Rackley Delves into AM IBOC - Radio World

Rackley Delves into AM IBOC

Few members of the engineering community are as familiar as Ron Rackley of du Treil, Lundin & Rackley Inc. For this first issue of HD Radio News, we decided to check in with him on this thoughts about AM IBOC.
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(Note: This is the original text of an interview that appeared in shortened form in RADIO WORLD newspaper's March 26, 2003 edition.)

Few members of the engineering community are as familiar as Ron Rackley of du Treil, Lundin & Rackley Inc. For this first issue of HD Radio News, we decided to check in with him on this thoughts about AM IBOC.

Rackley worked as a consulting engineer with Palmer Greer & Associates and Jules Cohen & Associates, and as a senior RF systems design engineer with Kintronic Laboratories, before starting a consulting practice with Bob du Treil. In 1988, du Treil-Rackley Consulting Engineers and A. D. Ring & Associates combined to form his present firm.

He is involved with a number of industry technical groups, has authored numerous articles, has adjusted directional antenna patterns as complex as 12 towers and as powerful as 2 megawatts, and has designed diplexed AM antenna systems. He is a registered Professional Engineer.

Radio World contributor Mario Hieb interviewed Rackley.

RADIO WORLD : How is AM-IBOC different than analog AM?

RACKLEY: AM-IBOC is different than analog AM in that encoded digital information - which a properly equipped receiver can use to re-create the audio signal - is sent along with the conventional AM signal in the hybrid mode or in place of the conventional AM signal in the all digital mode. It is expected that the hybrid mode will be implemented first and that stations will begin to migrate to the full digital mode as IBOC receivers become commonplace.

The digital information must be carried on additional digital sidebands that extend out to approximately +/- 15 kilohertz in the case of the hybrid mode and approximately +/- 10 kilohertz in the all-digital mode. These additional digital sidebands raise the noise level for analog reception somewhat, which can appear as a faint "bacon frying" effect underneath the audio of the host station and become more pronounced for adjacent channel stations. Concern over how this noise might "pile up" under skywave propagation conditions is the reason that the FCC has authorized AM-IBOC for use during daytime hours only at this point.

When the hybrid IBOC mode is in use, the analog transmission is limited to audio frequencies no higher than 5 kilohertz. The impact of this will be minimal, if noticeable at all, for most analog mode listeners, since there are very few receivers in use capable of receiving greater than 5 kilohertz audio. Much attention was focused on the receiver bandwidth issue more than a decade ago and a compromise was reached to allow reception of bandwidth approaching 10 kilohertz with receivers having sufficient bandwidth, even though there were none commonly available on the market at that time. The "NRSC" response standard was enacted into the FCC Rules with the idea that receiver technology would evolve in that direction, but it really hasn't. In order to avoid adjacent-channel "splatter" interference with the 10-kilohertz spacing of our AM band, the bandwidth of receivers tends to be limited to 5 kilohertz or less. The NAB funded an extensive research and development project in the early 1990s which resulted in an "AMAX" car radio that had a frequency response of around 6.5 kilohertz, but the technology has not been embraced by many receiver manufacturers because the cost of adding the 1/3 octave range above 5 kilohertz is not attractive from an economic standpoint. I believe that reducing the frequency response of the transmitted analog signal to 5 kilohertz, while it sounds like giving up a lot "on paper," will be judged to be of minimal consequence once people hear the difference on real radios - and the loss is offset by the improved response that will be possible with the new IBOC-equipped radios.

Another way in which IBOC transmission differs from the present analog system is that the process of converting the audio to digital and back causes a delay of approximately 9 seconds. In other words, the audio comes out of a receiver about 9 seconds after it goes into the line at the studio. This delay may be more of a nuisance for off-the-air monitoring than anything else at some stations, but it will add to the telephone delay that is normally employed by stations that run talk programming and it will be a significant "tuneout factor" for audiences that listen while watching live events - such as ball games.

The industry faces a decision not unlike the one it faced 40 years ago when FM stereo was adopted in the United States. Monophonic FM has an advantage in signal-to-noise ratio that is impressive "on paper" and can be quite noticeable in areas of poor signal. The noise disadvantage was a matter of significant controversy at the time FM stereo was being considered for adoption and some broadcasters were reluctant to "go stereo" because of it for at least a decade. The industry decided to take advantage of the advancement in technology that stereo offered, however, and advances in receiver technology eventually made it less of an issue. AM broadcasters must now weigh the issues that come along with the IBOC system - the increased background noise level, 5 kilohertz analog audio transmission, and delay - against its advantages, which are 15 kilohertz stereo reception where the digital signal is good, 10 kilohertz mono where it is marginal, and a low background noise level.

RADIO WORLD : How will AM transmitters perform with AM-IBOC?

RACKLEY: A lot will be required of AM transmitters that carry the IBOC hybrid signal. They must have very good linearity, low intermodulation distortion (IMD), low incidental phase modulation (IPM), and wide amplitude and phase response. A modulator bandwidth of 50 kilohertz or more is needed to provide acceptable IBOC performance and minimize out of band emissions.

The older transmitters employing vacuum tubes, such as high level plate modulated models, are not expected to be able to pass the IBOC signal.

Transmitters that employ single-phase pulse-width modulation (PWM -also known as pulse duration modulation or PDM) such as models employing vacuum tube final amplifiers and modulators, and some early-generation solid state designs, are not expected to pass the IBOC signal without significant modification of their modulator circuits and high-level pulse filters. Such modifications may not be economically justifiable.

Solid state transmitters that employ poly-phase PDM - which includes several models that have been designed by major manufacturers in recent years - are expected to be compatible with IBOC, although some may require relatively minor modification.

The most modern digitally modulated solid state transmitters are expected to be IBOC-compatible without modification.

RADIO WORLD : How will AM antenna systems perform with AM-IBOC?

RACKLEY: AM antenna system performance is important in two ways. First of all, the input impedance must be sufficiently flat and symmetrical above and below carrier frequency to allow the final amplifier of the transmitter to "see" a load to which it can deliver the digital sidebands in their proper relationships without interference from the analog signal. With good input impedance bandwidth, a high quality hybrid IBOC signal can be put into an antenna system. Where it goes after that is related to the other way in antenna system performance is important - radiation pattern bandwidth. Because the IBOC system uses both amplitude and phase modulation, both the magnitude response and the delay characteristics of the transmitter-output-to-far-field process are important.

I expect that many, if not most, of the non-directional antennas in use today will be able to perform well with hybrid IBOC without any modification and that most of those that do not will only require the installation of a new phase-rotation network between the transmitter and the antenna tuning unit to improve the symmetry of the sideboard load impedance's at the transmitter's final amplifier in order to perform well. The exceptions would mainly be shunt-fed towers that happen to have been built with high-Q wire skirts (such as can occur when the skirt of a high impedance tower is shorted to the tower at a point which forces the input resistance to be 50 ohms without regard to the slope of the resulting reactance-vs-frequency curve) and towers that are fed through filters that have a non-symmetrical effect on the input impedance (such as are found at some diplexed sites). Modifications beyond the installation of a simple RF network, such as changing the skirt feed arrangement or installing new filters, may be required in such cases.

Multi-tower directional antennas are another matter. The complexity of their feed systems and the large variety of pattern shapes that are in use make it impossible to say much about them in general. I can say that the impedance bandwidth issue will be a major factor for a larger proportion of AM directional antenna systems than will be the case for non-directional antennas and that, when the installation of a phase rotation network is not enough, good solutions will be more evasive for them also. On the pattern bandwidth issue, directional antennas present a whole new level of complexity, since a pattern shape depends on characteristics like transmission line length, tower spacing and tower height, which change in terms of the critical parameter - wavelength - with frequency, as well as the reactances of the inductors and capacitors in the phasing and coupling system, which also change with frequency.

Assuming that it is possible to get a high quality IBOC signal into most AM directional antenna systems, and I believe that will be possible if the necessary phasing and coupling system modifications for good impedance bandwidth are made , the percentage of coverage area where IBOC reception is possible will still be highly variable among stations because they use such a large variety of directional radiation patterns with differing pattern bandwidth characteristics. Some stations that happen to have excellent pattern bandwidth will have good IBOC reception throughout their coverage areas, including in most of their directional pattern null regions, while others will put out an IBOC signal that receivers can decode only in the central regions of the major lobes of their radiation patterns. I'm afraid that the only way to attack pattern bandwidth problems involves redesigning the complete phasing and coupling system, meaning that the networks within a phasor cabinet and at the tower bases will have to be extensively modified to an engineering-intensive custom design to make any improvement.

Although I believe the prospects for providing acceptable hybrid IBOC service through directional antenna systems to be good, the corrective actions that will be required for those having problems that cannot be solved by the addition of simple RF networks at their inputs might be considerably more costly than will be the case for non-directional antennas. I also believe that there will be some cases where it will be necessary to construct replacement directional antenna systems using a combination of more land, more towers, and taller towers before acceptable performance can be achieved. [I would expect these to be mostly arrays that employ short towers with short spacings (in terms of wavelength) between them, which are more commonly found near the lower end of the AM band.] It is entirely possible that some broadcasters will not find transmitting IBOC to make sense economically.

RADIO WORLD: How will AM phasor systems perform with AM-IBOC?

RACKLEY: Having discussed phasor systems already, I'd like to say that there may be another solution for good pattern bandwidth that doesn't require a phasor system at all. I have always been interested in an idea that has come up from time to time over the years that I've been in this business - to put separate transmitters with appropriate power levels at the various tower bases of a directional antenna system and do away with the conventional phasing and coupling system altogether. I remember that this was discussed extensively among my peers back in the 1970s, when Westinghouse had a modular solid-state transmitter under development and the possibility of having the modules spread out among the towers was thought to be an attractive one. The idea was that the RF phasing would be done at very low power with networks using small components in a rack-mounted unit connected through small diameter transmission lines to the RF power modules at the tower bases, with the tower ratios adjusted by adjusting the powers of the various RF modules. It was an interesting idea that never went beyond the discussion stage - the fact that the FCC would have to be convinced that the correct total power could be maintained was daunting enough, and then there was the problem of having the RF modules at the tower bases operate into loads that change impedance due to mutual coupling every time the parameters are adjusted. I did see one design overseas some years ago that had two high power transmitters feeding two towers to produce a two-tower pattern without a phasor, but that was a rather special case because the pattern had two towers with identical power, which greatly simplified things, and no FCC Rules to deal with. No one, to my knowledge, ever came forward to fund work on the various hurdles that would be required to use such a system domestically.

This idea has been resurrected recently with the new twist of using digital control for the system. This has me thinking on the subject again. I believe that it should be possible to construct modulated RF units that may be installed at the tower bases of a directional antenna system and have them serve as either current sources or voltage sources, as necessary depending on tower height, to establish the desired radiation parameters and, to the extent that they act like perfect sources, keep the parameters constant at sideboard frequencies to improve pattern bandwidth. For that matter, it might be possible to control the parameters at sideband frequencies as necessary to compensate for the changing array geometry with frequency to maintain the pattern nulls at precisely their correct azimuths to provide even better pattern bandwidth than can be achieved by merely holding the parameters constant with frequency. It might be necessary to dissipate a significant amount of power and use complicated RF networks at some tower bases to simulate perfect-source behavior over the operating impedance ranges that are encountered, but it might still be worthwhile to do so if there is no other way for a station to transmit an IBOC signal short of developing a new transmitter site. If the power can be determined to the FCC's satisfaction, and computer control may make that easier than it would have been years ago, the main hurdles will be persuading a transmitter manufacturer to provide a transmitter in pieces and developing the necessary hardware to turn them into sources to drive the towers. Who knows, the economics just might work out for such a scheme this time.

RADIO WORLD : It's been said that AM systems that can pass AM stereo are AM-IBOC ready. Your thoughts?

RACKLEY: I assume that we are talking about antenna systems here. There are a lot of transmitters that could be made to work acceptably well in AM stereo that will never pass the IBOC signal, because of the much wider modulator bandwidth required to carry the amplitude components that are necessary to produce the IBOC waveform. Although the same antenna system performance characteristics that degrade AM stereo will also degrade the IBOC signal, I believe that the IBOC system will be more sensitive to them for the simple reason that, when it "crashes," an IBOC radio will default to the noisy, narrowband analog signal while a stereo radio simply loses the spatial separation of that noisy, narrowband analog signal.

RADIO WORLD : Anything else you may want to add....

RACKLEY: I should disclose that I was a consultant to Ibiquity's predecessor, USA Digital Radio, for many years and helped them with the conceptual issues of getting the IBOC signal with which they were experimenting at that time through AM transmitters and antenna systems. I was also responsible for the engineering aspects of licensing various experimental facilities in different cities during the 1990s.

I can tell you that the hybrid IBOC signal that they now propose is much more sophisticated than those that were tested in the early years. Significant changes have been made to improve the signal's "survivability" in the "real world" of noise and interference that exists in the AM band. Although I am not a specialist in signal theory - especially digital signal theory - I believe that it probably represents the best that can be done under the present circumstances.

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