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A Case Study in Harmonic Interference FM 8th harmonic interference to a wireless facility not radiated by the FM station
◗WHITE PAPER BY DAVID MAXSON
David Maxson, CBRE, WCP, is cofounder of Isotrope LLC. This article first appeared in the Proceedings of the NAB 2020 Broadcast Engineering and Information Technology (BEIT) Conference, https://nabpilot.org/beitcproceedings/. It is reprinted with permission. ABSTRACT It is well known that 7th and 8th harmonic FM broadcast emissions have been a source of contention between cellular carriers and FM stations. Such harmonics are rarely radiated from the broadcast antenna itself. Usually, the cabinet radiation of the FM
transmitter is seen as the culprit, even if the cabinet radiation is within the Part 15 specifications. In a special case, the proximity of the FM and cellular antennas can create an overload situation where the cellular carrier generates its own interference. An FM station in Massachusetts was accused of causing 8th harmonic interference. The carrier involved the FCC in the matter. A methodical approach had to be taken to eliminate the assumed causes of the interference and implicate overload within the carrier’s receiver. This paper is a case study of that complaint. It shows how the possible causes had to be unpacked to satisfy the complainant and the FCC that the FM station was not the primary cause. Various mechanisms for generating 8th
harmonic energy are explored. The calculations and measurements required to exonerate the FM station are presented. INTRODUCTION In 2018, a wireless service provider (“WSP”) notified an FM station (“Station”) operating on Cape Cod in Massachusetts that there was interference from the FM transmitter on the WSP’s frequency.
The WSP frequency is the 8th harmonic of the FM frequency. To simplify the math, a frequency of 100 MHz is used in this paper as a proxy for the Station’s frequency, with a nominal 8th harmonic at 800 MHz. Cellular systems occupy channels in the 700 and 800 MHz bands, among others. The process of resolving the issue took a year and a half and
Maintaining Quality in Digital Audio Chains by Bob Orban and Greg Ogonowski See Page 10
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A Good Plan Is Key to a Successful Project Accounting for variables reduces pressures BY CRIS ALEXANDER
In the summer of 1983, I was working as the chief (and only) engineer of a startup UHF TV station in the panhandle of Texas. It was a shoestring operation, a spinoff of a more established TV station in a nearby market. The transmitter was a 1970s-vintage RCA, installed in the room adjacent to master control and visible through the large window separating master control from the transmitter room. Just about everything that aired on that station was on 3/4-inch U-Matic videotape; as I recall, Panasonic machines were used for playout of recorded material. The transmitter was interesting in that the aural AFC did not work for some reason, and a frequency counter was kept on top of the transmitter cabinet showing the aural carrier frequency. Several times a day, the operator on duty would have to go in the transmitter room and tweak the aural exciter while watching that counter to keep the frequency within the FCC tolerance. GUESS WHAT WE’RE DOING TODAY There was, however, a much bigger problem at the station. From time to time, RFI would cause the videotape machines to behave unpredictably. An operator walking through master control would sometimes put the playout machine into fast-forward. That played havoc with the program schedule and occasionally resulted in missed or botched spots. The director of engineering and his sidekick from the other market had spent a lot of time trying to cure the problem before I came aboard. Their efforts included covering the roof of the building with copper screen in an effort to create a Faraday cage of sorts. I think the effort actually made things worse, as we’ll see.
Much of my effort in my short tenure there was spent shielding the videotape machines themselves, lining the cabinets with foil and taking other RFI mitigation measures. While this did help, marginally, it didn’t cure the problem, much to my frustration. One Friday morning, I got to the station to find a handyman, someone the owner used for site maintenance, framing in the garage door. I asked what he was doing, and he told me that we were moving master control into the garage that weekend. (continued on page 4)
THIS ISSUE DECEMBER 16, 2020 A Case Study in Harmonic Interference. . . . . . . . . . . . . . . . . . . . . . . . . . . 1 A Good Plan Is Key to a Successful Project. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Power Up Hybrid Radio With RadioDNS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Maintaining Quality in Digital Audio Chains. . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Is It Time for Radio to Restore Dynamic Range?. . . . . . . . . . . . . . . . . . . . . . . . 16 Radio World (ISSN: 0274-8541) is published bi-weekly with additional issues in February, April, June, August, October and December by Future US, Inc., 11 West 42nd Street, 15th Floor, New York, NY 10036-8002. Phone: (703) 852-4600, Fax: (703) 852-4583. Periodicals postage rates are paid at New York, NY and additional mailing offices. POSTMASTER: Send address changes to Radio World, P.O. Box 282, Lowell, MA 01853.
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GOOD PLAN
plan takes all that out of the equation and makes the project much more of a “paint-by-number” affair. Such a plan makes a project, big or small, a much more relaxed endeavor. It takes off a lot of the pressure, and while it cannot absolutely guarantee a good outcome, it does greatly increase the likelihood of such.
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What!? Nobody had said a word to me about it, and I had not done any planning for such a project. Anyone who knows anything about NTSC television knows that it takes a lot of BNC connectors, video cable, audio cable, audio connectors and planning to relocate an entire master control facility, at least if you expect it to work right. And we had none of that. I called the director of engineering and got confirmation of the project, asking him how we were going to do this. The answer was that we were just going to wing it. That did not sit well with me. I could see disaster coming and I wanted no part of it. We had a parting of the ways that day. I have no idea what happened with the master control move or whether it happened at all that weekend. (The cause of the RFI, by the way, was very likely cabinet leakage from the transmitter, which was just a few feet through the glass from the videotape machines and switcher in master control. By screening the roof, the issue was made worse by reflecting RF back into the building.) DO YOUR THINKING AHEAD OF TIME The point of this story is that even as young and inexperienced as I was at the time, I knew that embarking on a big technical project without a good plan was a recipe for disaster. I’m glad I didn’t stick around. I’m sure I would have been at least partially blamed for the inevitable outcome. Prior to that episode and following, I was involved in all kinds of projects, from simple studio builds or rebuilds to construction of huge towers and antenna sites, and in each case, there was a plan in place. With each project, I learned something, usually the hard way. Some omission or something forgotten or not accounted for would result in delays. Something not thought through carefully required a last-minute revision of the plan. It was always something. And with each completed project, I got a little bit better at planning and thinking things through. In my company, there is no shortage of projects underway at any given time. As I write this, in the last quarter of 2020, we have several omnibus studio projects in the late planning stages, with equipment scheduled to be on site very shortly. These projects will be challenging, as we’re replacing cluster infrastructure while having to keep all the stations on
All the tools for a project plan: stacks of spreadsheets, a wire labeler, Ethernet switches all configured up, and good coffee.
After the project, post your accurate documentation in a handy place for easy reference.
By controlling as many variables as possible, your plan has a much greater chance of success, and that means making allowances for the variables you cannot control. the air and generating revenue … at one of the busiest times of the year! (We’re doing this now because we were delayed by several months by the pandemic.) As I was discussing these projects with our CFO, he made the observation that it sounded like we were going to be changing the tires on a car while it’s moving. Exactly! So how will we pull it off without affecting the on-air product and revenue? The answer, if you hadn’t guessed already, is to have a very detailed plan that will take us through the entire
project. The plan accounts for every piece of equipment, every wire and every signal in the facility. It really takes all the thinking out of the actual work. Throughout the decades of my career, that’s the one of the most important things I have learned about project work: do the thinking ahead of time and avoid having to figure things out on the fly. It’s much harder to think under pressure, and it’s especially hard when you’re tired. A good, well-thought-out
STUFF HAPPENS So what’s the recipe for a good plan? I start with a spreadsheet — a workbook, really, with multiple tabs for different parts of the project. In a studio project, every input and output (or source and destination in AoIP parlance) is accounted for and assigned. Self-laminating wire labels are printed and ready to install. IP addresses are printed on labels to be affixed to equipment as it is installed. Signal names are defined, and default routing is mapped out. The layout of every equipment rack is planned, and port assignments on Ethernet switches are made. Nothing is left to be decided on the fly. In a transmitter project, it’s much the same, although in addition to Ethernet and AES cables, there will be coaxial cables to deal with, both high-power and low, and there will undoubtedly be remote control connections as well, hopefully SNMP but maybe discrete control, status and telemetry signals. Think it all through, plan it out and label everything. Obviously, the time to do all this planning is not on the eve of the actual project. It should be done far enough in advance that the time pressures of the project deadline don’t factor in. Make allowances for material order and shipment, which means that you should check stock and delivery time on equipment, connectors, wire, etc. very early on and make adjustments as needed. As with any project, build in contingencies. Stuff happens (with amazing regularity), and you have to be prepared for that. Prussian Field Marshal Helmut Von Moltke is credited with saying, “No battle plan survives first contact with the enemy.” There is truth to that, but by controlling as many variables as possible, your plan has a much greater chance of success, and that means making allowances for the variables you cannot control. In my experience, project work is one of the most enjoyable aspects of broadcast engineering — that is, if the pressures and stress are reduced. The way to do that is with thorough planning. W.C. “Cris” Alexander, CPBE, AMD, DRB, is director of engineering for Crawford Broadcasting and tech editor of RW Engineering Extra.
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HYBRID RADIO
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December 16, 2020
Power Up Hybrid Radio With RadioDNS Learn new tech and save $$$ by doing it yourself BY TOM VERNON
Hybrid radio is just around the corner. Is your station ready? This fall, Audi and BMW are selling vehicles with hybrid radios in the United States and Canada. This technology enables both over-the-air and internet radio reception in cars. Impressive as that is, these receivers will also be able to display a station’s logo as well as station information such as “now playing” and other metadata from the web stream as part of a vehicle’s on-screen station guide. Additionally, the Audi radios will offer service following, which means they will be able to switch from the FM signal to streaming audio when radio reception becomes iffy, effectively extending your station’s coverage area. The receiver will enable the most seamless transition possible, matching timing and levels between the two signals, so long as the time difference is 30 seconds or less. In order for stations to reap the benefits of hybrid radio, they must first create an XML file with station information, host it on a publicly-accessible web server, create some DNS records and register with RadioDNS. Any number of service providers can do this for you, but in these challenging economic times, you can save some cash by doing it yourself. It’s not difficult, but like most things, it takes some study, advance planning and organization to ensure a happy outcome. Founded in October 2009, RadioDNS is a non-profit organization based in the U.K. that promotes the global use of open technology standards to enable hybrid radio. In addition to HD Radio, the standard supports VHF/FM, DAB, DRM and AMSS. RadioDNS manages the internet-based technologies that can connect hybrid radios to radio stations providing internet content like streaming audio URLs. DIY To assist the do-it-yourselfers, NAB PILOT offers a free on-demand webinar that walks you through the process. It’s hosted by David Layer, vice president, advanced engineering in NAB’s Technology Department. Panelists include Christian Winter, development engineer, radio, media at Audi AG; Nick Piggott, project director and co-founder RadioDNS Hybrid Radio; and Andy Buckingham, creative technologist at Togglebit.
How hybrid radio works. RadioDNS pulls unique station identifiers from RDS. It rewrites these into a format that looks like a domain name. This gets passed to the nearest DNS server. The receiver then makes a direct connection to a station’s stream using the IP address.
Help is also available on the RadioDNS website. There you can find step-by-step guides to implementing RadioDNS functionality, as well as presentations from technical conferences explaining how to manage RadioDNS hybrid radio application systems. There are also discussion boards where you can post your questions. This fall’s Radio Show virtual event, co-produced by NAB and RAB, featured a session on “How Radio Broadcasters Can Support RadioDNS.” It was hosted by Layer with panelists Piggott; Jason Ornellas, regional director of engineering, Bonneville International; and Mark McConnell, system administrator and digital content manager, Bonneville International. Layer began the Radio Show session by noting a September press release from Audi, announcing a collaboration with iHeart Radio, which is making more than 600 of its radio stations hybrid radio capable during its initial integration phase. The release, according to Layer, underscored an important point. “Auto manufacturers are going to great lengths to introduce hybrid radio-equipped cars, and now the onus is on radio broadcasters to do their part by making their stations capable of utilizing this new service.”
The small web icon in the middle of the display indicates the station, along with its associated metadata, is being received via the web stream rather than over the air.
Layer adds that no fees are collected for registering your station with RadioDNS; it is a free service. However, there are costs involved in setting up radio stations to make use of RadioDNS technology. RadioDNS is supported financially by its members worldwide, of whom NAB is one, as are several broadcast groups in the U.S. Broadcasters who want to help move this technology forward should consider joining RadioDNS. The XML file with station information previously mentioned is called the Service Information (SI) file, and is the main resource for conveying basic
information about radio stations to the hybrid radio receiver. To create this, you’ll need a database containing the station metadata from all stations being supported. For each station that should include call sign, name, description, genre, logo URLs and audio stream URL. Also needed is information on the station’s RDS PI code (for analog FM stations) or the facility ID (for HD Radio stations). NAB PILOT has created a way to automate the collection of the PI code/ facility ID and other “bearer” information. It’s called the “Radio Call-signs API.” For more information, contact (continued on page 8)
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frequencies. If your group has numerous channels, you may be able to register (continued from page 6) with a wildcard (*) entry, David Layer via nabpilot.org. Station and you won’t need to list logos as well as a web hosting service them all. Otherwise, freto put the SI and logo files on are also quencies need to be entered required. as a five-digit number, i.e. Next, you’ll need to create the XML 08850 for 88.5 MHz, or file to make it all work. Most of the 10790 for 107.9 MHz. examples on the NAB webinar are creIn summary, each FM ated using the PHP scripting language, channel will information but other languages can also be used. entered in the form frequency.pi.gcc, for example PREPARATION IS KEY 08850.pi.BA0. The first step is to collect all the If you’re operating with station information from the database. HD Radio, you need to Next, all the images need to be prebe transmitting your FCC Audi radios offer service following, where the driver is given the option of switching to a stapared. Facility Identifier (ID) in tion’s stream when the FM signal becomes marginal. RadioDNS specified five different hexadecimal format, padresolutions to support different receiver ding with leading 0s to create a fivethey wanted to make Bonneville program guide links, including social display resolutions, including 32 X 32 digit number. This should be followed International an early adopter in the media, website and studio line links. PX, 112 X 32 PX, 128 X 128 PX, 320 X by the country identifier (CC), which is six markets the company serves. Their The result was the overarching 240 PX, and 600 X 600 PX. This step 292 for the USA. And if you’re transjourney took them to places they never framework for the Bonneville station’s is really a matter of converting your one mitting multicast, all of this needs to be expected. SI files, and it was left to station engilarge image file into these smaller sizes. preceded by MC. Ornellas said, “Initially, we just neers to fill it in, using the Sacramento Then, you’ll need to ping the Radio An example of a multicast HD Radio wanted to be a part of it. Then Mark station as an example. All of the comCall-signs API to get back your broadentry would be MC.id.cc, for example began looking online and realized we pleted files reside in the Bonneville cast station details. Finally, you must MC.10C21.292. can do so much more than just have International corporate server. build the SI file document and save it. Completed applications should be an online presence. The Capital FM Piggott said getting hybrid radio to After the coding is complete, you need e-mailed to registrations@radiodns. example inspired us to put all the extra work involves teamwork. to register your station with RadioDNS. org. These should include your fully information out there for our listeners.” “The implementation of hybrid radio A bit of preparation is necessary. You’ll qualified domain name (FQDN) and McConnell adds, “The two big things brings people from two sides of the need to gather some information about your broadcast parameters for FM and that jumped out for us were their use of business together. It’s the intersection the parameters in your signal. In order to HD for each station being registered, the PI content code, as well as their use of broadcast engineering skills, such get into the RadioDNS hybrid radio regyour .zone file as an attachment (if you of an electronic program guide which as making sure PI codes are being istry, analog FM stations must be transhave created one), your name and telehad links to their social media. But transmitted on RDS encoders, but mitting RDS, including the PI (Program phone contact number, the registered the entire structure is XML-based, so also knowledge of how to set up DNS Identifier) code. name of the radio station(s), and finally there’s a lot of flexibility to add what records and put files on web servers.” It is further recommended that you the name of the authority that issued you want.” Ornellas added, “You need to have transmit the ECC (Extended Country the broadcast license(s). RadioDNS will The second big discovery for that collaboration with digital and Code). This can help improve the accuacknowledge the change request via Ornellas and McConnell was how much engineering to really streamline this, racy of locating your DNS entry. For the email, within 48 working hours. New data could be gleaned once the server especially the artwork and stream United States, the ECC is A0. and changed entries take up to 24 hours logs are combined with owner data from URLs. Some engineers have access to The DNS utilizes a GCC (Global to propagate through the Domain Name car dealers. that, others rely on the digital folks. Country Code), which comprises the Service. “These can be used to create some We’re unique at Bonneville in that we first digit of your RDS PI code followed powerful real-time analytics,” says already work together so closely that by your ECC code. For U.S. stations, PI ON THE AIR Ornellas. “The age and sex of the driver, this was a seamless process. Moving codes begin with A, B, D and E, so valid Some stations have already taken the for example, can be combined with forward, I believe digital and engiGCCs would be AA0, BA0, DA0 or EA0. plunge into this new technology. where and when they tune in, how long neering are going to become very When you register, RadioDNS will As soon as Ornellas and McConnell they listen, and what station they evenintegrated, and hybrid radio is a percreate a DNS entry for each of your heard about hybrid radio, they knew tually switch to. fect example.” “There’s probably more Radio never stands still, nor should to be discovered. The chalyour SI file. Stations change logos, lenge for us is to take this formats, call signs and even ownership, wealth of data, which is often with little advance notice. It’s best already parsed out by stato think of creating the SI file, as well tion, and create a dashas your other digital assets as a process board where it can be easirather than a one-time event. This may ly accessed and understood be another reason to learn how to create by sales, promotion and and manage them yourself, rather than management.” relying on a service provider. McConnell began the The solution at Bonneville is a virprocess by using a PI tual machine located at company headcode look-up tool, which quarters. When new logos or other obtained the PI codes necgraphics are created, they are immediessary for analog FM staately uploaded. Station engineers also tions. A few things were keep backups of their SI files and other left to be added by hand, engineering data there. The selected station, KITS, along with its logo and ‘now playing’ information, is being received including the ETSI content Tom Vernon is a longtime contribufrom the webstream. CS codes and electronic tor to Radio World.
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AUDIO ESSENTIALS
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Maintaining Quality in Digital Audio Chains Tips and truisms about a subject that is often misunderstood BY ROBERT ORBAN AND GREG OGONOWSKI
The following is excerpted from “Maintaining Audio Quality in the Broadcast and Netcast Facility.” In this excerpt, the authors deal with the many-faceted and often misunderstood subject of quality in digital audio chains. In digital signal processing devices, the lowest number of bits per word necessary to achieve professional quality is 24 bits. There are several reasons for this. Digital audio workstations need headroom to accommodate gain adjustments and mixing of several sources. Moreover, there are a number of common DSP operations (like infinite-impulse-response filtering) that substantially increase the digital noise floor, and 24 bits allows enough headroom to accommodate this without audibly losing quality. (This assumes that the designer is sophisticated enough to use appropriate measures to control noise when particularly difficult filters are used.) If floating-point arithmetic is used, the lowest acceptable word length for professional quality is 32 bits (24-bit mantissa and 8-bit exponent; sometimes called “single-precision”). In digital distribution systems, 20-bit words (120 dB dynamic range) are usually adequate to represent the signal accurately. Twenty bits can retain the full quality of a 16-bit source even after as much as 24 dB attenuation by a mixer. There are almost no A/D converters that can achieve more than 20 bits of real accuracy, and many “24-bit” converters have accuracy considerably below the 20-bit level. “Marketing bits” in A/D converters are outrageously abused to deceive customers, and, if these A/D converters were consumer products, these bogus claims would be actionable by the Federal Trade Commission. SAMPLE RATE CONTROVERSY There is considerable disagreement about the audible benefits (if any) of raising the sample rate above 44.1 kHz. An extensive double-blind test using 554 trials showed that inserting a CD-quality A/D/A loop into the output of a high-resolution (SACD) player was undetectable at normal-to-loud listening levels by any of the subjects, on any of four playback systems. The noise of the CD-quality loop was audible only at
This is at KTBI(AM) 810 in Ephrata, Wash., one of three AM stations owned by American Christian Network. Running 50 kW daytime, it covers Spokane over 100 miles away.
very elevated levels. Moreover, there has been at least one rigorous test comparing 48 kHz and 96 kHz sample rates. This test concluded that there is no audible difference between these two sample rates if the 48 kHz rate’s anti-aliasing filter is designed appropriately. However, in 2016, a controversial “meta-analysis” of existing tests comparing high-resolution and CD-quality audio was published in the AES Journal. According to the author, “Eighteen published experiments for which sufficient data could be obtained were included, providing a meta-analysis that combined over 400 participants in more than 12,500 trials. “Results showed a small but statistically significant ability of test subjects to discriminate high resolution content, and this effect increased dramatically when test subjects received extensive
training. This result was verified by a sensitivity analysis exploring different choices for the chosen studies and different analysis approaches. “Potential biases in studies, effect of test methodology, experimental design, and choice of stimuli were also investigated. The overall conclusion is that the perceived fidelity of an audio recording and playback chain can be affected by operating beyond conventional resolution.” Assuming perfect hardware, it can be shown that this debate comes down entirely to the audibility of a given antialiasing filter design, as is discussed below. Far before the publication of the 2016 meta-analysis, in a marketing-driven push the record industry attempted to change the consumer standard from 44.1 kHz to a higher sampling frequency via DVD-A and SACD, neither
of which succeeded in the mass marketplace. The industry is trying again with Blu-ray audio, and it remains to be seen if they will be more successful than they were with DVD-A or SACD. FM STEREO Regardless of whether scientifically accurate testing eventually proves that this is audibly beneficial, sampling rates higher than 44.1 kHz have no benefit in FM stereo because the effective sampling rate of FM stereo is 38 kHz, so the signal must eventually be lowpassfiltered to 17 kHz or less to prevent aliasing. It is beneficial in DAB, which typically has 20 kHz audio bandwidth, but offers no benefit at all in AM, whose bandwidth is no greater than 10 kHz in any country and is often 4.5 kHz. Some A/D converters have built-in soft clippers that start to act when the (continued on page 14)
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KTWO(AM) 1030 in Casper, Wyo., a Townsquare Media station. With 50 kW daytime omnidirectional and 50 kW directional night, it covers 75% of the state of Wyoming.
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input signal is 3–6 dB below full scale. While these can be useful in mastering work, they have no place in transferring previously mastered recordings (like commercial CDs). If the soft clipper in an A/D converter cannot be defeated, that A/D should not be used for transfer work. DITHER Dither is random noise that is added to the signal at approximately the level of the least significant bit. It should be added to the analog signal before the A/D converter, and to any digital signal before its word length is shortened. Its purpose is to linearize the digital system by changing what is, in essence, “crossover distortion” into audibly innocuous random noise. Without dither, any signal falling below the level of the least significant bit will disappear altogether. Dither will randomly move this signal through the threshold of the LSB, rendering it audible (though noisy). Whenever any DSP operation is performed on the signal (particularly decreasing gain), the resulting signal must be re-dithered before the word length is truncated back to the length of the input words. Ordinarily, correct dither is added in the A/D stage of any competent commercial product performing the conversion. However, some products allow the user to turn the dither on or off when truncating the length of a word in the digital domain. If the user chooses to omit adding dither, this should be because the signal in question already contained enough dither noise to make it unnecessary to add more. Many computer software volume controls do not add dither when they attenuate the signal, thereby introducing low-level truncation distortion. It is wise to bypass computer volume controls wherever possible, and if this is not possible, to maintain unity gain through the volume control. Microsoft Windows Media Player and Adobe Flash Players should be operated at 100% (0 dBFS) at all times, and level control should be done either at the amplifier volume control or console fader. In the absence of “noise shaping,” the spectrum of the usual “triangularprobability-function (TPF)” dither is white (that is, each arithmetic frequency increment contains the same energy). However, noise shaping can change this noise spectrum to concentrate most of the dither energy into the frequency range where the ear is least sensitive. In practice, this means reducing the energy around 4 kHz and raising it above 9 kHz. Doing this can increase the effec-
December 16, 2020
tive resolution of a 16-bit system to almost 19 bits in the crucial midrange area, and is standard in CD mastering. There are many proprietary curves used by various manufacturers for noise shaping, and each has a slightly different sound. It has been shown that passing noise shaped dither through most classes of signal processing and/or a D/A converter with non-monotonic behavior will destroy the advantages of the noise shaping by “filling in” the frequency areas where the original noise-shaped signal had little energy. The result is usually poorer than if no noise shaping had been used. For this reason, Orban has adopted a conservative approach to noise shaping, recommending so-called “first-order highpass” noise shaping and implementing this in Orban products that allow dither to be added to their digital output streams. First-order highpass noise shaping provides a substantial improvement in resolution over simple white TPF dither, but its total noise power is only 3 dB higher than white TPF dither. Therefore, if it is passed through additional signal processing and/or an
imperfect D/A converter, there will be little noise penalty by comparison to more aggressive noise shaping schemes. One of the great benefits of the digitization of the signal path in broadcasting is this: Once in digital form, the signal is far less subject to subtle degradation than it would be if it were in analog form, although in fixed point form it is still subject to clipping. Short of being clipped or becoming entirely un-decodable, the worst that can happen to the signal is deterioration of noiseshaped dither, and/or added jitter.
further stages can be used downstream. Sample rate converters can introduce jitter in the digital domain because they resample the signal, much like A/D converters. Maintaining lowest jitter in a system requires synchronizing all devices in the audio chain to a common word clock or AES11 signal. This eliminates the need to perform cascaded sample rate conversions on the signals flowing through the facility. Good word clock generators have very low jitter (also known as “phase noise”) and allow the cascaded devices to perform at their best.
JITTER Jitter is a time-base error. The only jitter than cannot be removed from the signal is jitter that was added in the original analog-to-digital conversion process. All subsequent jitter can be completely removed in a sort of “timebase correction” operation, accurately recovering the original signal. The only limitation is the performance of the “time-base correction” circuitry, which requires sophisticated design to reduce added jitter below audibility. This “time-base correction” usually occurs in the digital input receiver, although
BUSTING THE MYTHS There are several pervasive myths regarding digital audio. One myth is that long reconstruction filters smear the transient response of digital audio, and that there is thus an advantage to using a reconstruction filter with a short impulse response, even if this means rolling off frequencies above 10 kHz. Several commercial high-end D-to-A converters operate on exactly this mistaken assumption. This is one area of digital audio where intuition is particularly deceptive. The sole purpose of a reconstruc-
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tion filter is to fill in the missing pieces between the digital samples. These days, symmetrical finite-impulseresponse filters are typically used for this task because they have no phase distortion. The output of such a filter is a weighted sum of the digital samples symmetrically surrounding the point being reconstructed. The more samples that are used, the better and more accurate the result, even if this means that the filter is very long. It’s easiest to justify this assertion in the frequency domain. Provided that the frequencies in the passband and the transition region of the original antialiasing filter are entirely within the passband of the reconstruction filter, then the reconstruction filter will act only as a delay line and will pass the audio without distortion. Of course, all practical reconstruction filters have slight frequency response ripples in their passbands, and these can affect the sound by making the amplitude response (but not the phase response) of the “delay line” slightly imperfect. But typically, these ripples are in the order of a few thousandths of a dB in highquality equipment and are very unlikely to be audible. The authors have proved this experimentally by simulating such a system and subtracting the output of the reconstruction filter from its input to determine what errors the reconstruction filter introduces. Of course, you have to add a time delay to the input to compensate for the reconstruction filter’s delay. The source signal was random noise, applied to a very sharp filter that bandlimited the white noise so that its energy was entirely within the passband of the reconstruction filter. We used a very high-quality linear-phase FIR reconstruction filter and ran the simulation in double-precision floating-point arithmetic. The resulting error signal was a minimum of 125 dB below full scale on a sample-by-sample basis, which was comparable to the stopband depth in the experimental reconstruction filter. We therefore have the paradoxical result that, in a properly designed digital audio system, the frequency response of the system and its sound is determined by the anti-aliasing filter and not by the reconstruction filter. Provided that they are realized with high-precision arithmetic, longer reconstruction filters are always better. This means that a rigorous way to test the assumption that high sample rates sound better than low sample rates is to set up a high-sample rate system. Then, without changing any other variable, introduce a filter in the digital domain with the same frequency response as a high-quality anti-aliasing filter that would be required for the lower sample rate. If you cannot detect
the presence of this filter in a doubleblind test, then you have just proved that the higher sample rate has no intrinsic audible advantage, because you can always make the reconstruction filter audibly transparent. Another myth is that digital audio cannot resolve time differences smaller than one sample period and therefore damages the stereo image. People who believe this like to imagine an analog step moving in time between two sam-
differences between “upsampled” audio and the original are either imagining things or hearing coloration caused by the added image frequencies or the frequency response of the upsampler’s filter. They are not hearing a more accurate reproduction of the original recording. This also applies to the sample rate conversion that often occurs in a digital facility. It is quite possible to create a sample rate converter whose filters are
Provided that they are realized with high-precision arithmetic, longer reconstruction filters are always better. ple points. They argue that there will be no change in the output of the A/D converter until the step crosses one sample point and therefore the time resolution is limited to one sample. The problem with this argument is that there is no such thing as an infiniterisetime step function in the digital domain. To be properly represented, such a function has to first be applied to an anti-aliasing filter. This filter turns the step into an exponential ramp, which typically has equal pre-and postringing. This ramp can be moved far less than one sample period in time and still cause the sample points to change value. In fact, assuming no jitter and correct dithering, the time resolution of a digital system is the same as an analog system having the same bandwidth and noise floor. Ultimately, the time resolution is determined by the sampling frequency and by the noise floor of the system. As you try to get finer and finer resolution, the measurements will become more and more uncertain due to dither noise. Finally, you will get to the point where noise obscures the signal and your measurement cannot get any finer. However, this point is orders of magnitude smaller in time than one sample period and is the same as in an analog system with the same bandwidth. A final myth is that upsampling digital audio to a higher sample frequency will increase audio quality or resolution. In fact, the original recording at the original sample rate contains all of the information obtainable from that recording. The only thing that raising the sample frequency does is to add ultrasonic images of the original audio around the new sample frequency. In any correctly designed sample rate converter, these are reduced (but never entirely eliminated) by a filter following the upsampler. People who claim to hear
poor enough to make images audible. One should test any sample rate converter, hardware or software, intended for use in professional audio by converting the highest frequency sinewave in the bandpass of the audio being converted, which is typically about 0.45 times the sample frequency. Observe the output of the SRC on a spectrum analyzer or with software containing an FFT analyzer (like Adobe Audition). In a professionalquality SRC, images will be at least 90 dB below the desired signal, and, in SRC’s designed to accommodate long word lengths (like 24 bit), images will
often be –120 dB or lower, assuming a 24-bit path (which is capable of representing low-level energy down to –144 dBFS). Taking full advantage of highperformance sample rate conversion is another reason to use 24-bit audio for production and to reduce the bit depth (if necessary for applications like burning audio CDs) only as the final step, using appropriate dither. A good reference on sample rate conversion performance can be found at http://src.infinitewave.ca/. LESS IS MORE! And finally, some truisms regarding loudness and quality: Every radio is equipped with a volume control, and every listener knows how to use it. If the listener has access to the volume control, he or she will adjust it to his or her preferred loudness. After said listener does this, the only thing left distinguishing the “sound” of the radio station is its texture, which will be either clean or degraded, depending on the source quality and the audio processing. Any program director who boasts of his station’s $20,000 worth of “enhancement” equipment should be first taken to a physician who can clean the wax from his ears, then forced to swear that he is not under the influence of any suspicious substances, and finally placed gently but firmly in front of a high-quality monitor system for a demonstration of the degradation that $20,000 worth of “enhancement” causes! Always remember that less is more. Comment on this or any article. Email rweetech@gmail.com.
A FREE RADIO WORLD EBOOK EXPLORES
TRENDS IN PROCESSING FOR RADIO
Find it at radioworld.com/ebooks.
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COMMENTARY
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Is it Time for Radio to Restore Dynamic Range? Hybrid radio could be the impetus for backing off over-the-air FM processing BY JOHN KEAN
The author is senior engineer with Cavell Mertz & Associates Inc.
LOUDER WAS BETTER I hate to be nostalgic, but FM was once considered a “high fidelity” medium (I’m old enough to remember!). Consumers used to buy exquisite, expensive tuners to get the best FM sound for their living room systems. Today a number of my non-technical friends don’t even hook up the antenna on their multimedia receivers. What happened to that reputation, and is it connected to FM’s gradual loss of listeners to online media? A look at the General Electric transmitter two-page ad in a 1945 issue of Broadcasting Magazine says a lot about FM’s change. The signal-to-noise ratio of the new FM system promised to deliver “double the Dynamic Range” of AM and remove “the unreality of artificially controlled sound levels that compress a fortissimo.” Using an ingenious size comparison between AM and FM (using a photo of an all-woman orchestra during World War II) GE touted the “contrasts of sound intensities … in all its glorious realism.” Along the way, years ago, FM radio got the idea that dynamic range had no value, and louder was better. The development of stronger and stronger FM audio processors began. That seemed to work for FM for many
Getty Images/JGI/Jamie Grill
Audio processing has reached a level of performance where audio content can have high loudness without the traditional artifacts of audible clipping, pumping, intermodulation distortion, etc. Of course, audio processing in a broadcast medium is justifiable for overmodulation protection and combatting noisy listening environments. Due to freedom from distortion in processors and loudness wars, however, much of radio has reached a state of hyper-compression, where alreadycompressed popular music is fed to multiband compressors and limiters that aggressively reprocess the audio. This situation is hard to reverse in broadcast, where competitive loudness remains a concern, but I believe minimal processing may be the right direction for online radio media.
Younger listeners play music and shows online and from digital personal collections. My research finds that this music is distributed almost entirely in its original, unprocessed form.
The 2000s brought a newer medium: online digital audio that could be delivered to smart phones, as well as home computers. years — after all, it was a portable and in-car medium with lower noise and wider frequency response than AM, as well as stereo. However, the 2000s brought a newer medium: online digital audio that could be delivered to smart phones as well as home computers. LISTENER PREFERENCE While FM’s decline of listeners may be due to a combination of causes, online audio (streams, podcasts and ondemand playout) have flourished. Online audio is a 16-bit digital system having a dynamic range greater
than 90 decibels, regardless of the bit rate, and lossy compression codecs have continued to improve in sound quality. Younger listeners play music and shows online and from digital personal collections. My research finds that this music is distributed almost entirely in its original, unprocessed form. This is true of major on-demand music services, and some are now are offering high-fidelity channels with higher bit rates and even “lossless” coding. The tracks are simply normalized (gain offset) to a common loudness target, without touching the dynamic range of the content.
In a recent project for a major radio group, I found that some online distributors of live station audio are using substantially less processing than their on-air broadcasts. Perhaps some are learning that “artificially controlled sound levels” are not preferred by listeners. Similarly, podcasts — the fastest growing segment of online audio — are produced and delivered with little or no audio processing. The target loudness of the online industry is changing to a lower value to permit greater dynamic range. RETHINKING THE TARGET I have the privilege of chairing a drafting committee at the Audio Engineering Society, which is writing a new technical document for online audio parameters. These interim specifications will evolve to a profile with even wider dynamic range to match audio-for-video
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standards — and we know how much dynamic range video services deliver! Broadcasters are now faced with another choice if they adopt “hybrid radio,” which provides a streaming alternative to radio reception as listeners drive outside the broadcast coverage. FM stations could choose to match the audio processing of their stream to the (hypercompressed) broadcast audio, to avoid changes as the dashboard receiver switches between off-air and stream. Or should they? Perhaps radio should reconsider what it broadcasts and move with the audio industry and away from heavy compression. When hyper-compressed audio is normalized to the same integrated loudness as lightly-processed audio, a heavily-compressed stream sounds weak and flat by comparison. Compressing a stream to sound like air can’t compete with natural, dynamic sound. Considering this, wouldn’t it be wonderful if the FM stations, too, returned their own air audio to a high-fidelity condition, as FM promised 75 years ago? Advertising from a 1942 issue of Broadcasting touts the advantages of FM over AM.
HARMONIC (continued from page 1)
involved the Federal Communications Commission. Seventh and 8th harmonic FM interference to cellular services is a well-reported phenomenon [see References 1, 2 and 3 at the end of this paper]. Around 2013, newly repurposed TV channels acquired by wireless carriers at auction were inaugurated to launch 4th generation (LTE) wireless technology. The harmonic interference phenomenon was previously identified in the 800 MHz cellular and specialized mobile radio bands, which have been in operation for decades. The more recent inauguration of LTE services on the new 700 MHz frequencies created a rash of new interference complaints. The Station was collocated with the WSP in 2009, well before the 700 MHz wireless auctions. The affected frequency is in the 800 MHz legacy wireless bands, so the WSP had been coexisting with the Station for more than a decade. The WSP only recently identified the interference and informed the Station. To receive the signals from distant low-power handsets (the “uplink”), wireless base stations must be very sensitive. Low levels of harmonic energy from nearby FM stations can interfere with reception of weak uplink signals. Reports of other FM harmonic interference
issues have pointed to two primary sources: antennaradiated (“AR”) and cabinet-radiated (“CR”) harmonic emissions. The Station’s situation is a variation on this theme. While the two obvious likely sources of harmonics are potentially radiated by an FM Station — AR and CR — a third factor played a role: generation of 8thharmonics by a device external to the FM transmitter. Powerful FM signals can stimulate other devices to generate intermodulation and harmonic products. Published reports about FM station harmonic interference to wireless services have gravitated to the FM station as the source of radiated harmonics. Harmonic CR is the dominant culprit because it tends to be much greater at the point of radiation than harmonic AR. FM transmitters use amplification modes that are non-linear, which increases amplifier efficiency. With nonlinearities come harmonics. FM transmitters have harmonic filters to limit harmonic AR. Moreover, the FM antenna is a tuned element and is a poorer radiator of the harmonics. In addition, it is best practice for FM transmitters at multi-user sites to also employ bandpass filters, which not only protect the transmitter from ingress of other signals into the output of the amplifier circuits, but also the filters further limit harmonic emissions from the transmitter to the antenna.
With non-linear amplification, harmonics generated inside the transmitter can radiate from the cabinet or be conducted on cables from the cabinet. CR is regulated by the FCC in Part 2 of its regulations [Reference 4]. Section §2.1053 requires cabinet emissions to be below certain levels, based on what a dipole would radiate to produce the measured level. This is covered in detail below. Other well-known mechanisms that could cause harmonic interference have typically not been acknowledged in stories about the issue. These include receiver overload-induced interference (ROII) and passive intermodulation (PIM). Powerful FM signals are capable of stressing receivers (generating ROII), depending on how well filtered they are. Also, in the presence of strong radiated energy, nearby conducting surfaces with semiconducting connections to ground can generate interference (PIM). Intermodulation between two radio signals is the familiar product of both the PIM and ROII mechanisms. However, these same mechanisms generate harmonics of the fundamental stimulus. In the present case, the best indications are that the WSP was experiencing ROII in its own receivers, though PIM could not be ruled out. This paper describes the process and results of the effort to get to the bottom of the issue. (continued on page 18)
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HARMONIC (continued from page 17)
THE SITUATION Antenna Configuration The Station is a Class A FM facility broadcasting from an antenna less than 200 feet above ground level. The two-bay FM antenna is mounted at the same elevation as the WSP and on the same tower. The horizontal separation between the FM antenna and the nearest WSP antenna is about 10 feet (3 m). This is exceptionally close spacing between an FM antenna and a wireless facility. The WSP has two antenna arrays (i.e., two “sectors”) mounted off two faces of the tower, favoring two legs (southeast and northeast). The coverage of the cell site is essentially bidirectional (N-S). The west tower leg was unoccupied until the FM Station moved to the tower. Symptoms The interference symptoms are measured by WSP network applications that monitor receiver noise levels. The CDMA channel is centered at 800 MHz (nominal value for discussion purposes; actual frequency is in a wireless 800 MHz band). The WSP received level is monitored by an RSSI (received signal strength indication) value. This information comes in the form of a periodic single measurement on each of two diversity antenna branches of the CDMA receivers. Being RSSI measurements, these measurements include the total
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of a modulated fundamental FM signal becomes 8 times as wide, or about 1.2 MHz at the 8th harmonic. A portion of the modulated harmonic fell outside the wireless CDMA receive channel. With the FM signal unmodulated, the full power of the carrier’s harmonic lies entirely within the CDMA channel. The variability of the noise measurements also diminished. The unmodulated harmonic noise was received at –66 dBm, while the background level noise with the FM transmitter turned off was –88 dBm. Most results quoted herein are based on the unmodulated signal measurements, which exaggerates the impact on the WSP CDMA receivers. SEQUENCE OF EVENTS Below is a summary of the key events of the process. 1. Dialog. The Station and the WSP discussed the issue intermittently. 2. Investigation. The Station had an experienced regional broadcast engineer and a tower professional with interference-hunting experience each investigate the complaint. Cabinet radiation was suspected, as there were measurable levels inside and outside the transmitter shed. Equipment grounds were verified. The Station had no postfilter transmission line sample, preventing measurement of conducted harmonics. 3. F CC Contact. Lacking successful action from the Station, the WSP contacted the FCC.
The Station is a Class A FM facility broadcasting from an antenna less than 200 feet above ground level. The two-bay FM antenna is mounted at the same elevation as the WSP and on the same tower.
energy: background noise, interference and incoming CDMA signals. Since there is no way to distinguish whether any desired signals are included in the RSSI measurement, it takes observing multiple measurements to infer whether a measurement is strictly a background noise-and-interference level. To reduce uncertainty about the noise and interference levels on the CDMA channel, another application collects data that is labeled as “noise” and reports every 10 seconds. This metric subtracts the total power of each received CDMA signal (multiple CDMA signals can be received simultaneously on the same frequency). The WSP shared noise tables that show 10 minutes of samples collected every 10 seconds. Each 10-second sample is the 75th percentile noise value of many samples taken in the 10-second period. The most-affected sector, the Alpha (or northerly) sector, was used as the reference. The Beta sector experienced lower harmonic interference (about 4–8 dB lower). To make measurements that were more consistent and repeatable, the modulation was removed from the FM signal during testing. The conversion to unmodulated carrier increased the WSP noise levels by 6 dB. This is due to the nature of the 8th harmonic of a modulated signal: the roughly 150 kHz bandwidth
4. FCC Inspection. An FCC inspector made an unannounced site visit and filed a Notice of Violation (NOV) on the Station. 5. Irrelevant Violation. Oddly, the NOV was for alleged FM spurious emissions at 112 MHz and 198 MHz. There was no mention of 8th harmonics or of the WSP complaint.
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Possible. No substantive data on the receiverside characteristics is available. 13. FM Cabinet Radiation Not the Prime Cause. Observed noise level at WSP receiver is greater than that which the cabinet emissions on the ground could be causing. 14. External Mechanisms Not Testable. Other potential sources of on-tower harmonic generation were not eliminated, but not considered likely. 15. 8th Harmonic Confirmation. A temporary shift of the Station’s unmodulated carrier frequency up 100 kHz shifted the noise by 800 kHz, from the WSP’s 3G carrier to its 4G carrier, as expected. 16. WSP Continues to Press FCC. WSP responded negatively to funding more significant work to mitigate cabinet interference and to pursuing receiver filtering. WSP expressed the opinion the Station remained responsible for the issue. Station asserts its radiated harmonics are not the dominant cause. An FCC letter went to both parties requiring them to cooperate and report back to the FCC. 17. Collaboration Continues. A WSP corporate interference specialist was brought into the dialog. The author briefed the specialist. Specialist acknowledged the proximity of the FM antenna to the WSP antenna was a likely issue. Station agreed to foil-shield the transmitter shed. WSP contributed some shielding materials. 18. Station Shielded. Continuous foil screening in transmitter shed reduced cabinet/shed emissions by 9 dB. No corresponding change in received interference was noted. 19. WSP agreed to withdraw its complaint. Station agreed to maintain the foil shield in the shed. FCC closed the complaint. 20. The NOV on the unrelated claims was dismissed internally but the Station was not formally notified of the dismissal (in writing) until it requested same from the FCC. SUMMARY OF FINDINGS In this section, the results of various experiments are summarized.
10. Dummy Load Test of FM Transmitter.
• Shutoff of FM Equipment Eliminates Noise: Shutting of the FM transmitter eliminates the noise seen by WSP. This confirms that the phenomenon is related to 8th-harmonic energy stimulated by the Station FM signal in an unidentified device. While shutting off the FM transmitter may appear to “prove” the FM station is at fault, a fully compliant FM signal might be stimulating an external element to generate harmonics. Further testing is required to determine what is generating the 8th harmonic product.
11. Minimal WSP Receiver Data. Assumptions have necessarily been made about the WSP system topology because of a lack of useful information.
• Unmodulated Carrier: Testing with an unmodulated FM carrier improved the repeatability and dynamic range of some tests, while increasing the WSP’s measured noise by 6 dB.
12. Calculating Receiver Susceptibility Not
(continued on page 19)
6. I rreproducible: The author was called in to assist. No spurious emissions were found on the 112 and 198 MHz frequencies and the FCC was informed. 7. C abinet Radiation Measured. 8. Transmitter Harmonics Measured. 9. Parties Meet Several Times.
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HARMONIC (continued from page 18)
• Frequency Shift of FM Carrier: While unmodulated, the FM carrier was briefly shifted up 100 kHz. The 8th harmonic was thereby shifted 800 kHz into the WSP’s LTE channel. Noise on the WSP CDMA system was eliminated. Noise on the WSP LTE system increased. This confirms an 8th-harmonic of the FM signal is involved. It also confirms two different receivers experience the interference. Of interest is the fact that the noise level recorded by the LTE receiver was less than the noise level recorded by the CDMA receiver. Considering the same antennas are used for both systems, this could indicate ROII, assuming there is slightly different susceptibility between the two WSP receivers. • Compliant Cabinet Radiation: Cabinet radiation of the 8th harmonic is compliant with FCC certification requirements. • Compliant FM Antenna Radiation: The 8th harmonic content on the transmission line, and therefore the FM antenna radiation of the 8th harmonic, is less than –115 dBc, well below the FCC limit. • On-Tower Generation of Noise Is Primary Source: Running the FM transmitter into a dummy load made the greatest improvement in WSP noise: an 8 dB reduction. With the load, the FM antenna ceased radiating. This suggests the FM antenna is stimulating something on the tower to generate 8thharmonic energy (or that the FM antenna is radiating the 8th-harmonic energy, but see next point). The shielding of the shed had not been installed yet. • FM Transmitter Power Adjustment: Varying the FM transmitter power on the FM antenna yielded a stepped change in 8th-harmonic cabinet radiation. This is probably due to the deactivation of various power amplifier modules as the transmitter power is adjusted below certain thresholds. There was an observed but ambiguous change in the received noise. • Cabinet Radiation Received by WSP Antenna Predicted to Be Low: The path loss between transmitter cabinet and the lower reception angles of the WSP antenna was estimated. The 8th-harmonic cabinet emissions, as measured at ten feet from the cabinet, are predicted to be too weak when they propagate 160 feet above the transmitter. • Possible Overload of WSP Receiver by FM Fundamental: The radiated FM signal produces a strong field at the at the WSP antennas. This high signal level invites a variety of unintended consequences. The WSP’s duplexer filters do not necessarily provide enough protection for the CDMA receiver. The level of the FM fundamental at the CDMA receiver is entirely dependent on the amount of attenuation by WSP’s antenna and filter. There is no data to refine this estimate. • Possible Overload of WSP Transmitter (or others) by FM Fundamental: Because the WSP transmitters and receivers are isolated by duplexers, the prospect of WSP transmitters stimulated by a strong FM signal generating the 8th-harmonic
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product and that product then leaking into the receiver seems the least likely scenario. There is no duplexer data to confirm this assumption. • Possible Generation of Passive Intermodulation on Tower: Passive intermodulation (PIM) is usually the product of two or more radio signals mixing through conduction by a semiconducting surface. However, harmonics are also generated by this mechanism. With a strong field strength predicted near the WSP antennas, there is a risk of passive harmonic generation on some structural or electrical elements on the tower. At the 8th harmonic, however, such a product will be several orders of magnitude lower than the fundamental incident field and, for example, simple third-order intermodulation products. STUDY DETAILS In this section, the investigative effort is described in more detail. Cabinet Radiation Complies with FCC Requirements Measurements show the Station transmitter cabinet emits some energy on its 8th-harmonic frequency. Nevertheless, the level of cabinet radiation (CR) is in conformity with FCC requirements for the manufacture of such devices. Part §2.1053 requires the measured field strength of spurious cabinet radiation to be reported as “the relative radiated power of each spurious emission with reference to the rated power output of the transmitter, assuming all emissions are radiated from halfwave dipole antennas.”
December 16, 2020
• R is the distance (radius) between antennas (3 m) • f is the frequency, Hz (8 x 108) The unknown is the transmitter power, and (1) is easily adjusted to solve for PT. Also, converting (1) to decibels to simplify the calculation: PT = PR + 20Log(4πRf/c) – GR – GT
(2)
–14.3 = –52.2 + 40 – 0 – 2.15
(3)
Equation (2) is expressed in decibels. Equation (3) shows that about –14.3 dBm equivalent transmitter power produces –52.2 dBm received power for the conditions described. This level of 8th harmonic power can be compared to the “rated power output of the transmitter,” which is 5 kW or 67 dBm. Thus, the cabinet-radiated 8th harmonic at 10 feet was 67 – (–14.3) = 81.3 dB below the reference based on the rated power of the transmitter. §73.317 requires spurious emissions beyond 600 kHz from the carrier to be “at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier.” Substituting the licensed ERP of 33.3 dBW, the spurious emissions limit is –76.3 dBc. Thus, the measured cabinet emissions were 5 dB below the maximum allowed under FCC rules. Cabinet-to-Tower Path Loss Path loss calculations from cabinet to WSP antenna suggest cabinet radiation is indeed a lesser component, if not entirely insignificant. The WSP provided a limited amount of information on its reception architecture to help with the calculations. The calculations, detailed
Cabinet radiation was suspected, as there were measurable levels inside and outside the transmitter shed. Equipment grounds were verified. The Station had no post-filter transmission line sample, preventing measurement of conducted harmonics. At 10 feet, the power of the 8th-harmonic cabinet radiation received on a reference antenna, normalized to an isotropic antenna, was –52 dBm. This must be converted to the units used to verify conformity. While the measurement was taken only 10 feet from the transmitter, the short wavelength at 800 MHz (~0.375 m) allows the assumption this is a sufficiently far-field measurement for a free-space calculation. Because this is an in-situ measurement, the effects of reflections (ground and other equipment and structures) must be assumed to be immaterial. It is reassuring that multiple measurements at different azimuths from the transmitter produced similar results. To obtain the estimated radiated power of a dipole necessary to produce the received 8th harmonic signal strength, the Friis Equation [Reference 5] is used: PR = PTGTGRc2/(4πRf)2 (1) • PR is received power (linear equivalent of –52 dBm) • PT is transmitter power (the unknown) • GT is gain of the transmitting antenna (dipole by definition: linear equivalent of 2.15 dBi) • GR is gain of the receiving antenna (PR already normalized to isotropic: factor of 1 or 0 dBi) • c is the speed of light: 3 * 108 m/s
below, suggest the WSP antennas would receive cabinet-radiated 8th-harmonic energy at a level below the –88 dBm noise floor of the WSP receivers. It would not be visible to the WSP. Moreover, the predicted receive level of the 8th harmonic CR is substantially below the –70 dBm received noise level (modulated) observed on the WSP receiver. The Station transmitter cabinet emissions at ground level were –52.2 dBm normalized to an isotropic antenna at 10 feet from the cabinet. Assuming free space path loss between the transmitter shed and the WSP antennas and a distance of 160 feet, the additional free space path loss would be increased by the square of the ratio of 10 ft to 160 ft, or LI = 20 * Log10(160/10) = 24.1 dB additional path loss
(4)
Starting with the measured –52.2 dBm and incorporating another 24.1 dB path loss, an isotropic antenna is likely to receive no more than –76.3 dBm of the 8th harmonic CR from the ground. Because the WSP antenna has gains of –10 to –35 dBi in the direction of the transmitter shed, the expected receive signal level of an 8th harmonic CR emission would be near –111 dBm, or in the worst case, –86
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December 16, 2020
dBm. Recall the WSP measured noise level with the 8th harmonic product was –66 dBm (unmodulated) and –70 dBm (modulated). Thus, it does not appear possible for CR to cause the interference. Post-Shielding Measurement and Calculation Before and after the foil shield was installed inside the transmitter shed, measurements were taken 20 feet above ground, which is 15 feet radially from the shed. The 8th harmonic CR was –79 dBm (isotropic) before and –88 dBm after. Using –88 dBm as a starting point, the free space path loss from the shed to the WSP equipment at ~160 ft above ground would increase by about 20 Log(160/15) = 21 dB. The WSP could expect the cabinet/shed radiation to be approximately –88 – 21 = –109 dBm (isotropic) at its alpha-sector antenna. The WSP sector antenna pattern exhibits less than isotropic gain at low elevation angles, which suggests the received signal level of the –109 dBm signal would be further attenuated by the antenna, perhaps by 10 to 35 dB (discussed above). Deduct a conservative 10 dB for low-elevation-angle antenna losses, totaling –119 dBm. An additional loss not included in the –119 dBm estimate is the difference between the full-bandwidth measurement of the actual cabinet radiation and the partial bandwidth measurement of the energy within the WSP CDMA channel. An incoming wideband modulated –119 dBm signal should be partially captured by the CDMA receiver at about –125 dBm. This was not tested, as it would have required importing the dummy load again to isolate the FM antenna from the variables. Moreover, at this point, the WSP quit the effort. This final estimate reveals a substantial margin between the 8th harmonic cabinet radiation, with shielded shed, and the noise levels of the WSP receiver: nominal –88 dBm background noise level / –70 dBm modulated 8th harmonic noise level. This estimate is based on a measurement from above the shielded shed (–88 dBm at 15 feet vertically). In contrast, the previous calculations above are based on a measurement close to and horizontally from the shed (before shielding, –52 dBm at 10 feet horizontally). The previous estimate was more conservative, showing a smaller margin between the estimated received noise level and the noise floor of the receiver.
Fig. 1: Transmitter internal harmonic filter response
Dummy Load Test The dummy load test suggests that cabinet radiation is not the primary issue. The FM antenna was disconnected and a borrowed dummy load was connected at ground level. If cabinet radiation were the primary issue, the noise would be unchanged while the transmitter runs on a dummy load. With the dummy load, cabinet radiation was constant but the noise experienced by the CDMA receiver was reduced by at least 8 dB. This suggests cabinet radiation might contribute perhaps 15% of the elevated noise experienced by the CDMA receiver. Variations in FM Power Inconclusive While the FM antenna was active (no dummy load), transmitter power levels were changed. The CR and the WSP noise levels were monitored. With the power reduced by 5 dB, the CR and the WSP noise levels were unchanged. A 10 dB reduction in FM power lowered the WSP noise about 6 dB, while the cabinet radiation also dropped by about 6 dB. Alone, this data could suggest the cabinet radiation is a major contributor because the cabinet radiation and the WSP noise levels seemed to track. However, power reductions also changed the radiated FM power and the 8th harmonic AR (if any), in addition to the 8th harmonic CR. This test did not isolate the cabinet radiation as a variable. Moreover, the dummy load test shows that disabling the FM broadcast antenna has the greatest impact, implying the radiated fundamental is directly affecting receivers on the tower. Reference Noise-Power Levels The analysis assumes that the WSP-measured noise levels are normalized to the passive levels coming from the antenna into the WSP’s radio head. It is conceivable that there is unreported gain in the WSP receive signal path. Such active gain might yield a measured noise level of –66 dBm, while the actual noise level corrected for, say, 20 dB of gain might be –86 dBm at the antenna terminals. The active-gain hypothesis makes the potential contribution of CR to the noise problem more credible. It also makes the self-induced ROII idea more credible, because an active amplifier before the WSP detector could be suffering from gain compression
or other effect from the unwanted FM fundamental frequency. The analysis was significantly hampered by a lack of information on the design of the WSP receivers. FM Antenna Radiation Complies with FCC Requirements Measurements indicate that if there is 8th-harmonic energy conducted up the FM transmission line to the FM antenna, it is compliant with FCC requirements for antenna-radiated emissions. The 8th harmonic conducted on the transmission line is at least 115 dB below the carrier power level (≤ –115 dBc). This is FCC-compliant by a large margin because §73.317 requires this value to be more than 76.3 dB below the fundamental carrier power reference. The 8th-harmonic measured value of ≤ –115 dBc conservatively overstates the actual level on the transmission line. This is because the measurement is as low as the measurement system can go in this situation. The test apparatus employed one three-pole FM notch filter and two FM band-stop filters to filter the Station’s fundamental frequency from the line sample to provide enough dynamic range to confirm FCC compliance by a large margin. Filter insertion loss and sensor frequency response at the 8th harmonic were incorporated in the measurement. This measurement from the FM transmission line using a calibrated RF sample element resulted in an uncertain blip in the instrument noise floor at the 8th harmonic frequency. Despite the substantial attenuation of the fundamental frequency, the analyzer was turned up to view a noise floor that was also close to its internal intermodulation threshold. The blip could have been an analyzer artifact. It is reasonable to assume that the FM broadcast antenna, tuned to part of the FM band, will further attenuate harmonics as they are radiated. This maximum potential antenna-radiated 8th harmonic emission is consistent with the combined attenuation of the transmitter’s internal harmonic filter and the external two-cavity bandpass filter. According to manufacturer sweep data (Figs. 1 and 2), the combined loss near 800 MHz is about 180 dB. Even if the 8th harmonic in the transmitter were equal to the full
Fig. 2: Dual-cavity external filter response
(continued on page 21)
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HARMONIC (continued from page 20)
power of the transmitter (an obviously extreme worstcase estimate), the harmonic would theoretically be filtered to –180 dBc. While such low levels are beyond the capability of field measurement, there is almost certainly negligible 8th harmonic energy on the transmission line feeding the broadcast antenna.
RadioWorldMagazine
December 16, 2020
• S is the power density (W/m2) • PEIRP is the effective radiated power (licensed ERP times 1.64, W) • R is the radial distance to the other antenna (m) S = 31.2 W/m2 (7) and E = (S x 377)1/2 = 108 V/m
(8)
dBi (negative) gain of the WSP antenna at 100 MHz, the FM signal on the WSP antenna port could be a substantial +13 dBm. Assuming the WSP duplexer rolls off the out-ofband energy by, say, 40 dB, the level of the 100 MHz FM signal impinging on the CDMA receiver could be between –27 and +3 dBm. These are challenging levels for a receiver to tolerate. A –40 dBm desired uplink signal is considered very strong. If the WSP architecture is like this assumption, the 100 MHz FM signal could be overloading the WSP receiver and promoting internally generated harmonics.
where Estimating Worst-Case Antenna Radiation of 8th Harmonic • E is field strength (V/m) Using the safe-side estimate of the 8th harmonic on WSP NOISE DATA The WSP’s data collection of “noise” in the • 377 is the impedance of free space (ohms) the transmission line based on the –115 dBc noise floor, CDMA channel statistically collects an unidentiwhat is the worst-case radiated 8th harmonic received fied number of samples over 10 seconds. Those Equations (7) and (8) show the approximate magby the WSP? samples are tabulated and the 75th percentile values nitude of the power density and field strength at the Assume a separation of 10 feet (r = 3 m) from FM are reported as a single data point in each table. nearest WPS antenna. It is only approximate because to WSP antenna. Also assume the FM and WSP antenTable 1 shows avernas have 0 dBi gain at ages of twelve 10-sec800 MHz in their respecond samples (two mintive orientations. Deduct While this case may be exceptional because of the close utes) from the CDMA the 1.3 dB transmission noise data tables line loss at 800 MHz for 10-foot spacing between FM and WSP antennas, it is instructive collected during this test. net radiating-antenna After each change of input level of less than that there is the potential for a wireless facility to be susceptible to mode of the FM trans–116 dBc. The 65 dBm the fundamental, not the harmonics, of a nearby FM station. mission (power, frequenEIRP of the fundamental cy, load), data was colplus the –116 dBc marlected for at least three gin yields –51 dBm EIRP minutes. The most recent worst-case 8th harmonic 12 data points are represented AR. Returning to Equation (3), in the “2-min Average” column. enter –51 dBm for PT and solve for Using the noise baseline of the PR with gains GT and GR at 0 dBi. CDMA receiver with the FM The result is –91 dBm worst-case transmitter turned off, a differreceived 8th harmonic power from ence is shown in the last column. AR. This difference represents the rise This estimate is subject to in the noise level for each test potentially greater gain on the mode. Bear in mind that meaWSP antenna than assumed, surements 3 through 7 are with though its main beam is pointed no modulation on the FM carrier. north, away from the FM antenThis artificially raises the level na. It is also subject to a subbut provides a more consistent stantial reduction in FM antenna noise reading for comparisons. efficiency at 800 MHz. Assume these adjustments, if they could REGULATORY DISCUSSION be known, cancel. The –91 dBm There is some ambiguity [1] estimate is well below the –66 as to how the FCC addresses dBm received noise power. The complaints like the one docudifference is substantial, allowing mented here. for some error in the estimated GT There is a general principle of and GR values. Moreover, the ini“last-in” that places the onus on tial –115 dBc estimate of 8th harFig. 3: WSP and station antennas on the tower newcomers to a site to address monic energy on the transmission interference concerns with incumline is extremely conservative, as bent licensees. This is not always the way these discussed above. the WPS antenna may not be in the far field of the issues are handled. In the present case, it would This alternative analysis of potential 8th harmonic FM station antenna. The mounting arrangement on the have been difficult to argue the FM Station, having AR confirms that the FM antenna is incapable of raditower is shown in Fig. 3. This is an unusual arrangebeen collocated on the site for a decade, would be ating enough 8th-harmonic power to cause the noise the ment. Typically, wireless antennas are placed above subject to the “last-in” principle, especially considWSP is experiencing. or below FM antennas, which increases the electrical ering the fact that wireless service providers rouisolation between them. tinely upgrade their base station gear. Instead, the FM Fundamental on the Tower The following observations are speculative, based FCC approach was to require the parties to cooperate The case for the FM fundamental stimulating ROII on general knowledge while lacking the specifics and respond jointly within 60 days. is considered. Assuming the FM and WSP antennas necessary to fully analyze the receiver architecture. It was difficult to pursue any possible on-tower are 10 feet horizontally spaced, the FM field strength 108 V/m is a substantial field strength that could mechanisms external to the FM transmitter. The is about 108 volts per meter at the WSP antenna. A have an impact on the WSP antenna or receiving WSP had much less of an understanding of its far-field power density estimate [6] is obtained with (6): apparatus. Assuming the WSP antenna is neutrally equipment architecture than a broadcaster typically sensitive (0 dBi) to the fundamental at 100 MHz, the (6) does. It was not possible to model, bench-test or S = PEIRP/(4πR2) signal picked up by the WSP antenna could be +43 field-test the WSP receivers for susceptibility to dBm. This is an extremely high signal level to have on FM broadcast signal overload. Without such eviwhere the WSP antenna feed. Even if we allow for, say, –30
radioworld.com | RADIOWORLD ENGINEERING EXTRA
December 16, 2020
No.
WSP 75th percentile CDMA Noise Power over 10-second Intervals (dBm)
FM Transmitter Mode
2-min Average
Increase from baseline
1
FM transmitter turned off
2
Existing condition, modulated FM broadcasting at full power on antenna
-71.4
16.6
3
FM transmitter on at full power on antenna with modulation removed
-65.8
22.1
4
FM frequency shifted +0.1 MHz (harmonic +0.8 MHz), unmodulated
-87.95
0
5
Returned to normal frequency, unmodulated, reduced transmitter power 5 dB
-65.9
22.0
6
Returned to normal frequency, unmodulated, reduced transmitter power 10 dB (an additional 5 dB)
-71.5
16.5
Transmitter placed on dummy load, full power, unmodulated
-73.7
14.2
7
-87.95
Table 1: CDMA noise data with changes in FM transmitter mode
dence, it was difficult to press back on the WSP to consider additional filtering or other mitigation to address the on-tower component of 8th harmonic generation. Toward the end of the engagement, the FCC Wireless Bureau and Media Bureau jointly sent a letter to the parties requiring them to report to the FCC in 60 days. This action drove the par-
TECHMART
ties to halt their experimental work and draw a conclusion. The WSP withdrew its complaint and the Station agreed to maintain the shielding in the transmitter shed. Meanwhile, it is perplexing how the FCC informed the Station of the issue. The Station was working with the WSP, in an intermittent effort by both entities. Then comes the FCC NOV
REFERENCES [1] “ Harmonic Convergence? FM Interference to 700 MHz LTE Service,” Comm Law Blog, Peter Tannenwald, June 23, 2013 [2] “ Investigating Interference Between 4G LTE And Broadcast,” Radio World, Michael LeClair, Dec. 2014 [3] “ Can You Hear Me Now? LTE Interference,” by Jeff Welton, Nautel, support.nautel.com [4] T itle 47, Code of Federal Regulations [5] S ee e.g., www.antenna-theory.com/basics/ friis.php accessed Feb. 2, 2020 [6] FCC, Office of Engineering and Technology, Bulletin 65, p. 19 equation (4)
alleging spurious emissions on entirely unrelated frequencies (112 and 198 MHz). The station was forced to pursue the NOV, taking away Station resources from working on the as-yet undisclosed complaint. The alleged spurious emissions below 200 MHz were not reproducible. The NOV was abandoned without informing the Station, until the Station asked for a written dismissal during the final discussion among the parties and the FCC. While this case may be exceptional because of the close 10-foot spacing between FM and WSP antennas, it is instructive that there is the potential for a wireless facility to be susceptible to the fundamental, not the harmonics, of a nearby FM station.
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