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We chat with the winners of this year’s NAB Radio Engineering Achievement Award

Author: Randy J. Stine Published: November 3, 2021

Dave Hershberger's work on transmitter exciters and modulators and low-level signal processing is considered legendary by many broadcasters. In fact, he co-developed the world's first digital FM exciter as an experimental prototype and tested it in the air in 1986.

The National Broadcasting Corporation Association awarded Hershberger the 2021 Radio Engineering Achievement Award in recognition of his work at Harris Broadcast, Grass Valley Group, Axcera and Continental Electronics.

The senior engineer even researched X-band uplink transmitters for the JPL/NASA deep space network. He retired in 2017 as a senior scientist at Continental.

Hershberger occasionally consults, but mainly enjoys retirement. Radio World asked him about his career, the equal distribution between radio and television design projects, and his views on current events in the field of radio engineering.

Radio World: Where did you grow up and what inspired your interest in radio engineering? Dave Hershberger: I grew up in Sycamore, Illinois, just west of Chicago, but outside the suburbs. When I was 13 years old, I got an amateur radio license. After several years of amateur radio, my neighbor suggested that I should study radio stations. This is what makes me move forward.

[Read: Hershberg won the 2021 NAB Engineering Award]

When I was 16, I studied and obtained my FCC First Phone commercial license. That is my ticket to enter the broadcast. After graduating from high school, at the age of 17, I found my first radio job at WCLR in Crystal Lake, Illinois. AM 850. They have a directional antenna and need a First Phone license holder on duty.

RW: Please describe your early days at Harris Broadcast in 1975. What is it like in Quincy? Hershberg: Harris is very interesting. Quincy, Illinois, not so. But this is where the work is.

I started with the TV transmitter group, but after talking to Geoff Mendenhall who works at FM, we started brainstorming ideas at lunchtime about what we wanted to do in the new FM exciter.

TE-3 is very old, has high manufacturing costs and has its shortcomings. Geoff worked with the management and was authorized to start the development of a new FM exciter. I was transferred to FM. We added some engineers to the project, and then we left.

We broke new ground with a new type of stereo generator, overshoot control filter, improved PLL dynamics to eliminate the skew of low-frequency square waves, and many other features.

RW: What do you think is your most important contribution to Harris? Hershberger: That might be a low-pass filter for overshoot control. When we developed the MS-15 FM exciter, we heard that West Coast was making a popular new stereo generator with built-in audio processing. It is called Optimod. One of its most important functions is the ability to control the overshoot in the 15 kHz low-pass filter integrated with the stereo generator.

Bob Orban came up with a brilliant solution to deal with this problem. He adopted a systematic approach to closely integrate filtering and filter overshoot correction with audio processing.

We are also developing stereo generator options for FM exciters. But if we don’t also solve the overshoot problem, then we won’t be able to sell many stereo generators. So my job is to find a solution to the problem.

This is a difficult problem, while controlling the amplitude and frequency spectrum. One way to start solving the problem is to filter, then cut out the overshoot, and then filter again. This will reduce overshoots to some extent, but will not completely eliminate them.

In theory, this process can be repeated: filtering, editing, filtering, editing, filtering, editing and continuing to do so until the overshoot is low enough. Of course, such a system is not practical.

Eventually I found that I needed more than just editing. The limiter can be analyzed as a gain reduction device, which reduces the gain only during the overshoot period. If I can reduce the gain more than the gain required to accomplish simple clipping, then I can make the overshoot controller converge in the primary filter-clip-filter.

So the trick is to remove the overshoot, cut them off, amplify them with a gain of about 2, and then subtract them from the clipped waveform. The linear phase filtering of the signal almost completely eliminates the filter overshoot, while still providing a sharp 15 kHz cut-off frequency.

The advantage is that this is not part of the audio processor. This is an independent function. Therefore, you can use any audio processor you want and run it into our stereo generator, and the 15 kHz low-pass filter will not overshoot and produce overmodulation. Any audio processor can get the overshoot control capability of Optimod.

In early 1977, we conducted the first live test of the MS-15 exciter at WGEM (FM) in Quincy. Overshoot control and greatly improved low-frequency dynamics have resulted in a substantial increase in average modulation.

Brian Cox is one of the engineers of the MW-1 AM transmitter. He left Harris and worked for another company in Quincy. He would tune the car radio to WGEM, and one morning he drove to work. He turned on the radio and Brian said it was too loud and blew him into the back seat. That was the first morning we aired the prototype.

RW: What are the highlights of Continental Airlines? Hershberger: In general, digital signal processing is deployed in several different product lines: FM exciters, ATSC exciters (including linear and nonlinear adaptive equalization), and VLF and LF transmitters.

RW: Is there a career project that stands out? Hershberger: Probably the coolest project is the JPL/NASA Deep Space Network uplink transmitter. They generate up to 80 kW in the 7 GHz range

These are non-broadcast transmitters, but this is one of the most interesting and challenging projects. JPL wanted very low phase noise-not for communication, but so that they could also use the transmitter for scientific experiments. These include searching for gravity waves by detecting phase bumps and bistatic radar imaging. The source of the bistatic radar signal is on the earth, and the receiver is on the spacecraft.

So everything we do needs to pay attention to low noise performance. RF amplifiers, klystron bundle power supplies, focusing magnet power supplies and filament power supplies, and even water cooling systems require very low noise.

The first production launcher was installed at Goldstone Station and was used for the first flyby of Pluto in 2015. In addition to communication, it is also used for bistatic radar mapping of Pluto.

RW: Over the years, you have written articles about HD Radio implementation in Radio World. Can you discuss this work and evaluate the status of HD Radio in the United States and where it might go next? Hershberg: I'm quite disappointed. There are interference problems. Audio codecs cannot be upgraded to more modern technologies. There is no "oh, wow" factor—such as letting your radio find out your preferences, and then find and record programs that it thinks you might like—some features in DVRs.

Based on age, interest or location, there is almost no ability to provide different business announcements and music to different audiences. There is no non-real-time transmission and storage capacity.

There are many such functions that should be part of the new digital sound broadcasting system. In fact, we only have ordinary real-time, single-stream radio, which is transmitted digitally.

RW: You once advocated moving AM radio to TV channels 5 and 6 below the FM band, but this did not happen. What do you think about this and the future of AM broadcasting? Hershberger: Unfortunately, but it is foreseeable that it will not and will not happen. But now there are similar opportunities. ATSC 3.0 includes the ability to carry pure audio programs. And these pure audio programs can be broadcast through optimized and robust coding for mobile reception, independent of the modulation and coding of the video signal.

I hope to see AM radio start simulcasting ATSC 3.0 signals. Except for TV programs, a single ATSC 3.0 transmitter can carry all AM signals in the market. Can make the car radio receive at least the pure audio stream in ATSC 3.0. This will be a solution to the problem of electric vehicles. In this problem, the transmission system will generate so much electrical noise. Because of the need for EMI suppression, the price including AM radio is too expensive. ATSC 3.0 is also a way to allow AM programs to be accepted by families again.

At the same time, I am participating in the AM Improvement Working Group of the National Radio System Committee, which is studying ways to keep analog AM feasible.

RW: In the NAB acceptance video, you thank Geoff Mendenhall and Dan Dickey for their support over the years. Are there other mentors? Hershberg: Of course. There are many. In Harris, there are Hans Bot, Tony Uytender, Terry Hickman, Bob Willasser, Hilmer Swanson, Tim Hulick, and more . In Continental, there are José Sainz, Grant Bingeman, Michael Pugh, Howard Butler, etc. The late Dr. Steve Reyer, an EE professor at the Milwaukee School of Engineering, is undoubtedly a mentor.

RW: If you were the chairman of the FCC, which technological change would you like to make? Hershberger: If I can only make one change, it is to enforce radiated and conducted emission limits. Don't let cheap and noisy power supplies and chargers enter the country. Mandatory recall of products that seriously violate the regulations. Let AM radio listen at home again!

RW: What do you think is the most important trend or latest development in radio technology infrastructure management? We have heard a lot about the centralization of the engineering department. Hershberger: I am not a manager or accountant. But I am disappointed to see that the decision made by the management will greatly increase the suspension time when there is a problem. I was disappointed when I heard the radio station with audio problems lasting for several years.

There is an urgent need for better engineering. It costs money, but it's well spent.

Declining broadcasting infrastructure can save money in the short term, but not in the long run. You can do the same with your car — don’t do maintenance — but it will eventually come back and bite you.

RW: What do you think are the most pressing technical issues facing radio broadcasters today? Hershberg: Interference. Switching power supplies, chargers, lighting, etc. will not only affect the AM frequency, but also VHF and even UHF. Mistakes in distribution and supervision will cause legal interference and will only increase the problem.

RW: When you talk to fellow engineers, what are their most common complaints or professional challenges? Hershberger: Not many broadcasters want to do things the right way. They want to spend as little money as possible, even if performance and live broadcast products are degraded.

RW: What advice do you have for young people entering such a technical field? Hershberger: If your education is limited to digitalization, don't expect to be proficient. You still need to understand analog concepts, even if they are all implemented digitally. Learn about control and feedback systems, filtering, modulation theory, signal processing and complex mathematics.

In addition, beware of intrusion into the computer engineering philosophy of radio engineering. As more and more digital technologies are applied to broadcasting projects, cultural conflicts continue to evolve into real technical issues. Computer engineers are interested in sending bits in the correct format without errors.

So much is fine. But computer engineers usually do not pay attention to the phase noise in the clock, or choose a clock frequency suitable for broadcasting stations, or the frequency accuracy of such clocks. Why are they doing this? Even if the clock is noisy or deviates from the frequency, data can still be transmitted without error.

Most importantly, do not assume that the digital signal has low enough phase noise and frequency accuracy to meet broadcast standards.

RW: You and your wife must have been together for a long time. Is there any marriage advice you can provide? Hershberger: Sandy and I have been married for 44 years. She was a music teacher, a software engineer, and won an Emmy for it, as well as a licensed marriage and family therapist.

As for the marriage proposal, I think I would say compatible changes. We all change with age and it is important to consider our partner when doing so.

Besides, I think it involves some luck. I feel very lucky. We are fortunate to have a gifted 17-year-old daughter who is eager to pursue a career in science.

RW: Retirement in California seems to be good for you. Why do you like it? Hershberger: We live in a wooden house built on rural land near the Tahoe National Forest near Nevada, California. We have deer, bears, foxes, coyotes and other animals for neighbors to use. My voluntary hobbies include amateur radio. My involuntary hobbies include property maintenance—cutting, hauling and chopping wood from fallen trees, and plowing snow with a tractor. We have alternative energy sources-solar power and diesel generator backup-for our frequent power outages, but it is beautiful here.

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