All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Simon Meyers: Briantist's software doesn't really handle the many stages of switchover at some transmitters. These maps only provide 'before' and 'after' views, with the state as at 2007 for the 'before' and the final state at the end of 2013 (as far as it is currently known) for 'after'.
The only exceptions for the 'before' picture I'm aware of are the major changes that occurred at Rowridge, to allow the south-west switchover to start, and at Sheffield, to get out of the way of the HD service from Emley Moor.
For 'after', Brian has incorporated some of the known interim states (particularly Mux D/Arqiva B) but not all of them. This can give a confused picture.
In addition, The Wrekin has always been listed as two sites, with the second set of frequencies (using different aerials, covering a different area to the first set) shown as The Wrekin B. There are a couple of other 'B' transmitters in the system, e.g. Winter Hill B and Dover B. These are again different aerials on the same mast (Winter Hill B shut down when Winter Hill switched over at the end of 2009, but Dover B is still going).
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Russell Wolak: I assume you're asking whether the HD services will break up. Digital UK's predictor reckons the chance of reliable service at present is about the same as for the BBC's SD multiplexes.
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Anne Poultney: Your Sky+ box should act as a Freesat-from-Sky box, if you can run a separate feed from the dish to the kitchen. You will need to keep the viewing card in the box, and the recording and time-shift features will no longer work.
It needs to be an independent feed, as the box sends control signals to the dish along the cable. Each output on the LNB itself (the box at the front of the dish that the cables connect to) can be controlled independently. Splitting a cable means that the LNB either only gets control signals from one box - meaning that the other box is limited to only channels matching what the first box has selected - or it gets a mixture of signals, possibly selecting the wrong result for both boxes.
Your freesat box might have an 'LNB 1 Out' connector on it, but this has the same problem of only allowing the second box to have full control if the freesat box is in standby. Otherwise you're limited, again, to watching channels that match the settings for the channel that the freesat box is set to.
If the freesat box is a recorder, you can get the same problems when it's recording two programmes when in standby.
There is a slightly different range of channels on Freesat-from-Sky compared to Freeview. See the comparison at Compare Freeview and Freesat-from-Sky TV | ukfree.tv - independent free digital TV advice .
If you provide a full postcode, we can see if Freeview is likely to provide a full service.
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Eddie: You're predicted to have a very high probability of reliable reception. If you have problems, try removing the booster.
You will need to retune this Wednesday 8 June, as the current low-power BBC multiplex will close down and the new high-power one starts up in place of analogue BBC Two. You should do a full reset (also called first time installation, default setting or full retune) to ensure that the TV forgets about the old location for these channels and stores whatever it finds.
You then need to retune again on the 22nd to complete the process.
The transmitter shuts down at just after midnight (usually when whatever programme is on finishes) and is expected to be back up by 6am. You should retune after 6am on both days to ensure that all services are running before retuning.
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NottsUK: The Symbol Rate, or baud rate, is the fundamental speed at which it changes from one 'symbol' to the next. The symbol is the combination of bits currently being conveyed: in QPSK, two bits are conveyed by each symbol. The symbol rate is the same as the bandwidth occupied by the signal, less a roll-off factor. DVB-S mandates a 0.35 roll-off factor, DVB-S2 allows 0.25 and 0.20 to cram more symbol transitions into the same space.
To avoid hitting the signal from the transponders above and below, the transmission bandwidth must be limited to about 30 MHz. Using the 0.35 roll-off at 22.0 Mbaud, you get 29.7 MHz. 23.0 Mbaud would be a roll-off of around 0.29. 27.5 would be a roll-off of only 0.08, far too low and damaging the transmissions from tp48 (various BBC services) and tp52 (a Sky-operated multiplex).
The higher symbol rates are only available in the upper band, from Eurobird 1, Astra 2A and 2B, because the transponders are spaced further apart.
There's nothing in the DVB-S or DVB-S2 specifications limiting broadcasters to using specific symbol rates. Annex C of the DVB-S spec gives 8 different rates for 8 different transponder bandwidths - none of which match any settings that have been used 'in the wild'. The User Guidelines for DVB-S2 document refers to many different symbol rates, again, not really matching anything used in the wild.
The DVB-SI (Service Information) specification's 'Satellite delivery system descriptor', which I believe forms the basis of the EPG information telling the box where to find the channels, uses 7 numeric digits to encode the rate with a resolution of 0.1 kbaud, maximum 100 Mbaud.
Panasonic really should not have restricted their equipment to only work with three fixed rates.
DVB-S does not even carry any information on how the data was encoded - the receiver has to figure that out for itself. DVB-S2 tells the receiver using binary phase shift keying (one bit per symbol, just for the information period) which error-correction mode is in use and how many bits are carried per symbol.
Briantist: DVB-S and DVB-S2 use one carrier modulated very quickly, it is not an OFDM system with lots of carriers modulated slowly, unlike DVB-T and -T2.
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Ian Gunson: Could you post this on the Emley Moor transmitter page? You won't get a useful answer on this page, which is for the Durris transmitter near Aberdeen.
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k.flood: Does your aerial point east to Crystal Palace or west to Hannington?
Hannington does not currently broadcast a significant amount of digital signal to the east. You might get enough signal for it to work sometimes, but not reliably. A professional installer should have known this and advised you accordingly. The analogue signals will be coming in at full strength, because there is no restriction, which means that if you tried to boost the signal (or the aerial has too much gain) you'd probably just overload the amplifiers.
At switchover, digital signals move to the main aerial at the top of the mast (replacing analogue), and you're then expected to have a good chance of reliable reception from Hannington.
The chances of reliable reception from Crystal Palace are excellent both now and after switchover.
After only six months, I'd get the installer back in to correct it free of charge.
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Peter Short: Normally I'd say yes, everything comes from the same site, but in this area three transmitters - The Wrekin, Lark Stoke and Bromsgrove - operate as a so-called Single Frequency Network. Or, at least, the public service multiplexes all do currently. The commercial multiplexes will do from September, but the allocated channels are currently in use at Sutton Coldfield so for now, the signals from Bromsgrove remain on pre-switchover channels and reduced power levels.
A SFN uses the same frequencies at multiple transmitters, and your TV receives a contribution from each of them. The transmission from each transmitter is carefully timed so that the signals are approximately coincident, where they would be strong enough to interfere otherwise.
The power levels at the top of the page, against the channels on each multiplex, show the final situation. Actually, Mux C is still broadcasting at 50W on C34 (inherited Mux 1's configuration) and Mux D/ArqB at 25W on C29, at least according to Ofcom.
If the PSB signal levels are on the high side, you could be suffering from intermodulation, which causes frequency-shifted copies of the signals to be added, causing errors. The transmission contains error-correcting information but there's a limited amount of error that can be corrected - once this amount is exceeded, the picture starts to break up. The copies closest to the original signal are strongest so tend to affect adjacent channels worse. The ArqB mux is adjacent to the new HD mux.
You could ask your landlord or agent to see if it's possible to adjust the levels, to ensure that they're neither too low nor too high. Still, you may have to wait for September.
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Steve: No, it does not mean you will not be able to get Freesat. You will not lose any signal - Sky have no ability to turn off the transmissions for just one viewer. You should continue to receive all free-to-air services listed on the 'Freesat-from-Sky' comparison page at Compare Freesat and Freesat-from-Sky TV | ukfree.tv - independent free digital TV advice .
You can continue with your Sky box if you want. The encryption card remains necessary for some channels - again, see the comparison page for the services that are 'free-to-view' (requiring the card) rather than 'free-to-air'.
However, Sky can and do disable recording, timeshifting, and even playback of previously-recorded programmes on Sky+ and Sky+ HD boxes.
If you want to continue with recording you can replace your Sky+ box with a Freesat+ recorder, but you will then lose the channels listed on that comparison page.
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Saturday 4 June 2011 4:09PM
Nigel: At that location, you are predicted to have variable to poor reception from Sutton Coldfield on all multiplexes. It is likely to stop working at least some of the time, with very little you can do about it. It's simply down to the variability of signal propogation from the transmitter to you, and of signals from other transmitters as well. A bit lower propogation from Sutton Coldfield to you, and a little more from others, and it could well reduce signal quality enough for some services to stop working. This variability - which broadcasters call 'fading' - is usually different on different frequencies, so some services may be failing while others are working, then swapping around at different dates.
It is predicted that you have a high likelihood of reliable service from Sutton Coldfield once it has switched over. You will need to retune on the 7th and 21st of September.