The three-element Yagi mounted at a height of 4.4 meters on the mast. The parabolic dish provided a reliable one-kilometer IP link over Wi-Fi (100mW ERP). Credit: Ofcom.
LONDON — Ofcom, the broadcast regulator in the United Kingdom has found that low-power local DAB can be inexpensive and easy to transmit through the use of off-the-shelf technology and open-source software. This potentially opens the way for a new tier of community and specialist small-scale digital radio stations.
Rashid Mustapha, an engineer from Ofcom’s Spectrum Planning Group, broadcast an experimental DAB multiplex for four months on a test and development license to the city of Brighton, on the south coast of England. He put together the equipment in his spare time and funded the experiment himself.
REDUCED COSTS
Ofcom reports that the successful broadcast of seagull bird calls “demonstrated that a standalone software-defined radio approach to DAB multiplexing and transmission can deliver high availability and high-quality results at costs that are near to parity with an FM transmitter system carrying a single service.”
The experiment ran on what is described as an “old dual-core 2 GHz Pentium desktop PC,” with later testing showing that it would also have worked on a £30 (US$45) Raspberry Pi educational computer. The PC received audio data via an IP stream, and converted it into MPEG-2 format. This was then fed into the open-source CRC-DABMUX DAB/DMB multiplexer, developed by the Canadian government-funded Communications Research Centre.
The transmission site was a residential tower block in the center of Brighton already used by a local FM station, with a tank room beneath the roof housing the DAB equipment. The transmitter consisted initially of a laptop running a customized Linux operating system; this received the ETI (Ensemble Transport Interface) feed via a Wi-Fi link and then produced the modulated DAB signal.
Later, both of these PCs were replaced by a single “server grade” computer, which combined all of the network elements — encoding, multiplexing and modulation — in just 1U depth of the rack. In the fully integrated test, the ensemble services were acquired via a public Internet connection. The Ettus Research B100 transceiver card fed the custom 0.5-watt driver, which in turn drove a final power amplifier. A three-element Yagi aerial pointing north was installed on the rooftop mast.
Mobile measurement route — reception was available at all logged points. The colors denote the field strength, adjusted to the likely indoor field strength. Green=58dBμV/m, Orange=52dBμV/m, Red=42dBμV/m. Credit: Ofcom. The results of the Ofcom test highlighted what it calls the importance of “site over might.” It says that low-power transmitters sited in urban population centers 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 center. Coverage was broadly as predicted, which Ofcom suggests means that these standalone transmitters can be planned in a similar way to current DAB transmission.
It was 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 project also expected Adjacent Channel Interference (ACI) to other DAB services, but none occurred, and it was therefore concluded 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.
POSSIBILITIES
Ofcom calculated that the costs for operating an entire multiplex are comparable to those of small-scale FM transmission. While existing wide-area DAB coverage has proved out of reach to most small stations, this method could open the way to small commercial, community and specialist stations operating DAB multiplexes, and even using them as a source of revenue from carriage of other services.
The new system could also help with the rollout of existing DAB networks to serve more remote population areas, where current transmission methods are not cost-effective.
Ofcom Logo However it is likely to be some time before any permanent services of this type go on-air in the U.K. Ofcom is reported to be considering a consultation on the issue, with the regulator noting that “significant further work” will be required to identify suitable spectrum, and to establish how any new multiplexes would be licensed.
The research comes at a time when new audience figures from RAJAR show that in the U.K., listening to digital platforms now represents 36.8 percent of all radio listening, up 16 percent year-on-year. Within that, DAB is now 24 percent of all radio listening, up 18 percent year-on-year, and listening to online/apps is now 6 percent (up 31 percent year-on-year), overtaking DTV listening. Analog listening in home fell below 50 percent in the U.K. for the first time; and is also now below 50 percent among 15 to 24 year olds.
Meanwhile, the U.K. government has released a long-awaited statement on its Communications Review. It has confirmed a commitment to making a decision in-principle on a radio switchover at the end of this year. It also set out that a positive decision on switchover would open the way for the licensing of a second national commercial DAB network, known as the D2 multiplex, to increase national DAB capacity.
Will Jackson reports on the industry for Radio World from Brighton, England.
