Report on a possible low-cost DAB platform for small stations
An experimental DAB multiplex was set up in central Brighton between 14th September 2012 and 27th January 2013 using new lower-cost technologies. The experiment was privately-funded, and the research conducted in the author's spare time. As no similar study had previously been carried out, the test was a valuable exercise to inform policy makers of the practicalities of low cost DAB solutions when used to serve small areas, particularly from a single transmitter. The transmissions were operated under a Non-Operational Test and Development licence issued by Ofcom.
As well as testing the viability and reliability of new wireless techniques for generating a Eureka 147 DAB compliant signal in a real world scenario and it also allowed various measurements, and comparisons with existing multiplexes to be made. The experiment successfully demonstrated that much of the infrastructure can now easily be implemented in software, and that integration with public IP networks improves accessibility while reducing the capital and operating costs quite dramatically. It was also demonstrated that a trade-off in power amplifier selection could remove the need for large and costly cavity filters to meet ITU spectral masks. The trade-off is a reduction in power efficiency, but the cost of this is negligible at low power levels. Some thought was also applied as to how a small-scale multiplex could fit in with the current and future radio broadcasting spectrum landscapes. It was also shown that a low-cost, low power approach could deliver a reliable, high quality service at minimal opportunity cost by using interleaved spectrum, which is unsuitable for use by larger networks.
The experiment also highlighted the importance of 'site over might'. Low power transmitters sited in urban population centres can often deliver the field strengths required for reliable indoor reception much more effectively than might be achieved with a higher-powered site on the periphery of the population centre.
Adjacent Channel Interference (ACI) to other DAB services was anticipated and thoroughly checked for, but none was found. It seems that ACI 'holes' are not created by low power DAB transmitters sited in an area where the wanted signal levels from other multiplex services transmitted from elsewhere are sufficiently high.
This work demonstrated that it is feasible to deliver DAB transmission infrastructure at much lower cost than currently required for equipment to deliver wide-area coverage. Nevertheless, significant further work is required to identify suitable spectrum for services making use of these technologies. In addition, it will be necessary to consider how they might be licensed to cover particular areas, especially in circumstances where there is a requirement to carry more than one service on the multiplex. When these issues have been resolved, these new techniques could find particular application for Community Radio or smaller scale commercial radio stations. These techniques might also assist in rolling out existing networks to serve more remote population areas where existing approaches might not prove to be cost-effective.
This report is intentionally light on engineering details (although it does use many technical terms) as the goal is to provide an outline of new concepts that can be understood by an audience with varying depths of understanding in technical matters. Although a wide range of different products and tools is given, their inclusion as a reference in this document should not be taken as an endorsement of these products and tools.