All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Clive Windy: Transmissions can travel a very long way, sometimes - under particular weather conditions - far further than planned. You can still get enough signal to detect even if it's not usable. I suspect the transmission on C52 is Emley Moor near Huddersfield.
Your mother's box must be one of those that simply stores the first version found rather than the best quality or strongest. See Digital Region Overlap for ideas on how to handle this.
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Matt: The prediction for Mux 1 at your postcode is variable - only 4% of locations within your grid square are predicted to get reliable reception. This may be because the Heathfield transmitter uses C34 for Mux 1, although that is fairly low power. Crystal Palace also uses C34 for Mux C, at the same power level as Rowridge.
It's shown as 'variable' rather than 'poor' as there's a very good chance of getting reception 50% of the time. It will depend on weather conditions whether it works at any given time.
Switchover at Rowridge starts on 7 March 2012. Until then, there really isn't anything much you can do. A slight adjustment in the aerial's position, moving it slightly clockwise or anti-clockwise, might change the level of interference. It might have been aimed a bit north of Rowridge to improve reception of analogue Channel 5 from Fawley (which was turned off two years ago to facilitate Rowridge's retune, in turn to free up channels for the south west region's switchover).
The commercial multiplexes will remain on low power on their current channels from 21 March to 18 April 2012, as the final channel allocations clash with low-power services at Crystal Palace until that date. You will need to retune again on 18 April. If, after the retune, you find that the commercial multiplexes are unreliable, it may be worth re-orienting the aerial for vertical polarization.
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billy: The symptoms you see are indeed the same as too little signal. The distorted signal pulls the 'value' of each carrier away from the value it should have, so it's interpreted incorrectly. Some carriers will not be disturbed enough to be misinterpreted, so error correction may be able to determine what most of the missing bits are, and reconstruct a block for the video and audio decoders to display it or play it back. Break-up happens when a block is detected to be uncorrectable or if the decoder runs out of valid data to play back (data is buffered, stored temporarily in memory; problems occur when the buffer runs empty). If the error rate is too severe, then the data which tells the receiver what channels are contained in this multiplex can't be decoded, and the receiver won't store it. In the most severe cases, the special Transmission Parameter Signalling carriers - which use a very robust scheme, transmitted redundantly, to indicate how the rest of the carriers are encoded - can't be decoded and the box won't even know the transmission is there.
There's really no way to tell whether it's too much signal without just trying to reduce the level and see what happens. Try disconnecting the aerial cable. If it works, you have grossly too much signal. If not, try an unamplified indoor aerial. Again, if it starts to work, too much signal. If you have a booster, take that out and see what happens. Amplified splitter in the loft or masthead amp? Bypass it, check the result on one TV.
Even if you have a signal strength meter, it may not be reliable. Each box seems to be calibrated differently. I strongly suspect that some have been calibrated to maximum possible input on one channel, but the distortion is caused by the SUM of all signals being too high, so you have to stay substantially below the maximum limit for 'safe' operation. I've seen reports that some boxes will report low signal levels when the levels are in fact high - my assumption there being that the box responds to not being able to decode by turning up the automatic gain control, and it's actually reporting the gain control level as the signal strength.
I usually look at Digital UK's postcode checker and make a guess. If they're shown as being within a short distance of a main transmitter and/or the prediction is 99-100% across the board, I usually assume too much signal :)
There are actually very few places in the country where you can have too little signal. Generally the problem is too much interference - the frequencies are very heavily reused across the country. It doesn't matter how much you amplify the signal, you amplify the interference just as much, and you add some extra noise from the amplifier itself. Amplification is only useful to offset losses along a cable, or occasionally if you have a very noisy receiver - see Usage and abusage of DTT TV booster amplifiers for how to use boosters correctly.
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lisa: It's very hard to say that you're 'on' Sudbury or Rouncefall, both have quite a large area of influence and the overlap area is large. If you do get ArqB on C50 now - the channels that Dave listed - you will lose them on Wednesday and will have to retune to get them back. If you don't get them now, you *might* get them by retuning on Wednesday. However, it remains on lower power, so you're still not predicted to get them (if you're still at the address you were before).
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lisa: As for *why* it's moving frequency: it's to give up C50 for Tacolneston to use from next week. The planned final home for this multiplex is C56, but this can't be used yet because Dover is still using it. There is another retune on 27 June 2012, the same day as stage 2 of Dover's switchover, when Sudbury's commercial multiplexes will all move to their final channels and power levels.
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Monty: At that postcode, the predicted coverage of the ArqA multiplex (which carries Challenge) from Belmont is good, though not quite as good as the others.
It's worth checking that your box hasn't tuned in Emley Moor on UHF channel 52 instead. This is expected to be significantly weaker. Some boxes just store the first version of a multiplex that they find. See Digital Region Overlap for ideas on how to resolve this.
If you didn't have digital before switchover, you might still have a Group A aerial, which won't pick up C53 or C60 very well at all. A wideband aerial is recommended. If the cable installation is old, it could be letting in water which increases the losses much more at higher frequencies than lower ones.
The ArqA and ArqB multiplexes from Belmont are still on low power for another week, until Tacolneston (which currently uses these two channels) completes its switchover. The problem could just correct itself next Wednesday - although given that prediction, I would have expected very occasional breakup, not complete loss of channels.
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John Plant: Derby only provides the three PSB multiplexes. Its purpose originally was to provide the East Midlands BBC One and ITV1 services to viewers using the Sutton Coldfield transmitter.
C48 is Derby BBC A and C46 is Sutton Coldfield D3&4. C29 is Waltham SDN. That suggests your box/TV just stores the first version of the channels that it finds.
The predictions are slightly better from Waltham than from Derby, so I suggest you do a manual retune using the channel numbers on the Waltham transmitter page. See Digital Region Overlap for more ideas on how to get the services you want tuned in.
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There is also a retune at Sudbury on Wednesday morning, with ArqB moving from C50 to C63. This releases C50 for Tacolneston ArqB next Wednesday, C63 having formerly been Tacolneston Mux 1.
Sudbury is still blocked from going to final channels until Dover completes switchover in June.
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Dave Caunt: I don't think anything has particularly changed. This week I can't get Mux C (C34) where frequently I'd be able to, but I'm having no more problems than usual with Mux A. I'm in Caversham, Reading.
A lot of tropospheric enhancement has been predicted over the last week: Tropospheric Ducting Forecast for VHF & UHF Radio & TV . This weather condition allows signals to bounce along between layers in the atmosphere without losing a lot of power. It could be interference from overseas stations, or possibly from Rowridge which also uses C32.
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Thursday 10 November 2011 2:22PM
Alex: As I understand it, manufacturers can put their updates into a queue managed by the Digital TV Group's Testing group - Book an over-air download | DTG Testing .
I would expect the protocol to be DVB-SSU - DVB standards can be found at DVB - Digital Video Broadcasting - Standards & BlueBooks .