IBOC Implementation Details Emerge

As the much-awaited deployment of in-band, on-channel nears, an IBOC implementation session at the recent NAB2002 convention offered a wealth of practical information.
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As the much-awaited deployment of in-band, on-channel nears, an IBOC implementation session at the recent NAB2002 convention offered a wealth of practical information.

As the much-awaited deployment of in-band, on-channel nears, an IBOC implementation session at the recent NAB2002 convention offered a wealth of practical information.

From theoretical papers that gave an inside look at IBOC signals, to nuts-and-bolts recommendations on how to make it work, this session was a must for those planning deployment of IBOC digital.

Of particular interest was an array of AM transmission system experts giving detailed presentations on the requirements for deployment of IBOC on the AM band. All experts agreed that making it work would require a system bandwidth much higher than needed for conventional AM.

"When IBOC comes along, we're going to be dealing with components out to about 40 kHz," said W.C. Alexander, director of engineering for Crawford Broadcasting and a contributor to Radio World.

G. Michael Patton, a regional consultant and systems integrator, agreed: "IBOC is a whole new ballgame when it comes to bandwidth," he said. "And the problem is if we don't make it, it just might not work at all."


Various experts defined the required bandwidth requirements differently.

Alexander suggested that IBOC transmission requires a maximum voltage standing-wave ratio, or VSWR, of 1.4 to 1 at 15 kHz above and below the carrier frequency.

Patton said VSWR must be kept below 1.25 to 1 out to 15 kHz. Glynn Walden of Ibiquity Digital Corp. defined adequate bandwidth as having "hermitian symmetry" for at least 5 kHz above and below center frequency. Hermitian symmetry means that antenna reactances are equal and of opposite sign on either side of the carrier.

To achieve this higher bandwidth, special attention must be paid to AM system design and adjustment. Many AM stations may require new antenna tuning and/or phasing equipment.

All the experts agreed on the basic design elements for a wideband antenna system. T-networks for matching impedances and shunt power dividers (for directional arrays) are preferred over older methods. Several also suggested the use of input components matched to the slope of the tower impedance to improve matching bandwidth.

However, broadcasters will find that in some cases it will be difficult to meet the new bandwidth requirements.

"Note well: all antennas are not going to work with IBOC," said Patton.

Short radiators, such as antennas less than 70 degrees in length, will be difficult to broadband due to their low base resistance. Alexander suggested that it might be possible to electrically lengthen these towers by extending them or using top loading to achieve a more favorable impedance.

Special situations

Skirted towers also present some challenges to broadband performance. Alexander recommended that the shorting stub on skirted towers be selected for optimal bandwidth rather than the approximately 50-ohm impedance commonly selected during installation.

"Then use a T-network to match the skirt," he suggested.

In directional arrays, negative power towers, especially those that operate with a small amount of negative power, can cause an array to have poor bandwidth. This type of negative tower sometimes can shift to positive impedance when driven at frequencies offset from the carrier frequency.

Alexander proposed replacing these towers with a resistor in some cases. "It can significantly improve the system bandwidth," he said.

With a series of charts, Bobby Cox of Kintronic Labs Inc. showed the dramatic bandwidth improvements that are possible with careful design. Using a case study of two AM stations diplexed on the same tower, he demonstrated how each element of the design and adjustment contributed to improvements in performance with bandwidth measurements made at each step.

The result was an antenna system with nearly perfect VSWR performance out to a very wide bandwidth.

Separate antennas?

An alternate means of FM IBOC implementation was addressed by Eric Wandel, director of product development at Electronics Research Inc.

Wandel proposed the use of separate antennas to deploy IBOC signals as an alternative to the use of high-power combining into a shared antenna. The second antenna approach offers advantages in efficiency, eliminating the losses required by a combiner attempting to match signals that differ in power by 10 dB.

He also suggested that a second antenna could function as an auxiliary for the main analog channel in an emergency.

The use of a second antenna is not without drawbacks, chief of which is the need for more tower space to mount another antenna. It also is important that the second antenna closely match the coverage pattern of the existing analog signal.

Field tests conducted by ERI have been favorable, demonstrating that the necessary isolation between transmitters demanded by IBOC is achieved easily with dual antennas, even with spacing as close as 20 feet. Initial field strength measurements also showed that digital and analog carrier strengths were reasonably matched in the coverage area. ERI also is planning tests of dual antennas on a master FM antenna system.

In a paper about the effects of combining audio compression algorithms, Simon Factor, sales manager for Audio Processing Technology, warned about potential audio problems in the deployment of IBOC.

"The PAC compression algorithm is an enabling technology for digital audio broadcasting and a new addition to the broadcast chain with regard to compression. In addition to the many benefits of compression, there are pitfalls associated with multiple cycles of certain types of compression," Factor said.

In particular he cautioned broadcasters to consider the entire broadcast chain to avoid audio problems from multiple passes through digital compression. Automation systems, ISDN backhauls, recording devices (such as MiniDisc), and studio-transmitter links can contribute coding that eventually can cause a severe loss of audio quality.

His recommendations included the use of higher data rates, the use of decreased compression ratios, the use of different compression techniques, and higher bit resolution at all points in the contribution layer. The goal is to preserve the highest possible quality so that the final compression used by IBOC will not result in loss of audio quality.

On the wave

Finally, Ibiquity Digital Corp. presented three papers about the theoretical aspects of the IBOC waveforms.

Stephen A. Johnson, technical team leader for the Development of AM IBOC, showed the methods used to achieve a robust IBOC signal for AM service.

Orthogonal frequency division multiplexing is central to the IBOC approach. In OFDM, many orthogonal carriers are used throughout the AM channel. Data is distributed amongst these carriers so that a complete digital signal can be reconstructed even in the presence of interference to one or many of these individual carriers.

Greater robustness also is achieved in AM IBOC with the use of slower data transmission rates on the individual carriers that are most subject to interference, those operating in the same channel area as the analog signal. On the outer sidebands, where digital carrier strength is greater, up to 64 QAM modulation is used to achieve the maximum throughput, while only QPSK is used in the analog modulation area.

Johnson also described the four possible service modes for AM IBOC that reflect the possible tradeoffs between data throughput and robustness of the signal or coverage.

Paul Peyla, a senior member of technical staff at Ibiquity, presented a similar paper on FM IBOC. The system has the advantage of greater channel bandwidth and accordingly can achieve much higher data rates than on AM.

Up to 19 possible combinations of service modes are possible with FM IBOC. Peyla discussed these options and the tradeoffs involved in implementing IBOC in both the hybrid and the digital-only versions.

The final paper of the day presented an overview of IBOC deployment from Jeff Detweiler, broadcast technology manager of Ibiquity.

"Consumers are asking for digital," he said. "This is an opportunity for radio to offer them digital."

Detweiler discussed the details of IBOC deployment, including the need for GPS receivers in the IBOC exciter to permit exact digital synchronization.

For those using an STL, he pointed out the need for increased STL signal capacity under IBOC due to the increase in frequency response up to 20 kHz and the need to consider any additional data that may be transmitted with IBOC.

Detweiler also reviewed the three methods proposed for IBOC deployment: common amplification, high-power combining and the use of separate antenna.

Tapes of this session are available from www.mobiletape.com. Click on Technology and follow the prompts for the NAB2002 show to the session "IBOC Implementation."


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