Recent tests have been performed by the NAB to assess the viability of the all-digital AM IBOC mode, MA3. Hybrid AM transmission using MA1 has been commonplace for several years now, but the all-digital mode presents some unique challenges. The peak to average power ratio of the signal increases significantly compared to a station broadcasting an analog AM or hybrid signal, requiring either a reduction in power or some form of peak control. A typical hybrid transmitter installation would have the transmitter optimized for analog AM performance, but a different set of criteria are required for the all-digital signal to optimize spectral performance and MER.
This paper will present an analysis of the MA3 signal and spectral mask, along with the implications for passing it successfully through both current and past generation AM transmitters. Suggested power levels for a given transmitter will be reviewed, along with how these could be increased by reducing the amount of power in the AM carrier. Finally, an innovative signal conditioning technique that reduces the envelope and phase frequency content will be presented, allowing the operator to choose a trade-off between MER and spectral performance into difficult loads.