The AM Digital Data Service (ADDS) is a newly proposed system that would allow AM broadcasters to transmit low-rate data very much like FM broadcasters currently do with the RDS system. AM broadcasters would be able to transmit call letters and a station slogan, along with program-related data. The only way for an AM station to do that now would be by means of a complete AM IBOC installation. ADDS is a much simpler, less-expensive alternative that supports text-only transmission.

The development work was undertaken by iBiquity Digital and the NAB Fastroad (Flexible Advanced Services for Television and Radio on All Devices) initiative.

ADDS is designed to allow the transmission of text messages and data associated with the program content. These text messages could include station service messages, alert messages and program service messages. These definitions are identical to the corresponding definitions in the AM IBOC service. In particular, there are two different categories of messages: SMS (Station Message Service) and PSD (Program Service Data). SMS includes: station call letters and station message.

PSD includes, but is not limited to: content messages, commercial messages, title, artist, album and genre.

ADDS may also provide EAS messages and even interactive advertisements: According to the report, 'These services can be layered in to the defined SMS or PSD data fields. Detailed definitions for these applications will require additional study and implementations.'

Application

Figure 1. Click to enlarge.

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Now that we know what ADDS is designed to accomplish, let's take a look at how it works. Figure 1 shows the spectrum of an AM/ADDS transmission with the proposed sideband frequencies and power levels.

As you can see, the proposed system uses pairs of digitally modulated subcarriers. Pair 1 is at 181.7Hz above and below the reference carrier frequency; Pair 2 is at 363.4Hz above and below the carrier frequency; and Pair 3 is at 545.1Hz above and below the carrier frequency. Pair 1 uses BPSK modulation; Pairs 2 and 3 can make use of either QPSK or 16-QAM. It should be noted that Pairs 2 and 3 are optional, and as you can see from Figure 2, provide extra data throughput capability. According to the report, the maximum coded throughput in the proposed system is 1,098b/s.

As part of its design criteria, ADDS would be scalable to the broadcaster's needs, and therefore two different protocol and subcarrier mapping options are proposed.

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