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.
After that step, the formed composite digital packets are sent for OFDM mapping and generation. Finally, this signal is then transmitted along with the standard analog signal. The digital information appears in the spectrum above and below the regular analog sidebands.
Figure 2 shows the hybrid formats for both AM and FM. Note the FCC spectrum masks. In the AM signal, the normal analog signal appears with its sidebands ±5 kHz from the carrier frequency. There are two sets of digital channels. The primary channels extend from about 10 to 15 kHz above and below the carrier. The secondary channels extend from about 5 to 10 kHz above and below the carrier.
Also, tertiary sidebands exist in the ±5-kHz range "underneath" or in quadrature with the analog carrier. Overall, there are 81 OFDM carriers with a spacing of 181.7 Hz. The primary carriers use 64-QAM (quadrature amplitude multiplexing), while secondary sideband carriers use 16-QAM. Tertiary sideband carriers employ quadrature phase-shift keying (QPSK). The digital audio bit stream is 36 kHz for a final audio bandwidth of about 8 kHz.
The FM spectrum is a bit more complex. The analog signal, with its multiple FM sidebands, appears 130 kHz above and below the carrier frequency, as shown in Figure 2. The digital data, contained in OFDM carriers, runs about 130 to 200 kHz above and below the analog sidebands. The upper and lower digital spectrum each contains 10 partitions, with each of those featuring 18 subcarriers. Use of these carriers depends on the kind of service provided. Spectrum plans for all-digital AM and FM modes also exist, and they differ from the hybrid plans described here. These future formats can be seen in the iBiquity and NRSC literature.
HD Radio will make its greatest impact on the FM side, where most of the action in radio is today (about 80%). At those higher frequencies, more bandwidth is available, and higher digital rates can be achieved. AM radio has an adjacent channel interference problem that can occur over long transmission distances.
FM transmission is usually limited to line-of-sight distances up to about 100 miles, which significantly minimizes the interference problem. It also allows frequencies to be reused across the country with adequate spacing. With AM, propagation is via short-distance ground-wave during the day, so interference is minimal. But at night, ionosphere changes make sky-wave-refracted signals common over very long distances. Banning AM HD broadcasts at night solves that problem.
HOW TO DESIGN IT
If you're going to design an HD radio, you must first contact iBiquity for licensing information.
The company owns the technology and licenses it to transmitter and receiver manufacturers. It also has a full testing and certification program that ensures compliance with the standard, designated NRSC-5A. It's available from the National Radio Systems Committee, which is co-sponsored by the National Association of Broadcasters (NAB) and the Consumer Electronics Association (CEA).
Given the state of IC technology today, making an HD Radio is relatively easy despite the technology's complexity (Fig. 3). You'll need a front end or tuner that translates the AM and FM signals to an intermediate frequency (IF), an analog-to-digital converter (ADC) to digitize the IF, and a DSP chip to handle all of the remaining radio functions. Digital-to analog converters (DACs) then drive the stereo power amplifiers. An internal embedded controller is needed to run everything, including the tuning functions and LCD display.
Several sources of front-end tuners are available. For example, Atmel's T4260 biCMOS chip was certified by iBiquity for AM/FM automotive or home desktop radios. It has two RF paths, one for AM and the other for FM. The signal from the antenna goes to the mixer, which upconverts the AM band to an IF in the 10- to 25-MHz range. Usually it winds up in the popular 10.7-MHz setting, where many ceramic IF filters are available. In the FM path, the signal is down converted to 10.7 MHz.
A phase-locked-loop (PLL) synthesizer supplies the local oscillator signal to both mixers. Incremental tuning is 1 kHz for AM and 12.5/25/50 kHz for FM. Separate fast automatic-gain-control (AGC) sections for AM and FM are supplied. Gain can be changed via internal DACs in 1-dB increments for RF circuits and 2-dB steps for IF circuits. Also included is a three-wire serial bus for tuning and control.
Philips Semiconductor's TEF6721HL resembles the Atmel chip. But it also covers the 31-, 41-, and 49-m shortwave bands and the VHF FM weather band. An I2C serial interface is used for tuning and control. Both of these ICs require the necessary antenna, impedance matching, and other front-end components.
Alps Electric's TDGA2 fully integrated and shielded tuner is designed for IBOC signals in the AM and FM bands. The IF output is 10.7 MHz. An I2C bus is provided for tuning and automatic-gain and other control. The IF signal from the tuner must then be digitized in an ADC. One of the most popular circuits is Texas Instruments' AFEDRI8201. It passes the 10.7-MHz IF signal to a programmable gain amplifier and then to the ADC. This 12-bit converter samples the IF signal at 80 Msamples/s and supplies the interface to the external DSP chip. The 8201 also includes a DAC that controls the AGC in the tuner.
The DSP performs all of the decompression, de-interleaving, decoding, and demodulation. Texas Instruments, the primary supplier of DSP chips for HD Radios, offers its TMS320DRI300/350 baseband chips incorporating all of the licensed iBiquity HD Radio software. They handle all AM and FM functions and provide audio post processing and MP3 and Windows Media Audio (WMA) support. They also feature interfaces for the external embedded controller, boot EEPROM or flash, and de-interleaver DRAM. The output is designed for compatible DACs.
Though TI is currently the primary supplier of DSPs for HD, Philips has its share of chips. The SAF3550 HD Radio digital baseband processing chip uses a powerful ARM946 processor core with on-chip cache. It also includes programmable baseband input and audio output samplerate converters. It's compatible with the TEF6721HL tuner mentioned earlier.
Philips' SAF7730 is a flexible dual-IF radio with audio DSP. Designed for car radio applications, it integrates most of the radio on one chip. Its programmability lets designers differentiate their own design. The digital outputs from the DSP usually go to external DACs available from multiple sources. The DACs drive the stereo power amps, which typically are class AB linears or class D switching amps for some auto applications. Completing the picture is a power supply for ac-operated units or a power-management chip for auto battery operation.
Testing requires an HD Radio exciter, which can be had from transmitter manufacturers Broadcast Electronics or Harris. Also, iBiquity can supply information about its testing and certification program.
One of the main issues these radios face is the EMI problem caused by the DSP and embedded processors. To reduce the interference with sensitive RF receiver circuits and to meet FCC class B radiation tests, most manufacturers must spend time designing a board layout that minimizes EMI. Filtering eliminates most of it, but in some cases, shielding is required.
So far, iBiquity has blessed the Radiosophy MultiStream HD desktop radio. It uses Alps' tuner and TI's chips. FM signal-to-noise ratio is 78 dB, and AM's is 50 dB. The MultiStream's multicast capability lets uers select multiple streams from the FM signal. The AM and FM antennas are built in, with connections available for external antennas. Audio comes from dual-channel 8-W amplifiers with 4-V speakers. The 128-by 64-pixel backlighted LCD display shows time, frequency, and scrolling messages as stated in the HD standard.
Other interesting features include a stereo headphone mini jack and a USB port that will upload software upgrades into the DSP via the Internet and a PC. It also integrates RCA phono jack outputs in Sony/Philips Digital Interface (S/PDIF) digital format. An S/PDIF Toslink optical output jack sends the digital audio directly to an external stereo amplifier with larger speakers.
HD Radio is here. Unfortunately, only a handful of people have it, and not many more are even aware of it—despite the fact that it's present in multiple stations in most areas.
Recognizing this, major broadcast corporations have joined together to form the HD Digital Radio Alliance. This organization is devoted to publicizing HD Radio and lobbying auto manufacturers to add HD to their standard radio offerings. With more home radios on the way, users will soon be able to sample multiple new programs and services.
HD Radio was born with the initial goal of improving radio audio quality and reception reliability, which has been achieved. But with the new multicast capability (called HD2), stations are lining up lots of new shows and programs, such as specific music channels as well as talk shows.
Once they're in place, HD Radio will give satellite radio a run for its money. HD won't replace satellite radio, with its hundreds of channels of programming. But it will offer far more local programming choices, all for the price of a new radio. Look for high-defintion to explode once the word gets out and radios become affordable.
For more, see "Few Radios, Yet" at Drill Deeper 12192, "100 Years On The Air" at Drill Deeper 12193, and "Need More Information?" at Drill Deeper 12213 at www.electronicdesign.com