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Digital Radio
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| DRM: Ready for Global Deployment |
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by Daniel G.P. Mansergh,
7.18.2007
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Daniel G.P. Mansergh is director of engineering of KQED Public Radio in San Francisco.
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In the world of digital radio, it’s a long road from working system to market penetration, as developers of digital radio technologies have discovered to their frustration over the past two decades.
Now, with three major systems in play in the global broadcasting marketplace, the differences in their design philosophies and development histories are leading them down distinctly different deployment paths.
HD Radio can be seen as a technological response to the American broadcasting business model constrained by limited spectrum. Eureka-147 grew from the soil of European national broadcasters in the luxury of new band allocations.
Digital Radio Mondiale’s global approach and multi-band support have forced it down a more pragmatic path, seeking wide regulatory authority and regional adoption.
The consortium of broadcasters and technology companies known as DRM is, as indicated by its hybrid Anglophone/Francophone name, a study in compromise and flexibility. True to this birthright, it has continued to change based on the needs of partners, broadcast users and market realities.
At the NAB convention in April, Don Messer, chairman of the DRM Technical Committee, presented an update on the progress of the consortium’s activities over the past year.
After early deployment of technology on a number of shortwave stations operated by consortium members, Messer presented DRM as a system now poised to make inroads into the most widely-used AM and VHF broadcasting bands in countries around the globe.
According to Messer, this represents the natural evolution of DRM, which was designed to be flexible in accommodating a range of frequencies, allotment schemes, transmission systems and programming types. “The DRM market is worldwide, by design, so it must be a versatile system,” he said.
Other countries deploy DRM
After initial rounds of proof-of-concept and large-scale coverage testing, the DRM consortium has more recently been devoted to fully characterizing and demonstrating the capabilities of the system to broadcasters considering deploying DRM in their countries.
Of great concern to many broadcasters is identifying a migration path from analog to digital broadcasting in heavily-populated existing bands, since new channel allocations or unused blocks of spectrum may not be available.
To address this issue and investigate the suitability of DRM in constrained signal environments, DRM conducted a round of field testing in Mexico City in 2006 to assess the performance and compatibility of DRM analog/digital simulcast operation in the AM band.
For the trials, DRM used the transmitter site of Radio Educación, a public station run by the Mexican Education Department located about nine miles outside the urban center of Mexico City.
Support for analog/digital simulcast with the DRM system is achieved through in-band, adjacent-channel, or IBAC, operation; in this specific case, the regular programming of Radio Educación was transmitted on its analog frequency on 1060 kHz at 50 kW, while a digital signal was broadcast on 1070 kHz at 1.25 kW, resulting in a total occupied bandwidth of 20 kHz.
These power levels represent the preferred design ratio of 16 dB for simulcast operation with the DRM system, according to Messer.
Because the performance and coverage of standalone DRM transmission on medium-wave frequencies have already been well-characterized, Messer said, the focus of the Mexico City tests was on simulcast reception and interference potential to in-band analog stations.
Tests were conducted on seven mobile routes and 36 fixed locations throughout Mexico City, representing a cross-section of reception environments typical of urban, industrial and residential areas of the city.
Comprehensive signal strength, signal-to-noise ratio and audio quality measurements were captured for a “typical” DRM simulcast operation throughout the three-week test period, Messer said, while several additional scenarios were tested separately.
These included comparison of a variety of more- and less-robust transmission coding parameters, four-channel multicast on the digital signal and 9 kHz operation of the digital channel. Additional tests studied performance of DRM reception quality as compared to AM quality in high-noise environments
For analog compatibility testing, four generally available consumer receivers were used by “expert listeners” to evaluate the analog service of Radio Educación subjectively at all of the measurement locations and in several indoor locations. In addition, spot measurements were conducted to assess potential interference to an adjacent-channel station on 1080 kHz, located north of Mexico City.
Messer reported that there was “no detected interference between analog and digital” either to the host analog station or to the adjacent-channel station.
Standard transmitter/antenna designs
Based on analysis of the collected measurement data and consistent with the results of previous field tests, Messer found that “DRM ‘FM-like’ reception works perfectly if the signal-to-noise ratio is at least 17 dB,” and that “DRM coverage is roughly equivalent to analog.”
DRM also caused no interference to the analog host during simulcast operation as long as the digital power was maintained at a level at least 13 dB below the analog signal, according to Messer.
From an implementation standpoint, Messer pointed out that although the expanded 20 kHz bandwidth requirement of DRM simulcast may not be achievable with more complicated existing directional AM transmission systems, it “can easily be accommodated with standard transmitter and antenna designs.”
Messer also discussed the status of the effort to seek regulatory authorization for use of the DRM system on FM frequencies. He explained that the current draft specification proposes to add a new VHF Mode E to the existing sub-30 MHz DRM Modes A through D, with a basic 100 kHz bandwidth for the digital signal.
Standalone DRM operation or an adjacent-channel simulcast (dubbed DRM Plus) would be allowable, with higher power levels available in DRM-only applications due to the reduced interference potential. To combat multipath and other propagation challenges in the VHF band, lower coding rates would be needed to introduce more robust error correction, Messer said, although even with this accommodation, he indicated that VHF DRM could deliver a bit rate of around 100 kbps with a typical set of transmission parameters.
In concluding, Messer characterized DRM as “a high-quality, high-reliability system on the AM and VHF bands, with no new spectrum required, and a large capacity for additional programs.” He also touted the system’s flexibility, noting that the specific use depends on the requirements of broadcasters in each country, driven by the needs of listeners in the region served.
Ultimately, though, Messer wanted broadcasters to know that DRM is ready for prime time, given the system’s use by several shortwave broadcasters and the ability to implement the system in several frequency bands.
“DRM is not an emerging technology,” he declared. “It’s already deployed.”
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