digital Broadcasting

Spectrum Availability

ITU Circular Letter CCRR/20 stipulates that DRM broadcasts are compliant with the GE75 frequency plan, if the power of the digital transmissions is 7 dB under the internationally assigned power. A 750 kW AM assignment gives a parallel right to 150 kW DRM providing the national authority issues a supplementary notification to the ITU Radiocommunications Bureau, and the broadcaster may alternate freely between transmitting in 750 kW analogue and 150 kW digital.

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Introduction Guide gives a practical overview.

Encoders and Modulators

Encoding of the DRM signal can take place in the modulator, or centrally as MDI (Multiplex Distribution Interface) for distribution via UDP/IP. Programming and data are then fed to a content server, with the modulator at the transmitter site acting as only a client. In addition to commercial hardware-based products at various price levels, non-commercial PC software of high quality for MDI or direct modulation of the transmitter is available.

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Fraunhofer DRM ContentServer R5 generates an MDI signal.

Transmitters and Antennas

Digital transmitters require a resistive load with symmetrical reactive components. Due to the high sideband energy content of digital modulation, antennas on medium and particularly long waves have difficulty achieving the necessary VSWR ≤ 1,4:1 ±5 kHz. For linear transmitters this is a minor issue, whereas non-linear transmitters cannot compensate for excess return power and may interfere heavily outside the channel spectrum mask.

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Smith chart with VSWR 1,4:1 limit of DRM transmitters.

Equipment Suppliers

Modern I/Q modulated transmitters, AM or SSB amplifiers can normally be converted to transmit DRM at 15-40 percent of the rated power, whilst older transmitters with A/RFP may have too low bandwidth. Manufacturers can often modify their existing transmitters, and the modulator/content server Spark is free for DRM test transmissions with simple linear exciters, or to generate MDI and I/Q signals.

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Audio configuration and station ID of BBC Devon trial.