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摘要

Like it or lump it, multi-mode highspeed data (including audio) seem set increasingly to take over our bands from HF to EHF, following in the footsteps of other telecommunications services. For broadcasting, there are already DAB (Digital Audio Broadcasting) multiplexes and DRM (Digital Radio Mondiale) presently used only on HF, although theoretically capable of being used on all AM broadcast frequencies up to 30MHz. Both DAB and DRM can also transmit other forms of data. Fig 1 shows a block outline of a typical DAB receiver DRM, it is claimed, overcomes the problems on HF of multi-path, Doppler and most forms of electrical interference, although severe fading may still cause more problems than the less dramatic increase in noise on analogue transmissions. On DRM, appreciable amounts of redundancy provide error correction to increase robustness, but there remains a critical signal level below which the audio drops out. However, transmitter power needs be only about a quarter of that required for equivalent AM coverage. Several coding schemes can be used to offer optimum quality at different rates and with different programme content. These include subsets of the MPEG-4 AAC, CELP and HVCX codecs, with optional spectral band replication.