A Case Study in All-Digital HD AM Broadcasting: Refinements, Performance Tests, and Lessons Learned

WWFD (820 kHz, Frederick MD), owned by Hubbard Radio and having engaged in a cooperative effort with Xperi Corporation to become the first AM broadcast station to transmit using the HD Radio MA3 mode full-time, commenced operations as such on July 16th, 2018. While initial performance met expectations in terms of coverage, a transmission problem occurred: the secondary and tertiary carriers, containing the stereo information and ancillary data services, were not being formed correctly and therefore suffering from excessive bit-error losses. Inadequate sampling of the baseband digital signal by the transmitter?s Pulse Duration Modulator (PDM) had led to excessive noise in the carriers and increased spectral regrowth in the transmitted RF signal. A second transmitter having a higher-phase PDM was installed, solving this issue. A further refinement was made by adding a phase rotation network to the day antenna, achieving Hermetian symmetry (already present on the night antenna), which presented a constant-conductance load to the transmitter. With the antenna system optimized, adaptive precorrection techniques were employed in the transmitter to minimize spectral regrowth, and improve signal robustness and receiver acquisition time. Day and night drive tests were conducted in the summer of 2019. Nighttime tests were repeated in the winter of 2019, to compare coverage with the reduced co-channel skywave interference conditions encountered earlier in the year.

With all-digital operation, the monitoring of licensed parameters must be revisited. Power measurement with regards to MA3 transmissions must be performed in a different manner than with standard analog AM transmissions, as pin-diode base current meters designed for AM do not accurately measure currents using the all-digital mode of operation. Procedures for performing power measurements with spectrum analyzers, power meters, and thermocouple RF ammeters are discussed.

Finally, some operational ?lessons learned? are presented. Techniques for optimal audio processing in conjunction with the high audio compression rate in the MA3 mode are discussed. A battery-protected power supply which keeps critical parts of the digital transmitter online during power interruptions was installed, thereby reducing system downtime during utility interruptions. It is hoped that the knowledge gained from WWFD?s experience will facilitate the conversion of other stations who are contemplating using the MA3 transmission mode.

Dave Kolesar | Hubbard Radio | Washington, DC USA
Mike Raide | Xperi Corporation | Columbia, MD USA