David Austerberry /
04.05.2012 09:36 AM
London readies for analog switchoff

London’s television transmissions are about to undergo their final frequency allocations in the long-term project to move from analog to digital transmission.

The analog switchoff will take place April 18, 2012, with the tower being specially floodlit for the event. The lightshow will use 200kW of lighting (7.5 billion candlepower) weighing in at 6 tons to light up the London skies at night. The Arqiva transmitter site at Crystal Palace covers the London area, serving a population of 12 million. Erected in 1956 to provide a single analog VHF service, the tower will radiate six UHF channels after the switchover.

The six multiplexes carry four HD channels (DVB-T2), around 50 SD DVB-T channels under the Freeview banner and digital radio channels. UK viewers looking for more choice of HD programming can opt for the Sky satellite service (up to 60 channels) or Virgin Media cable with 23 channels.

The United Kingdom has had a protracted digital switchover for a number of reasons. The incumbent broadcasters sold off their transmission operations as separate companies, now Arqiva. The broadcasters now contract transmission from Arqiva. This much simplifies transmission, as a single tower serves all channels for a given locale.

Digital broadcasting (DVB-T) started around 10 years ago with a low-power service called Freeview. The service uses a single EPG for viewers to access all of the broadcasters’ channels on the multiplexes.

Being a densely populated country, and operating the high-power analog and low-power digital channels in parallel, has meant the switchover to a single high-power digital service needed to be carefully managed to avoid co-channel interference.

There has been inevitable retuning to reach the final frequency channel allocations. The 1200-man-year project started in 2008 and will be completed in October 2012, and it has required an investment by Arqiva of $1 billion. The company operates 1154 transmission sites across the UK, each of which now has new transmitters and antennas. Many of the towers had to be strengthened to take higher wind loadings from the antenna panels, and five new towers were erected.

At the Crystal Palace site, 30kW Thomson IOT transmitters provide an ERP of 200kW per multiplex channel, totaling 1.2MW ERP. This equals the power used for the analog service. The old low-power digital service had a 20kW ERP. The new high-power service will extend digital coverage to areas that had relied on analog or satellite.

Once completed, the new digital service will cover about 98 percent of the UK’s population. The countrywide project was complex, as existing services had to be maintained during the installation of new antennas and feeders.

(See the article, “RF Safety,” by Julia Clark in January 2012 Broadcast Engineering, world edition, that details how this was undertaken.) Another issue was that 2007, ‘08 and ‘09 were among the wettest summers on record, with many critical activities being weather dependent.

The final digital services now use Channels 21 to 31 and Channels 38 to 60. Once the project is completed, 16 UHF channels will have been freed, potentially for mobile data applications. This includes the 600MHz and 800MHz bands. There are also suggestions to free the 700MHz band, Channels 49 to 60, after 2018.

As recent purchasers of the Apple iPad have found, the UK has no LTE or 4G services. This is a great concern to many potential users who look enviously at other parts of the world.

The government regulator (OFCOM) is finally getting around to consultations about the freed spectrum, and it is hoped by many that spectrum auctions will follow. In the meantime, mobile users have to put up with the limited capabilities of a patchy 3G service. It is ironic, that in the rush to be first in Europe with digital TV, the UK is going to have to wait longer for 4G.



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