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How the Solent Wireless Mini-Mux Works — And Lessons for the Future

Behind the scenes at one of the UK’s miniature DAB multiplexes

Associated Broadcast Consultants provided the above predicted coverage map for the Solent Wireless transmitter at Fort Widley.

As reported recently in DRU, the �mini-mux� in Portsmouth, England, has recently celebrated its first year of being on-air.

DAB operates as an ensemble of different radio services, known in the UK as a multiplex and operated by a multiplex owner. Most local DAB multiplexes operate over a broadcast area designed to cover one or many heritage FM broadcast areas; and typically these multiplexes are single frequency networks with multiple transmission facilities to ensure a consistent quality of reception. The multiple transmission sites, and requirement for carefully synchronised broadcasts on a SFN, come with significant costs.

A year ago, Ofcom, the UK broadcast regulator, licensed an amount of trial, �small-scale� DAB multiplexes at lower signal strengths. These were seen as opportunities for smaller broadcasters to get onto digital at lower cost. Solent Wireless applied for a licence for Portsmouth, a city on the south coast of England. The area covered has a population of 300,000.

The existing DAB multiplex covering the area is designed to cover a much larger footprint of 1.2m people, and has six transmission sites. The transmitter covering Portsmouth runs a power of 0.33 kW ERP.*�

I spoke with Ash Elford, the Digital Development Manager of Angel Radio, the company that operates the mini-mux as �Solent Wireless�. I learnt that, in contrast, their multiplex runs at 0.2kW ERP. They use an antenna with 6dB gain, which, according to Elford means they can run a lower-powered amplifier, and that they cause very little adjacent channel interference.

A significant cost of a traditional multiplex is getting the radio station�s audio to the multiplex operator in the first place. For Solent Wireless, this is a place where savings can be made. �Stations get their audio to us over the public internet,� says Elford. �It works very well. Some stations have fibre connections, but there are a few on ADSL. It holds up�, he adds.

Use of the public internet extends to the transmission site, which uses wireless internet with a QOS to ensure reliability. Indeed, at least one station on the multiplex is entirely cloud-based, using Amazon Web Services for everything, including the playout system.

While there are ten trial mini-muxes of this type currently running, Solent Wireless appear to be the most innovative, adding DAB+ services and slideshow to broadcasts. �It appears that in the real world, we�ve found quite a few DAB sets support DAB+ without explicitly saying so,� says Elford. �Running DAB+ allows us to include as many different formats as possible whilst maintaining good audio quality. It appears that those stations who have launched in DAB+ or have converted to it have good numbers of interactions from listeners.�

However, some innovations may be a little ahead of their time. �We have slideshow running on nearly all of our services, but there�s been no feedback from listeners acknowledging its existence. But our philosophy is that, as some radios support it, and we can do it, we should do it.�

The low cost of these multiplexes is an important part of the trial, though Elford highlights that low cost is �not the same as no cost. We have to pay rental on our TX site, insurance, telecoms and maintain a float of spares, and these costs have to be passed on. But despite this, the main [non-content] costs for our broadcasters are fees from Ofcom and [music collection societies] PRS and PPL.�

These mini-multiplexes appear to be well received by broadcasters and the public alike, and many new trial formats have appeared. The day after I spoke to Solent Wireless, they announced a new, all-weather radio station, Weather 24/7: possibly the most British format you could think of. But whatever the weather, with new mini-muxes pushing technical boundaries, the outlook for British DAB listeners is bright.

*An earlier version of this article originally mis-stated the power level as 90 kW ERP.�We have now corrected our error, as pointed out by reader Glyn Roylance.