver constantly compares the two sidebands and decodes the cleaner one. If the signal fades, the receiver will maintain digital quality and full stereo separation as long as it can, then fall back to analog instead of simply cutting off, the way a pure digital system would. Even though the station's digital signal has only about 1% as much power as its analog one, listeners will hear digital stereo until the more powerful analog signal is nearly out of range.

On AM, where the transmission channel is only about one-tenth as wide as on FM and the signal is more prone to interference, things get trickier. Here, the duplicate sidebands carry only mono audio, while the stereo information is separate, as it is in FM, and is tucked underneath the main analog "carrier" signal (Click for PDF ). To prevent interference with the main carrier, the stereo data is transmitted 90 out of phase with it and at so low a level that an analog tuner can't tell it from background noise. If the signal fades, the receiver will first fall back to high-quality digital mono reception, then to analog.

For FM, the audio data stream available to HD Radio is 150 kbps (of which 96 kbps is used for main-channel audio), but for AM it's only 36 kbps. To squeeze good audio into such small spaces requires a perceptual coding system, which subtracts from the signal those parts least likely to be heard or noticed. iBiquity uses Lucent's Perceptual Audio Coder (PAC), the same one used by the Sirius satellite radio system, but at a higher bit rate on FM and with limited bandwidth on AM.

For programs originating at the station airing them, this shouldn't be a problem. However, programs originating elsewhere might have gone through a coder and decoder (codec) already, and audio that passes through multiple codecs can wind up sounding awful.

According to Glynn Walden, iBiquity's vice president of broadcast engineering, the major radio networks have upgraded to new satellites with higher-quality coding. They now use MPEG Audio Layer 2 at a 128-kbps bit rate. "Funneling that down to HD Radio's 96 kbps shouldn't be a problem," Walden said.

Cracking the CodeCompared with the broadcast technology, the architecture of HD Radio receivers looks simple. Like many existing AM/FM radios (such as Blaupunkt's DigiCeivers and some Visteon car stereos), the new receivers process signals digitally. In such receivers, the radio-frequency (RF) tuner chip selects the desired station and converts its signal to a fixed intermediate frequency (IF), all in the analog domain.

A "baseband processor" chip digitizes the IF signal and decodes it, digitally, into audio (and left and right channels, if it's stereo), RDS data (such as station call letters) displayed on Radio Data System-compatible gear, and HD Radio data. The audio then goes through a digital-to-analog converter before being sent to the receiver's amplifier, RDS data goes to the display, and HD Radio data goes to a dedicated chipset. That chipset decodes the data, feeding the audio content back to the baseband processor and the rest to a microprocessor, which controls the whole process and finally feeds text data to the receiver's display. Adding HD Radio to a receiver that has the basic digital architecture is expected to increase the price about $100 for now, and a lot less in the future.

Costs to the stations, of course, will be much higher. Yet HD Radio is unlikely to suffer from the indifference that afflicts RDS, an earlier and less expensive system for grafting text information onto analog FM signals. Stations have had little incentive to adopt RDS, and less to implement its fancy features (including the ability to tell the listener what was playing). For HD Radio, the incentives are strong: staying competitive in a digital world and possibly finding new income sources.

Ironically, HD Radio may give RDS a boost. When RDS started here in 1992, any display text had to be typed in by hand-an added expense. Today, radio stations store information about the music they play and things like traffic or weather conditions in databases. Stations that transmit such information via HD Radio are encouraged to transmit it via RDS as well-which could lead even stations that don't go digital to do the same.

If HD Radio catches on, everyone wins, stations and listeners alike. A respectable number of stations are already converting to digital, and the ones that aren't will be watching to see how their competitors fare. For listeners, it's simply better-sounding radio. For stations, it may be a last chance to make their fortunes blossom as they did when analog was king. S&V