Direct satellite sound broadcast

Broadcast Satellite Service (Sound), also known as Direct Broadcast Service Radio, or Digital Audio Broadcast (or DAB its digital version), is a service conceived for broadcasting sound directly to mobiles, outdoor and indoor portable sets and receivers using fixed outdoor antennas. DAB service applies to terrestrial or satellite only, or to hybrid systems comprising a synergistic combination of both media. Satellite transmissions cover a large geographical expanse, while terrestrial services cover areas where satellite services are unreliable, such as in dense urban environments. Through some pioneering work in Europe, the USA and by the CCIR, sufficient interest was aroused in the service by the late 1980s, leading to spectrum allocation in WARC 92. A frequency range between 500 MHz and 3 GHz was considered the most appropriate for the service due to spacecraft EIRP constraints, the relatively benign propagation environment and the developed technology status of these bands. Below 500 MHz, spacecraft antenna size becomes prohibitive, and above 3 GHz the satellite EIRP per channel gets too high for the desired flux density on the ground. The spectrum awarded to DBS (sound) in WARC 92 is shown in Table 10.4 (see the Radio Regulations for the most recent situation and full details). Some nations have applied certain restrictions in the use of this band until 2007.

From a technical standpoint, there are a number of similarities in direct sound broadcasts and MSS technologies. Their frequency bands are in close proximity, which allows similar RF receiver technologies; their operational environment and propagation characteristics are similar; and finally, there are similarities in user expectations in terms of service provision and receiver design.

Recent developments in digital systems in general, and computing/multimedia in particular, have opened interesting opportunities in the broadcast field. For example, it is possible to multiplex useful data with audio programmes for downloading to home PCs. Real time interactively, such as offered by direct television broadcasts, can be introduced through terrestrial feedback to the source, or feedback through low cost leased MSS channels. In view of such trends, it was concluded that future sound broadcast systems would be digital. The ITU has consequently recommended a number of generic requirements for DAB services (ITU, 1995). A DAB system is expected to:

*provide a range of receiver signal qualities up to CD quality; and stereophonic, two or multi channel sound;
*use state of the art source/channel coding, modulation and digital signal processing techniques;
*be more spectrally and power efficient than conventional analogue schemes in the presence of multipath and shadowing;
*operate with terrestrial systems synergistically through hybrid/mixed satel lite terrestrial systems and maximize commonality in dual mode receivers used in such systems to benefit from economies of scale;
*provide programme related facilities such as service identification, programme labelling, programme delivery control, copyright control, etc.;
*provide value added services such as business data, paging, graphics, etc, using a variety of data rates;
*provide coverage for national, regional or international services.

Two systems have received particular attention by the ITU in formulating recommendations. These are the European Eureka 147 system, called Digital System A, and another system called Digital System B, sponsored by the Voice of America and the Jet Propulsion Laboratory of the USA both of which are under evaluation. Eureka 147 is a European consortium comprising the CETT laboratory of France, the IRT laboratory of Germany, manufacturers, the European Broadcasting Union and several European governments. The European approach, based on COFDM (coherent orthogonal frequency division multiplexed; see Chapter 4) over a 1.S MHz band, is based on the hypothesis that frequency diversity of this order is necessary to combat frequency selective fading and multipath observed in land mobile systems; whereas the American developers intend to use techniques such as adaptive equalization to transmit within 50 kHz (monophonic FM quality) to 200 kHz (CD quality). Another approach, pursued under Project Acorn, conducted by USA Digital, intends to demonstrate simulcast FM and digital signals using COFDM on the same transmitter, with digital signals 30 dB lower than the FM signals (Messer, 1993).