AM Carrier Power Reduction in All-Digital Service Mode MA3

Some AM radio stations have turned off their analog broadcasts and converted to digital-only operations, with the possibility of better coverage, improved ratings, and more competitive features. Since a modulated analog signal is not present, it is possible to reduce the main carrier power in All-Digital service mode MA3 without adversely affecting audio quality and availability. This might not only decrease conversion costs but could also save broadcasters thousands of dollars per year in utility bills. Alternatively, the resulting power savings could be applied to the digital subcarriers, thereby enhancing the robustness of the digital signal.

Since receivers use the main carrier to demodulate the digital signal, reducing its power could potentially degrade reception performance. Specifically, HD Radio receivers use the main carrier to correct frequency differences between the received and transmitted signals, and to normalize the amplitude of the digital subcarriers across the allocated channel bandwidth. Xperi has analyzed and simulated the effects of reduced main carrier power to determine an acceptable decrease and quantify the resulting degradation. Results indicate that a 6-dB carrier power reduction, with a concomitant 4-dB reduction of the two OFDM reference subcarriers, should have a negligible impact on signal robustness.

The next logical step in assessing the viability of reduced MA3 carrier power was to validate these predictions by measuring performance in a real-world automotive environment, using Hubbard Broadcasting station WWFD in Frederick, MD. Field tests were performed over multiple routes to expose the signal to a variety of terrain, speed, interference, and shadowing conditions.

This paper will review Xperi’s analytical and simulated performance predictions and will present field test results that successfully validate them. Results indicate that reducing carrier power by 6 dB and reference subcarrier power by 4 dB has an insignificant effect on WWFD’s MA3 digital audio coverage area.

Paul Peyla | Xperi Inc. | Columbia, Md., United States
Jeff Baird | Xperi Inc. | Columbia, Md., United States
Dave Kolesar | Hubbard Broadcasting | Washington, D.C., United States