Digital radio comes down to earth

DN Staff

November 4, 2002

7 Min Read
Digital radio comes down to earth

Back in the early 1900s, engineers must have been pretty impressed with themselves when they first transmitted voice and sound over radio waves instead of the dots and dashes of Morse code. Today, many of their counterparts believe the best way to transmit information is to once again use a binary code. Digital radio is drastically changing not only the design of radio receivers, but their capabilities as well.

Granted, the modulation techniques engineers use today to send digital information are a bit more sophisticated than simply interrupting current flow in a circuit. By transmitting audio information as digital symbols, radio engineers can send more content with less interference than they can with standard amplitude or frequency modulation (AM or FM).

The latest additions to the digital radio arena are based on terrestrial transmissions. One method uses a new modulation scheme requiring broadcasters to upgrade their transmitters. The other leaves the AM and FM signals alone and uses DSP technology only in the receiver to tune in the signal and eliminate interference.

Digitizing AM and FM. Formerly called IBOC (In-Band, On-Channel), HD Radio is iBiquity Digital's proprietary technology for transmitting digital audio in the AM and FM bands and backed by several large broadcasters like ABC and Viacom. Recently, iBiquity received FCC approval for its new technology and expects to have radios on store shelves by early next year. By then commuters should be able to hear HD Radio stations in New York, Los Angeles, Chicago, San Francisco, Seattle, and Miami. Stations converting to HD Radio can expect to pay from $30,000 to over $200,000 to upgrade, depending on the condition of their current equipment. Listeners won't have to subscribe to HD Radio, but they will have to buy a receiver that iBiquity says will cost about $100 more than a standard AM/FM radio. That price difference should drop as the silicon used to create HD Radios becomes cheaper.

HD Radio employs the same transmitting towers and frequencies already in use by commercial AM and FM stations. Some HD Radio stations initially will operate as "hybrid" stations, broadcasting the digital signal along with the conventional analog signal. Thus listeners will be able to hear their favorite stations on a conventional radio. Some stations, however, may choose to be all digital, all the time. By eliminating the analog transmission, these stations may transmit their HD Radio signal at higher power levels than the hybrid stations.

In August, Texas Instruments announced the TMS320DRI200, the first digital baseband chip engineers can use to create HD Radio receivers. Samples of the DRI200 are available now for $50. iBiquity also announced the IBOC Digital Module (IDM), which combines the DRI200 chip with memory and appropriate interfaces on a credit-card size board and is available to licensed radio manufacturers.

HD Radio operates within the FCC "mask" allotted to each broadcast station by bracketing the anlog signal with two, lower-power digital side channels. Typically, these digital channels operate at 5% of the AM and 1% of the FM analog transmitting power levels to reduce interference with the analog signals. HD Radio receivers are designed to fall back to the analog signal if the digital signal degrades beyond acceptable limits.

TI's Worldwide Marketing Manager for Digital Radio, John Gardner, says HD Radio's biggest driver in the market is its ability to carry data as well as high-quality audio. This capability will allow the listener to see a displayed song's title and artist, and to hear local, on-demand, traffic and weather reports. The broadcaster, in turn, can send ads containing text and/or graphics to HD Radio receivers equipped with displays.

Symphony digital radio. Satellite radio and HD Radio require new transmitting equipment because they transmit signals in new, digital formats. Motorola Symphony Digital Radio takes a different approach by applying digital technology only to the receiving side of the equation. Receivers based on the new technology will be on store shelves by late 2003. Using newly developed algorithms in a 24-bit DSP-based chipset, Symphony Digital Radio tunes, filters, and processes the standard AM/FM signal. Motorola says the result is reduced interference and fading as well as a greater listening range.

John Hansen, a director of marketing for Motorola, says the goal was to build a better radio receiver without altering the existing broadcast radio infrastructure. As a result, there is no FCC approval issue, and any AM/FM signal will be compatible with the new radios. Some radio manufacturers are reportedly sufficiently impressed with Symphony Digital Radio prototypes that they have decided to phase out their conventionally tuned radios. That's no surprise, given that it can cost less to build a radio with Motorola's technology than with a conventional analog tuner because software replaces some of the analog components.

Although the price should drop as the silicon used to create HD Radios becomes cheaper, iBiquity expects its first ground-based digital radios to cost about $100 more than conventional radios.

Hansen cites one case in which an automaker discovered it could save $45 using Symphony Digital Radio technology in a luxury-class car with an optical MOST (Media Oriented Systems Transport) network. On the other hand, another car manufacturer reports a cost increase of $10.

Another advantage to using DSP technology to tune and process radio signals is that engineers aren't limited to just AM/FM radio. While that is the market Motorola is focusing on at the moment, at least one company is reportedly looking into how Symphony Digital Radio will play in radar detectors.

DSPs are the driver. Advances in DSP technology have been one of the main drivers behind affordable digital radios. As recently as 1995, iBiquity engineers were struggling just to get a dozen DSPs to work together to prove the concept was feasible. Today, a single DSP accomplishes the same task. One advantage of applying digital processing techniques to radio design is that radio engineers are using if-then-else statements instead of thin sheets of metal to eliminate interference from their designs. But once a signal is in digital form, a whole host of features becomes possible.

Auto engineers, especially, will welcome the flexibility. A car's interior is a constrained environment of engineering tradeoffs where even determining the location of a cup holder is a major design challenge. Thanks to DSP audio algorithms, engineers can shift a speaker's apparent location relative to the listener. Similar software games also allow engineers to reduce the size of or even eliminate the sub woofer while maintaining adequate bass performance.

In addition to CD-like sound and reduced distortion and interference, digital radio can shorten development cycles. That's because a tweak here or an enhancement there usually means changing a few lines of code instead of respinning the PC board and changing the bill of materials. At the moment, however, digital radio technology is not all pros and no cons. Digital radios require a higher-quality RF front end than conventional radios, which translates into higher-cost components and greater attention to PC board design. Also, since DSP-based digital radio technology doesn't lend itself to battery powered designs just yet, don't expect to find it outside the car or home.

That's bad news for people like Steve Kupec. The IBM manager says the only thing his PDA phone with voice recognition lacks is a stereo headset and FM radio. While some devices do come with an FM radio, other features are sacrificed in order to keep size and cost down. As digital technology advances, it should be possible to share a mobile phone's DSP for both phone and AM/FM-radio processing tasks. Such a phone may still need two RF sections, but someday Kupec will have it all: phone, PDA, voice recognition and recording, and a digital radio to get him through those endless PowerPoint therapy sessions, a.k.a. meetings.


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