NPR Labs says it’s working with member station KUOW(FM), Seattle on a new type of FM+HD booster system as well as new design methodology, which the R&D engineering group says could ease the way for stations to employ both FM analog and FM HD Radio boosters to improve coverage in areas where interference may be high due to geographical obstacles or overlapping signals. The study is based on a new hybrid, FM+HD booster system developed by Harris Broadcast.

“This study is intended to put an end to the questions about the optimal design of FM boosters,” NPR Labs Senior Technologist and Project Lead John Kean told Radio World. “In the past, engineers have had no hard information on the optimal design criteria for FM boosters. As a result, we’ve seen boosters that have been designed, built and turned on, but they’re ultimately disappointing because they suffer from these multipath distortion effects. What we’re hoping is that this study will put an end to that lack of technical information about the parameters for booster design.”

The first phase of the project is a study to predict, and avoid, multipath distortion effects that occur with signal boosters. RW readers are familiar with multipath distortion that causes FM sound to sound noisy, and with mobile reception multipath adds clicks and pops as the FM signal varies. Listeners can hear the multipath at a stop light, and then find that moving the car a few inches restores the signal by moving out of the point of multipath cancellation, according to Kean.

“This distortion of the received audio occurs because the signal from most boosters must overlap to some degree with the signal of its primary station,” Kean said. “As a result, there has been uncertainty over the deployment of FM boosters for many years because the effects of these overlapping signals have been unknown.”

To develop accurate data on the allowable signal ratios and modulation timing offsets, NPR Labs and Towson University will perform a series of listener-based tests using controlled simulations of overlapping signals. These tests will determine the necessary parameter limits for booster location and operation — before construction of the booster begins. These parameters will be used in advance mapping software developed by NPR Labs to geographically evaluate booster locations and optimize design.

The labs will also test HD Radio receivers to verify their Orthogonal Frequency Division Multiplexing “guard band” parameters, outside of which the digital reception may fail. The parameters will be used to ascertain the geographic operating area for KUOW’s digital coverage with and without the planned booster, according to Kean.

KUOW’s design is challenging because it operates with both FM analog and FM HD Radio transmission. To more precisely control the signal overlaps with hybrid broadcasting, Harris’ DSP-based gap filler system was selected. This system provides RF filtering and has the ability to separately control the power of each digital sideband, as well as the analog FM channel, according to Harris.

Another first for this system is the use of over-the-air relay. In comparison with booster designs that regenerate the IBOC signal locally at the booster, the over-the-air relay will reduce construction and operating costs and eliminate the need for a digital circuit from the primary transmitter, according to NPR Labs.

NPR Labs anticipates presenting a report on the design and testing process this spring at the Public Radio Engineering Conference.