RDS coming to an AM radio near you?


NAB Fastroad has released a report on a new AM radio data service concept, with the potential to provide analog AM broadcasters the ability to transmit low-rate data similar to what FM broadcasters can provide using the Radio Data System (RDS) subcarrier. This work was undertaken by iBiquity Digital and was co-funded by iBiquity and NAB Fastroad, the a multi-year, multi-million dollar technology advocacy program established by the NAB to seek and facilitate development and commercialization of new technologies that can be exploited by broadcasters using radio and television broadcast spectrum.

Dubbed the AM Digital Data Service (ADDS), this new technology supports delivery of low data-rate text for analog AM radio stations, which currently have no method for transmitting data such as song title and artist. “This technology could be easily integrated into future radio receivers,” said the report.

While aimed at analog broadcasters, on the product side iBiquity CEO Bob Struble tells RBR-TVBR that right now iBiquity and the NAB see this as a sub-function in HD Radio products, not analog receivers.  

We asked Struble, How much less interference would it cause from these subcarriers than a full AM IBOC signal? “The AM data service concept has only 6 subcarriers – all located under the analog AM signal.  The system is designed to fit within the +/- 10kHz bandwidth.  The subcarriers are low in power level and have been set to minimize any self interference with the analog host. The AM Digital Data Service report is a preliminary study and analysis that was submitted to the NAB for evaluation.  Much more work is required to finalize the definition, test, and commercialize.”

The digital signal is modulated using Orthogonal Frequency Division Multiplexing (OFDM). A maximum of three OFDM subcarrier pairs are defined to provide data transport capability of up to a maximum of 1098 bits per second. The proposed OFDM subcarriers would be transmitted under the analog audio broadcast at power levels to minimize any self-interference.

One of the benefits of this service is that it would offer AM broadcasters a simpler and less-expensive way to provide a limited digital service compared to a full AM IBOC installation which at present is the only option.

“With the deployment of HD Radio broadcasting in the United States, a digital data service can be created efficiently for analog AM broadcasters. A well-designed analog AM data service can provide limited text message capability to convey information related to the current program, station information, or local issues. These services are currently part of the HD Radio system for stations that have full digital capability. A much simplified digital system can be adopted at potentially lower costs. Such a service would provide analog AM broadcasters with a step toward a full digital broadcast,” said the report.

This report describes methods for digital signaling within an AM analog broadcast signal and options/tradeoffs for application layer implementation. The service would support existing analog audio programming. Although other AM digital signaling methods have been proposed world-wide, the method the report describes is intended to be at least partially compatible with an HD Radio signal, using a subset of the modulation techniques already designed for AM HD Radio transmission equipment and receivers.

The advantage is that this signaling can leverage the reuse of existing modulation and demodulation techniques in HD Radio receivers, which should facilitate adoption of the signaling method. This data service would provide broadcasters with an intermediate step toward a full digital IBOC implementation.

Analog AM radio stations in the US currently have no means of transmitting limited text and data services to receivers.

The IBOC modulation definitions used in HD Radio broadcasting have been proven as robust digital data carriers, said the report. “These same modulation techniques may be applied on a limited scale over a few data subcarriers to enable a robust, low-rate data service. With a digital messaging capability, analog AM stations would be able to transmit information (title, artist, program messaging, and other information) related to the analog audio program.”

Proposed ADDS use cases include station service messages, alert messages, and program service messages. These system definitions are identical to the corresponding definitions in the HD Radio system standard.

RBR-TVBR observation: With all of the AM HD Radio stations shutting down their digital carriers over the last few years, this seems like a good way to employ some of the digital sideband technology iBiquity developed and improve the AM broadcasters’ offering in future receivers. This service will also cause much less interference on the AM band.