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Once a year the BBC analogue transmitter network undergoes a very special test that checks that BBC One and Two will still reach all corners of the nation, even if there is a large-scale disaster to the network, such as a national power failure.
As the analogue network is now partly disabled (on Border Scotland), it may be the final end of for this particular 'cold-war' system test with the test on 12 January 2009 on BBC One 3am-4am and BBC Two 4.30am to 5.30am?
Each main transmitter in the country is fed with digital data feeds that provide the analogue TV broadcast. This provides the picture, mono and NICAM sound plus the teletext to be broadcast from the transmitter in perfect quality (well high quality 16-bit sound and motion-JPEG images).
Each main transmitter is fed with the same data using 'diverse routes' where all aspects of the data links are provided independently. They have different physical routes around the country, take different physical routes into the transmitter transmission suite, independent power and so forth. On-site equipment can change the sustaining feed if one becomes damaged within a single TV frame, 1/25th of a second.
Under normal circumstances this deals with almost any kind of network failure. However, a improbable catastrophic event means that the analogue system has another trick - to take a feed from another main transmitter that is still on air.
However, the main transmitters are located far apart so they do not interfere with each other, so are not really suited to taking off-air analogue signals and rebroadcasting them, so this has to be for emergencies.
Once a year, during the middle of the night, engineers switch the BBC One and Two feeds to 'Testcard J', the 4:3 test-card for these analogue tests accompanied by a spooky speaking-clock and tone.
With the testcard playing, at each transmitter in turn the remote diagnostic system is allowed to disable (digital optical-fibre delivered) analogue feed of BBC One and BBC Two, with the on-site automation switching to the re-broadcast system (RBS).
Over the testing hour each station can then be allowed to 'fail' and automatic measure is taken of the quality of the rebroadcast feed. It is then possible to simulate the network failing in many different combinations, from a single transmitter to the whole network.
It will be interesting to see if this is the final RBS test, at least in this form. With all of Border and Granada switching off their analogue signals in the next twelve months, this will break the link for Scotland and Northern Ireland.
It would be speculative to suggest that the analogue feeds may be switched to a stand-by Freeview box at the transmitter site until switchover is completed in all regions.
RBS TEST 2010 - It's January, the bleak mid winter and what have we to look forward to this time year. Of course it's the annual RBS tests... Re-broadcast standby is the resilience test where a main station will have regular primary and secondary feeds, but RBS is the fall back and as such engineered for all else fail, rather than full standard. I'm wondering will be any at all now the network is effectively broken. The auto-mated fallback test just doesn't have quite the same ring to it does it...
It still makes alot of sense to conduct these tests, we still albeit with some jury-rigging can get an analoguesignal from one end of the country to the other. Issues are now that the RBS tests we all knew over the years are probably dead and are now a series of tests, a separate test conducted in Scotland to ensure they can opt out under these conditions. Anyone living in a dso'd region i'm afraid won't get to see testcard J anymore, likewise in Sandy Heath's service area and hence any region re-broadcast from Sandy won't get the testcard either. Since last year all bbc analogue services have moved to dsat, and would get to see normal playback.
Two scenarios then unfold - Assuming loss of BBC distribution only, would a residual BBC1 work if derived from DSAT/off-air/microwave, in the face of full SI information from other muxes?
In the face of a complete loss of all DTT distribution, could the BBC mount a residual BBC1/News by translating dSAT, re-broadcast with only dummy SI data?
I believe all the transmitters now work to the island site setup and i believe interesting times are ahead on this one... I for one would be very happy to see the return of Testcard J... Your thougths are very welcome...
Is there a date for the January 2010 RBS test yet.
There are a few more years to go before Channel 5 and digital TV starts.
I see that ITV and Channel 4 have already stoped teletext on digital.
I can still get teletext on all 4 analogue channels.