All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Jack: The reasoning behind the 7 dB difference appears to be:
- For perfect reception, analogue required 43 dB of signal-to-noise ratio
- For error-free reception, digital requires about 17.3 dB of signal-to-noise ratio if line-of-sight is available (20.3 dB if not)
- Planners added 5.7 dB of 'implementation margin'
- Propogation varies over time, but 99% of this variation is within 12.8 dB of the predicted signal
Adding the required signal-to-noise, the fudge factor, and the variation, we get 35.8 dB, call it 36 dB. This is 7 dB less than the 43 dB required for PAL.
The commercial multiplexes don't have to meet the PSB multiplex requirement of 98.5% population coverage, matching predicted analogue coverage, nor do they have to make their signals reach the relay transmitters, which are nearly all fed off-air (they pick up the signal from a main transmitter and re-transmit it, usually on a different frequency). Reducing the power by half (3 dB) reduces the tolerance of variation at the fringe of the coverage area by about 5%.
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Heinz, Dale: We're trying to keep up with what's going on, but the only official documentation comes from Ofcom and they seem to be following what the broadcasters do, not leading.
There is more information - from which a lot of that commentary came - in the "Digital Terrestrial Broadcasting Stations for Multiplex Licences" documents at Ofcom | Supplementary licence documents in relation to DSO . These now form part of the licences to broadcast. However, the notes are truncated badly for version 2 and fairly badly for version 3 on Mux A/SDN.
The version 3 document confirms Mux C/Arqiva A on C54 at 3kW and Mux D/Arqiva B on C50 at 2.2kW from DSO 2. Digital UK's postcode checker (which is run by the broadcasters) confirms this, but indicates that both will change mode to 64QAM 2/3 from the current 16QAM 3/4. The new mode requires about 4 dB more signal-to-noise ratio than the old mode. A doubling of power is 3 dB, so there will be a small reduction in the covered area, but not as much as if the power levels were left unchanged.
At other sites Arqiva have swapped their multiplexes A and B around, so that Arqiva B (which carries Sky Sports) gets the better channel/the one which can get a power increase earlier. Before switchover Sky Sports 1 and 2 are carried by the BBC on their second multiplex; if ArqB doesn't go to 64QAM 2/3 at DSO 2, these channels have no home because BBC B is changed to DVB-T2 256QAM 2/3 for HD channels.
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Heinz: Digital UK's predictor shows that you are well within the covered area, a prediction of 99-100% across the board, and I would say that any problems that occur at switchover are likely to be down to too much signal. This is particularly the case at Sudbury, where the peak power reduction from analogue to digital is only 4 dB, whereas at most sites it is 7 dB.
The average digital power is actually higher than analogue, because the peak-to-average ratio is much smaller for digital than for analogue. The analogue transmissions could also be, and are/were, synchronized so that the channels were not all transmitting at peak power at the same time, whereas digital transmissions cannot be synchronized in this way. You can get an idea of how digital signals combine from BBC White Paper 156, DVB-T and Voltage Ratings of Transmission Equipment at BBC RD - Publications - White Paper 156 DVB-T and Voltage Ratings of Transmission Equipment , and how that differs from analogue in White Paper 126, titled 'Co-axial Cables' at BBC RD - Publications - White Paper 126 : Co-axial cables (see section 10 on p17).
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Alan: The problem for you is that the final COM channels from Durris clash with the PSB channels from Knock More. (The underlying cause is probably the governments' decision to release channels 31 to 40, and 60 to 68, meaning less space for digital TV transmissions.)
Knock More is closer to you - 23km vs 55km - but Durris is more powerful, 100kW vs 20kW. Distance has more effect than the transmitter's power - theoretical calculations show that you should receive slightly higher signal levels from Knock More than from Durris. It does also depend on the terrain - there are hills blocking line of sight to both transmitters, but I would say the situation with Durris is worse than Knock More.
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Will: If you already have a wideband aerial, you don't need to change it. However, you're right that a Group A aerial should provide more gain, for a given size of aerial.
There's a chance that any new interleaved services could start up outside Group A. The report produced for Ofcom's review on the subject constrained the channel selections to those within or just outside the traditional analogue group, including Channel 5. You needed a wideband aerial for analogue C5, so they could have selected any channel between 21 and 60, though in fact the channel producing the best coverage from Durris was C30.
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Nick: When you say the picture is smaller, how does it compare? Does it have black bars at top and bottom? At left and right?
It could simply be that the modulated output only carries 4:3 picture information and not full widescreen, and also possibly that the channel is broadcast in the compromise 14:9 ratio. See More than just a pretty face... , which mentions (scroll down to 'Wot no AFDs?') that Sky digiboxes do not send the Active Format Descriptor to the set.
SCART can signal that a picture is widescreen using the status/aspect ratio pin 8.
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Wallace Douglas: If you have picture, but no sound, you probably have the TV set to the wrong mode. You need to make sure it is set to PAL I, System I or UK, not B/G or any other option that might be available.
I'm assuming that the satellite receiver is an official Sky box and not some other free-to-air receiver. If it isn't a Sky box, check that the receiver is also set to PAL I or UK. If it's a plug-in modulator, note that some sold by satcure.co.uk came pre-set for PAL B/G and need to be adjusted for PAL I.
TV transmissions have differed somewhat around the world, with the sound carriers at different locations relative to the video information, and sometimes different ways of encoding the sound.
If you're interested, there's a list of how the picture and sound were transmitted at World Analogue Television Standards and Waveforms .
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M
Sandy Heath (Central Bedfordshire, England) Full Freeview transMonday 20 June 2011 2:06PM
Dunstable
Adam: UHF reception can vary greatly between locations only a small distance apart, so we really need a full postcode to see the situation for you.
Taking a postcode for the centre of Dunstable (Bing Maps gives a co-ordinate of 51.8858052045107,-0.52066370844841 which nearby.org.uk translates to LU6 3SH), I get a prediction of 100% on the PSB multiplexes, 73% on Mux A, 76% on Mux C and 92% on Arqiva B. This is the estimated probability of finding a location for a rooftop aerial that would give reliable reception, at that postcode. The simplified coverage checker for consumers will show that as 'good' on all six multiplexes (the threshold is 70%).
I would start by bypassing the booster and splitter and seeing what the situation is like on each TV connected individually to the aerial. If you can get all the channels, plug the splitter in without the booster and try again. If you find that you can't get all channels with the booster, but can without, leave it disconnected. Otherwise, check that the booster has just enough gain to offset the loss through the splitter. A 4-way splitter typically drops 8 dB.
It may all just fix itself as the switchover programme completes. The retune in August changes the prediction for Mux A to 97%, while that in mid-September puts Mux C to 93% and Arqiva B to 99%. The November retune and mode change turns Mux C into Arqiva A, for which the new prediction is 99%.
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Doreen: Can you provide a full postcode, please? Reception depends very heavily on your exact location and can change quite a lot with just small changes in distance.
Does your box still show an entry in the Guide for ITV3 on channel 10? If you enter '10' on the remote, does it show NO SIGNAL, or just not change channel? If the channel is entirely missing, try retuning - some boxes retune automatically, and if the multiplex carrying ITV3 was off-air or at reduced power when this happened, it may have deleted the channel.
There is some natural variation in signal levels due to how the signal travels through the air, and over terrain and around buildings, which sometimes can cause drop-out due to either too low or too high signal levels, or higher levels of interference from other transmitters using the same frequencies.
Changes in reception can also indicate the start of a problem with your aerial system or cabling. Check that analogue BBC One is clear, or at least as clear as you remember. If not, get the system checked, although note that there is currently engineering work going on at Sutton Coldfield which may cause periods of low power and interruptions. We don't unfortunately, know how long this will go on for, as Digital UK only publish notice of works a week at a time.
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Friday 17 June 2011 1:47PM
Jack: Since 2007 the transmitters have all been owned and operated by Arqiva. Before then, about half were owned by National Grid Wireless and half by Arqiva. The Arqiva half were originally set up by the Independent Broadcasting Authority and the NGW half by the BBC. Both were privatized, the IBA in 1990 and the BBC transmitter network in 1997.
Craigkelly was originally an IBA transmitter.
Permitted power levels and radiation patterns are regulated by Ofcom, but generally the broadcasters have been allowed to set whatever levels they feel are necessary. Most sites have had a reduction of 7 dB (one fifth) from their analogue signal levels, for the PSB muxes, with the COM muxes 3 dB (one half) lower than that. Some sites that previously had directional aerials to avoid interfering with other regions or other countries now have better directional properties on their new aerials, and are allowed relatively higher signal levels. For example, Rowridge only has a 4 dB (60%) cut, while Dover has a 1 dB cut (20%).
Craigkelly *does* transmit the full range of channels. If you can't receive some, that is either down to your location, or to your system setup. Digital UK's postcode checker shows a prediction of 100% probability across the board, which indicates that signal levels are likely to be high. Too much signal can also cause problems if the signals are distorted by any amplifiers in the system, including the amplifier in the TV, set-top box or PVR.
If you have an amplifier or booster, you should remove it. If not, or that doesn't help, add an attenuator to reduce signal levels, to avoid overloading the TV or box's input.
You are also expected to have a very good chance of reliable reception from Black Hill (after next week's final step there) and from Angus, while predicted levels from Durris are also high, so generally the received levels could be very high. It's worth checking that the box has tuned into the frequencies from Craigkelly and not weaker off-beam signals from one of the other transmitters. Some boxes store the first version that they find, even if it's poor, which could have happened with some of the muxes from Durris, as they're on lower channels.