Radio airs a digital future

Although it is a much older technology, broadcast radio lags television in the conversion to digital transmission. Spectrum allocation, broadcast standardization, and infrastructure problems interfere with a smooth transition from analog to the digital airwaves of the future.

By Warren Webb, Technical Editor -- EDN, 11/23/2000

Technically, digital radio is relatively easy. Simply modulate an RF carrier with a band-limited data stream, amplify the signal, and send it to a broadcast antenna. But the difficulty comes when you try to fit this new technology into a world with a severely limited RF broadcast spectrum, millions of analog receivers, no digital-transmitter infrastructure, no universal digital-broadcast standard, and potential customers that are reasonably happy with current analog radio.

So, what are the benefits of digital radio? Proponents claim that the major reason to yearn for digital radio is the reception of "near-CD-quality" audio. Digital error-detection and -correction codes allow users to establish quality-of-service standards for the transmission path even correcting for the multipath interference that plagues analog broadcast radio. The phrase "near CD quality" takes into account the errors that bandwidth-limited compression algorithms intentionally introduce. Digital radio also allows broadcasters to offer free or fee-based additional services, such as music lyrics, road conditions, stock quotes, and one-way Internet applications.

The first step in deploying a digital radio system is to define a transmission standard. Dozens of countries, including Canada, the United Kingdom, Germany, France, and China, have already adopted the Eureka 147 standard and have working digital radio systems (Reference 1). Eureka 147 is a fully digital system operating at any frequency from 30 MHz to 3 GHz. The broadcast signal, called a multiplex, simultaneously carries several digital services. The overall bandwidth is 1.536 MHz, providing a useful data rate of 1.5 Mbyte/sec. Each service is independently error-protected with a coding overhead of 25 to 300%, which the broadcaster selects depending on the transmitter coverage and desired reception quality. Multiple stations or data services may share a single-multiplex broadcast signal. The signal is divided into channels with bit rates varying from 8 to 384 kbps. Typically, each multiplex provides five to six high-quality stereo programs or as many as 20 restricted-quality, single-channel programs. Audio bit streams may also include an embedded program-associated data substream for digital-display information such as song titles, lyrics, and phone-in numbers. Each service may also include conditional access encryption for subscription data feeds or programs.

So far, the FCC (Federal Communications Commission) has not adopted a digital-radio standard. The FCC has balked at accepting the Eureka 147 standard because it would require the allocation of a new RF spectrum. The NAB (National Association of Broadcasters) is also against the Eureka 147 system, because its members would have to develop and pay for new transmitting infrastructure, which would essentially compete with their analog-radio stations. Both entities favor a standard that uses existing frequencies and transmission equipment.

Hybrid contender

The remaining contender for a US standard is IBOC (In-Band-on-Channel) technology proposed by iBiquity Digital (www.usadr.com). Lucent Digital Radio and USA Digital Radio recently merged to form iBiquity Digital, combining the major competing digital-radio technologies. IBOC combines the digital and analog signals within the spectrum allocated for a station's current analog signal. Depending on the final form of the standard, IBOC may use a portion of the "guard" spacing between adjacent stations. Like Eureka 147, error correction and data compression are part of the digital broadcast signal. A digital signal is more efficient than the corresponding analog signal, so it can be broadcast at much lower power levels. Therefore, the digital signal can fit into the side lobes of the FCC-designated FM mask without interfering with adjacent stations (Figure 1).

Regardless of the standard chosen, digital broadcast radio works best with a modulation method that spreads the signal over the available spectrum. OFDM (orthogonal frequency-division multiplexing) is a multicarrier technique that divides a high-speed data signal into tens or hundreds of lower speed signals, all transmitted in parallel, dividing the data across the available spectrum into a set of carriers. Each carrier is independent or unrelated (orthogonal) to all other carriers, so frequency information can overlap and still be extracted at the receiver. In addition to the bandwidth efficiency, you can modulate and demodulate subchannels with a discrete FFT (fast Fourier transform) at the transmitter and the receiver.

Multipath is one of the biggest sources of interference with analog FM signals. Delayed signal reflections from manmade buildings and natural hills or trees compete with the direct signal to cause frequency-selective fading. Because OFDM comprises many narrowband components, multipath interference degrades only a small portion of the signal and has no effect on the remaining frequency components.

One problem unique to digital radio is the dropout that listeners experience at the fringe of the reception area. The receiver's processor shuts off the audio when the error rate is too high. Fringe listeners prefer analog radio, in which a weak signal fades in and out of the static over digital radio's intermittent segments of silence. With IBOC, processors can switch over to analog reception when the digital error rates exceed a predetermined threshold.

Even with a standard in place, digital radio faces a tough future. Digital radio offers the consumer few benefits over the analog system. In a car with outside noises competing, it would be difficult for the typical user to tell the difference between current FM quality and "near-CD" quality. In addition, auxiliary subcarrier services are available now with analog radio but are seldom used. So far, most consumers in Europe, where digital radio is already operational, have resisted the urge to purchase expensive digital receivers (Figure 2).

Digital choices

Consumers that want high-quality audio also have choices other than broadcast digital radio. Sirius Satellite Radio (www.siriusradio.com) expects to be operational before broadcast digital radio and offers 100 channels of high-quality sound and seamless coast-to-coast coverage. Music channels are commercial-free. Users must purchase a satellite radio receiver or a $199 adapter for their AM/FM radio and pay $9.95/month. A series of terrestrial repeaters ensures unobstructed coverage in urban and mountainous areas.

3Com's Kerbango tuning service (www.kerbango.com) is also on track to deploy its Linux-powered Internet radio before broadcast digital radio becomes available. The radio allows users to tune in thousands of stations with a simple Internet connection. Because the Internet gives two-way communications, users can also query the data source to obtain detailed information on songs, artists, and advertisers.

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