The recent FCC adoption of IBOC as the U.S. digital radio broadcast standard is a pivotal event in the history of broadcasting. From its inception in the early 1990s, the broadcast trade press has followed closely and reported on the development and rollout of this rather complicated technology and the implementation of it called HD Radio.
Anyone interested in how IBOC actually works at a more technical engineering level has been hard-pressed to find much published documentation on the subject, other than white papers published by the technology developer itself. That challenge has now been solved in splendid fashion by David Maxson and the release of “The IBOC Handbook.”
For 20 years, Maxson served commercial classical station WCRB(FM) in Boston and was its vice president and director of engineering. At the time, Charles River Broadcasting owned the station; it changed hands from Greater Media to Nassau Broadcasting in 2006.
Maxson formed Broadcast Signal Labs, a precision frequency measurement and signal analysis service, with longtime friend and fellow engineer Rick Levy in 1982. He began to focus more on digital broadcasting projects in 1995 and has participated as a key member in National Radio Systems Committee standard-setting proceedings since 1998.
David Layer, NAB Science & Technology director of advanced engineering, is the book’s technical editor; Focal Press published it in association with NAB.
A comprehensive view
(click thumbnail) “The IBOC Handbook” was written to provide a comprehensive look at IBOC from the inside out.
The first two chapters lay down the building blocks and establish a clear context of how IBOC evolved and took its place as the industry’s digital broadcast platform choice for radio.
Every component of IBOC’s service feature set is explained, including Main Program Service (MPS), Supplemental Program Service (SPS), Station Information Service (SIS) and Advanced Data Services (ADS) with future data enhancements.
Maxson traces the rise of IBOC through its various stages: Project Acorn, USADR, the formation of Ibiquity Digital and the long and arduous testing on pioneering stations. He documents the role of the NRSC, managing the process by which the necessary protocols and specifications were defined and fine-tuned so that a standard eventually could be adopted for both the FM and the AM broadcast services.
Competing digital technologies including Eureka-147, DRM, CAM-D, DVB-T/H, FMeXtra and the satellite radio services are discussed in significant detail. This sheds valuable light on the big-picture context within which IBOC has emerged as the chosen standard.
The standards-setting process often is misunderstood by many in our industry, especially by those who have never participated. As an active member of the NRSC, Maxson is able to paint a clear picture of the rigorous NRSC regimen.
In his second chapter, “The Path to the IBOC Standard,” Maxson recounts the chronology of NRSC activities, including DAB subcommittee and working group meetings beginning in 1998. The objectives and goals established at official meetings are documented along with their follow-up results.
Key issues that confronted the NRSC during the standards-setting process included FM subcarrier compatibility, the codec crisis of PAC vs. HDC, multicasting and “Tomorrow Radio,” competing surround sound proposals, and the thorny problem of AM nighttime performance. The book offers Maxson’s insider view of how each was handled.
The dynamics of how digital communications systems actually work are heavily grounded in “protocol stacks,” the ordered implementation of software-driven rules by which the various layers of computer networking processes communicate. Maxson offers a plain language guide in his introduction:
“As one progresses from the information input at the top of the stack to the radio signal that comes out the bottom of the stack, the processing tasks require different types of engineering.” The eight chapters in Section 2 are devoted to the NRSC-5 IBOC protocol stack in exquisite detail.
Readers with a basic understanding of digital transmission theory will be able to navigate through this section more rapidly than those who are less technically inclined.
Keeping track of all the acronyms can make one’s head hurt, but Maxson does a good job of relating technical definitions and concepts in understandable language and metaphors. Following the journey of PDUs (Protocol Data Units) as the packaged carriers of digital information through each layer of NRSC-5 becomes fascinating.
Data, data, data
The NRSC-5 core standard was adopted by the NRSC in April of 2005 and focuses on how any developer would assemble the standardized IBOC digital signal for transmission.
All computer-based communications systems use protocol stacks derived from the legacy OSI (Open Systems Interconnection) model. The Handbook details and illustrates the overall system blocks and layers of the NRSC-5 OSI in Chapter 3.
Another chapter breaks down and describes the top transport layer of NRSC-5, Program Services and PSD or Program Service Data. The audio codec processing and main error protection techniques using Reed-Solomon coding and CRC checks also are explained in this chapter.
Chapter 5 tackles Channel Multiplexing, the last portion of transport layer 2 in the stack. Here, the role of Logical Channels within the RF channel of multiple subcarriers as well as the OFDM modulation scheme are explained and illustrated. That sets the stage for audio transport layer 1 where the vitally important component pieces of digital encoding including scrambling, folding in data, convolutional coding, matrix filling, puncturing, and interleaving are fully covered.
The next phase of generating the IBOC signal unfolds in “Populating the OFDM Waveform With Information.” The block diagrams, illustrations and discussion detail the structure of layer 1 including frame and block characteristics, the critical functions of symbols, latency, AM and FM time diversity, time synch, system control processing and differential encoding.
At the end of this chapter, the book summarizes the various Service Modes available in both AM and FM IBOC, which offer shared uses for different data payloads in the Logical Channels of the OFDM subcarriers.
Two chapters present analysis of the physical layers of the FM and the AM OFDM carriers. The primary, secondary and tertiary subcarrier maps are detailed along with a revealing discussion about symbol values, matrices and constellations.
Maxson offers mathematical analysis of the OFDM modulation vectors for advanced readers. While the equations are there to be dissected, the explanations of the concepts they represent are also valuable for those who glaze over when looking at algebra and calculus problems.
Transmission System Specifications are examined in Chapter 9. Timing issues and diversity delay alignment techniques are discussed.
Special emphasis is devoted in this chapter to spectral emission limits and the NRSC masks for both AM and FM. Valuable tips on how to fine-tune the IBOC transmission system to achieve mask compliance are presented.
Maxson completes his top-to-bottom analysis of the IBOC protocol stack in Chapters 10 and 11 with a detailed breakdown of the SIS Data (Station Information Service) and Advanced Data Services. ADS is the arena where future data applications will be developed and threshed out in the marketplace.
Title: “The IBOC Handbook: Understanding HD Radio Technology”
Author: David P. Maxson
Publisher: NAB/Focal Press
Price: The book retails for $89.95 and was available used $42.79 from Amazon.com through www.ibocbook.com. It’s available from the NAB Store at a member price of $79.95.AM nighttime tests
Maxson also outlines Use Cases, mapping of potential classes of future data applications that may appear in ADS, including results of the Impulse Radio study on this topic.
The third section of the book covers the part of IBOC that station engineers must face directly: implementation.
One chapter examines AM considerations, focusing on the demands IBOC places on the AM transmission chain, including transmitter output bandwidth, antenna load Hermitian symmetry, impedance bandwidth and antenna pattern bandwidth performance. Maxson draws from the resources of various consulting engineers to offer tips on how to measure and optimize load conditions to maximize performance.
Another chapter concludes with a discussion about AM IBOC nighttime operations, including a review of the rigorous tests Ibiquity performed to ensure compatibility with analog operations during skywave. With the recent rocky start of the full-time AM HD Radio rollout, it would behoove those with reservations about the issue of compatibility to take a close look at this section.
The considerations involving implementation of FM IBOC systems in Chapter 13 examine the various methods of launching and combining the digital and analog signals. Issues involving multiplexers as well as combined and/or separate antennas are covered.
Engineers who have installed or are planning to install FM IBOC are familiar with these choices and concerns. However there is a wealth of valuable additional information in this chapter that will benefit station engineers charged with maintaining optimized performance.
“The IBOC Handbook” completes its daunting mission of telling the consummate IBOC story with a chapter covering real-world transmission and reception issues. Maxson lays out the needs of a typical IBOC station transmission chain and interconnection requirements including audio processing, level matching, STL, Exgine topology, Importer and Exporter, plus network IP and security issues.
The closing section of the final chapter offers IBOC coverage observations based on charted data collected by the NRSC and more recently by NPR Labs.
Readers will conclude that in general, the first generation of consumer IBOC receivers do not achieve the early projections made by the 2001 NRSC tests concerning the limits of reliable IBOC coverage. Maxson does indicate that receivers should improve as the rollout continues.
An Appendix by Dave Wilson, director of technology and standards for the Consumer Electronics Association, offers a comprehensive overview of IBOC digital radio receiver fundamentals.
This is the best published resource on virtually everything you’d want to know about IBOC receivers I’ve encountered. The back of the book includes a useful glossary including those pesky acronyms engineers will find handy.
Maxson completed the book in June, one month after the FCC released the text of its “official” IBOC authorization, which was adopted in March. Full on-air implementation of the standard commenced on Sept. 14.
As a working engineer maintaining a cluster of FM IBOC stations, I found the book an engaging, though at times challenging, read.
I am left with the overall impression that while the data offerings of IBOC in the form of Ibiquity’s HD Radio are flexible and scalable to accommodate new products and opportunities, the standard will not be easily modified to “fix” perceived fundamental problems with this technology going forward.
Also, Maxson does not discuss criticism of the technology to any extent. Throughout, he tacitly accepts the prevailing conclusions of the NRSC committee and Ibiquity’s attitude about interference caused by IBOC operations, especially AM IBOC: that primary coverage contours are still protected and “bonus coverage” beyond that enjoyed by many stations, including skywave, just isn’t all that important anymore.
But engineers who have not investigated the design fundamentals of the IBOC system beyond equipment installation and performance concerns should greatly benefit by acquiring their copy of the IBOC Handbook. It will prove to be an invaluable resource.