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DIGITAL SWITCHOVER

FACTS AND FIGURES

Some of the latest technical facts and figures about Digital Swichover...

CHANNEL ALLOCATIONS

After analogue switch off there will be six multiplexes (muxes), just as there are today, but they will be designated slightly differently, as follows:

  • PSB-1
  • PSB-2
  • PSB-3
(BBC A)
(ITV/C4/Ch5)
(BBC B - HD)
   
  • COM-4  
  • COM-5  
  • COM-6  

These will be the commercial multiplexes used for non-public-services.

These will roughly equate to the six current muxes, the main exception being that Ch5 and (in Wales) S4C will move into a shared PSB mux with the BBC.

It is now "anticipated" that the post-switchover transmission mode for all six multiplexes will be 64QAM modulation (2/3 rate) with an 8k FFT size.

  • All 1154 present analogue stations will carry PSB1, PSB2 and PSB3.

  • After DSO the 80 sites which currently transmit DTT/Freeview at low power are required to transmit the three commercial muxes COM1, COM2 & COM3 at high power. Their coverage is expected to reach up to around 90% of the population after higher than expected ERPs were agreed by the ITU RRC in June 2006.

  • PSB muxes will be at ERPs typically 7dB less than analogue peak sync

  • COM muxes will be at ERPs typically 10dB less than current analogue peak sync though at some stations these will operate at reduced power to protect other neighbouring regions which are still running analogue services.

  • Where possible all three PSB muxes will occupy existing analogue allocations, i.e. 'in band' for any 'pre digital' aerials (and communal distribution systems).

RE-ENGINEERING THE NETWORK

It is understood that most main TV sites will be fitted with new main transmitting aerials, feeder cables and transmitters. In addition, most main sites will also have fully redundant reserve DTT aerials.

Most visibly at Blackhill, Caldbeck and   new replacement masts will be constructed. Some other sites - including Wenvoe, Sutton Coldfield and Divis - require mechanical strengthening work. To this end, work on a temporary mast began in Autumn 2008 at Wenvoe to allow the existing mast to be modified 'cold'.

MORE ROBUST SIGNALS

In order to maintain reception protection 99% of the time, some digital transmitters might ultimately run at just 4dB below current analogue levels. (Post-DSO most services will be at -7 or 10dB below current analogue levels).A likely contender is Rowridge, it suffers interference from the continent during 'lift' conditions. To povide more robust reception two ideas are under consideration.

  • A vertical component to Rowridge's signal, the idea being that a vertically polarised receiving aerial will discriminate better against distant interfering HP transmissions.

  • Additional relays, at 'greenfield' sites, possibly working in SFNs (single-frequency networks). If the latter is adopted, then 8k working will be needed in these areas ahead of a national switch.

Other south east sites are under consideration for a similar scheme, notably Dover and Sudbury.

All muxes will be 64QAM, probably at 8k. 8k working is necessary for SFN (single frequency networks) and this use will be brought in at DSO from 2011 in East Anglian and Southern areas where new sites will be built and SFN will be necessary to reduce the effect of interference from European transmitters.

TIMING

2-4 weeks before analogue shut down day at a given site, one of the 4 analogue transmiters will be shut down resulting in the closure of the BBC2 analogue service, which would be replaced by the high power BBC-A digital multiplex.

2-4 weeks later BBC1, ITV, C4 and if applicable, Ch5 will also shut down, along with the low power DTT muxes 2, A, B, C and D and would be replaced overnight by PSB-2, PSB-3, and COM-4, COM-5 and COM-6.

The night each main station is switched over, all of its dependant relays will also be switched.

Download the latest regional info from Ofcom (PDF)

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