HD Radio Technology — the science behind the sound

HD Radio technology works much like traditional analog transmissions (AM and FM are both analog signals).The difference is that the station broadcasting HD Radio technology transmits an extra digital radio signal, along with its normal analog signal. It can also broadcast a third signal for text data (click here to learn more about text data).

Your radio receiver receives the signal – just as it does an AM or FM signal. If you have a HD Radio receiver, it will decompress and translate the signal and viola! You get bright, clean, near-CD quality sound.

What happens if you don’t have one of these receivers? It’s simple. You hear your normal analog radio– AM or FM.

What to expect from digital AM and FM radio

AM radio has smaller sections of bandwidth than FM radio. This means there is not enough “space” to give AM stations the same near-CD quality as FM stations. But there is enough bandwidth that AM stations will be able to broadcast with the same clarity of signal as one of today’s analog FM stations. This performance boost is expected to make AM radio a better alternative to FM than it has been – to give you more listening choices.

Digital FM radio is less vulnerable to reception problems. Your HD Radio tuner’s digital processors will eliminate all those annoying pops, hisses, fades and static caused by interference.

What happens if you lose the digital signal for some reason? Really nothing. HD Radio technology defaults back to analog mode in much the same way as conventional radios switch from stereo to mono mode when the signal is weak. Then, when the digital signal again becomes available, your HD Radio automatically switches back.

If you just have to know “what’s behind the curtain”

Here’s a technical description of HD Radio as written by Larry Loeb, Principal, PBC Enterprises.

“Digital radio has been in development in the US since 1991. Digital Audio Broadcasting (DAB) is a system that has been in use around the rest of the world for years now. But US broadcast spectrum is different, and the VHF frequencies of 174 MHz to 240 MHz(or 1450 MHz to 1490 MHz) that DAB uses aren’t available.

“It wasn’t until 2002 that the FCC blessed a single method of doing digital radio for the US: iBiquity DigitalCorp’s IBOC, which stands for in-band on-channel. (iBiquity, by the way, wasformed by Lucent and Westinghouse as a spin-off.) The system uses the same radio spectrum that a station currently uses for broadcast, and (unlike HD TV) can allow a station’s content to be received by analog and digital radios with the same signal (in the “hybrid” mode). Of course, the digital signal will sound better, but the analog radio won’t sound any worse.

“If HD takes off, broadcasters can eventually go all-digital. But the IBOC signal fits into the power level/frequency spectrum mask that the FCC wants to see from a broadcaster right out of the gate, so no frequency reassignments will be necessary to implement the scheme. Use of existing broadcast spectrum is such a powerful advantage for the IBOC system that many European countries using DAB (except for Britain, where DAB seems to be entrenched) are now considering changing over to IBOC.

“Source material is provided by a broadcaster in layer 5, source encoded in layer 4, multiplexed into logical channels by layers 3 and 2, and finally formatted for RF transmission in layer1.

Figure 1. IBOC layers (courtesy of iBiquity)

Figure 1. IBOC layers (courtesy of iBiquity)

An AM IBOC broadcast can be either hybrid (which retains the mono AM signal in analog form) or all-digital. In the hybrid waveform, the OFDM (orthogonal frequency division multiplex) sub carriers are located in primary and secondary sidebands on either side of the host analog signal, as well as underneath the host analog signal in tertiary sidebands. In the AM system, due to the narrow bandwidth available, Quadrature Amplitude Modulation (QAM) is used on each of the OFDM sub carriers. Each sideband has both an upper and lower component. Status and control information is transmitted on reference sub carriers on either side of the main carrier. Figure 2 shows these relationships.

Figure 2: AM Hybrid waveform spectrum(courtesy of iBiquity)

Figure 2: AM Hybrid waveform spectrum (courtesy of iBiquity)

Two additional sub carriers exist between the primary and secondary and the secondary and tertiary sidebands on either side of the main carrier. These are called the IBOC Data System (IDS) sub carriers and are primarily used for low latency, low data rate applications such as SIS. The power level of each OFDM subcarrier, in the primary sidebands, is fixed relative to the unmodulated main analog carrier. However, the power level of the secondary, IDS, and tertiary sub carriers is adjustable.

Things change if the AM broadcast is all-digital. Figure 3 shows what the waveform spectrum would look like in this case.

Figure 3. AM digital waveform spectrum (courtesy of iBiquity)

Figure 3. AM digital waveform spectrum (courtesy of iBiquity)

Broadcasting FM

The FM IBOC system shares the same protocol layers as the AM system. There are differences due to the wider bandwidth available in the FM system. Three waveform types can be created: hybrid, extended hybrid, and all-digital.

The hybrid contains digital information transmitted in sidebands on either side of the analog FM signal, which can be mono or stereo. This is shown in Figure 4, with the sub carrier numbers and the frequency offset.

Figure 4. Hybrid waveform (courtesy of iBiquity)

Figure 4. Hybrid waveform (courtesy of iBiquity)

The extended hybrid adds more digital information to the analog signal with upper and lower digital sidebands. This is shown in Figure 5.

Figure 5. Extended hybrid waveform spectrum

Figure 5. Extended hybrid waveform spectrum

The all-digital waveform shown in Figure 6 takes the frequency space previously used by the analog waveform and expands the bandwidth of the primary digital sidebands while adding lower-power secondary sidebands. All four of the extended frequency partitions are present in each primary sideband of the all-digital waveform.

Figure 6. All-digital waveform (courtesy of iBiquity)

Figure 6. All-digital waveform (courtesy of iBiquity)

Performance and power

There are seven primary service modes in FM waveforms (rather than the four of AM), due to the addition of the extended hybrid mode. Additionally, there are four secondary service modes that only come into play with the all-digital waveform. These serve to set up the performance and power level configuration of the logical channels used to get the content through the first protocol layer. By allowing different combinations, a broadcaster can configure the system for signal robustness.

The first protocol layer has several functions in it. The data is first scrambled (randomized) in each of the logical channels to avoid signal periodicity’s that can cause degraded reception. The channels are then encoded with forward error-correction algorithms that add error correction bits that the receiver can use to correct a signal when it processes it. Interleaving will reorder the transmitted bits to minimize burst errors that can occur in a fading channel. The service mode will affect how the interleaving is done. System control mapping, and OFDM subcarrier mapping are the next to occur. The output of these steps is a frequency-domain representation of the input signal. The shaped time-domain OFDM signal is then generated and transmitted. In the transmission step, the analog signal (if present) is modulated and combined with the digital signal to form the final composite signal that is transmitted.

Simulcasts for podheads

FM bandwidths let a broadcaster carry simultaneous programs along with the main one. Simulcast programs are one of the primary business attractions of using IBOC, because the additional content can differentiate one station from another. The prospect of selling ads on alternate programs doesn’t hurt, either.

Of course, someone’s going to eventually figure out that this is a great distribution channel for podcasts (that is, user-generated audio that the broadcaster can get for free). Broadcasters can stuff the side-channels with “hip” content that will market well to people who already get their music from the ‘net. They can also play wireless content aggregator to all the stuff that’s out there. All those sites with user MP3 compositions? Content. IBOC gives the radio crowd a chance to compete with the ‘net by co-opting it, embracing it, and distributing it.

This kind of thing isn’t as far off as you would think. Texas Instruments (TI) is a primary supplier of DSP chips used in HD radios for most of the grunt work. TI’s TMS320DRI300 provides both MP3 and Windows Media Audio support. The hipsters looking for a new electro-toy are more likely to be early adopters if their tastes in entertainment follow them. Why download a podcast if someone is streaming if for you?

Or, just do a timed record onto your hard disk. There are a lot of possibilities here, and I’ve only skimmed the very top of the lot.

In conclusion

HD radio is coming soon. The first real receivers are out, and more are coming. Expect a steady stream of content gimmicks in the near future, as broadcasters try to jump-start consumer acceptance and spur the sale of HD radios. They’ve learned their lesson from the satellite guys, and are ready to start fighting back.
My big question is, who will be the Howard Stern of HD radio?

About the Author

Larry Loeb has written for many of the last century’s major “dead tree” computer magazines, having been — among other things — a consulting editor for BYTE magazine and senior editor for the launch of WebWeek. Larry’s newest book has the commercially obligatory title of Hackproofing XML and is published by Syngress (Rockland, MA). He’s been online since uucp “bang” addressing (where the world existed relative to !decvax), serving as editor of the Macintosh Exchange on BIX, and the VARBusiness Exchange. He’s also written a book on the Secure Electronic Transaction Internet protocol. His first Mac had 128 KB of memory. His first 1130 had 4 KB, as did his first 1401. You can e-mail him at larryloeb@prodigy.net.