All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
G.Hodson: Yes, it's quite likely you will be able to. Depending on the software on your box, when you retune, it might ask you which set of channels to store, it might store the strongest (presumably Waltham if your aerial points that way, although Belmont is more powerful and closer), or it might store the first version that it encounters. The second set might be stored somewhere else in the EPG.
If it stores the first version, you would probably get BBC A, D3&4 and Arqiva A from Belmont, and SDN and Arqiva B from Waltham, as the Belmont frequencies are generally lower than the Waltham ones.
See Digital Region Overlap for thoughts on how to get the services you want.
Note that Belmont Arqiva A and B don't reach full power until November, as they clash with services from Tacolneston. However, SDN, Arqiva A and Arqiva B have the same content at every English transmitter. Richard McCulloch: I KEEP telling Brian to remove those.
These represent tentative future allocations. They might be used for Jeremy Hunt's pet Local TV project. Or they might not. They have not yet been auctioned and there is no date set for an auction.
The frequencies chosen are those with the widest coverage area that fall into the traditional analogue aerial group, or just outside it. (Craigkelly is counted as Group K/Wideband because Channel 5 was up at C48, although many viewers wouldn't have changed their aerial for this.) A local TV service for a specific city or town might get a different allocation covering only that area, allowing the NEW7 and NEW8 frequencies to be used for something else.
You are not missing anything, you won't expect to find anything else if you rescan, and they might never be used for TV at all as there are plans to allow broadband providers to use these 'white spaces'.
The planning may have to be completely redone as these 'assignments' were based on old versions of the plan, before international agreement on the frequencies for Ulster, Tyne Tees and the south and east of England was reached, and predating the Europe-wide agreement to release C61 and C62 for 800MHz mobile phones/broadband. 
sugapriya: All providers are supposed to provide some way of receiving the Public Service channels without a subscription. However, I can't find any details from Virgin Media on how to do this.
The contract with Virgin Media states that the box is theirs and should be returned if you stop subscribing. However, you often find that they don't collect it, so you often see boxes on eBay. I don't know if it's possible to watch free channels just by plugging a box in.
You're certain that it's a Virgin Media cable connection, not a satellite dish connection? These use the same 'F' connectors, which screw into the box. Aerial cables are usually plain push-in 'Belling-Lee' types. Some aerial installers do use 'F' connectors for wall-plates, though, as it makes a more reliable connection.
At your location it should be easy to find a location to install a rooftop aerial that gives reliable results. It should only require a small aerial. Second best would be an aerial fitted in the loft, and worst an indoor aerial, though you might still get good results, as you're not that far from one of the largest transmitters in the country. Dale:
The switchover is intended to be complete by the end of 2012. Check 
Digital UK - When do I switch? for the dates for your transmitter, or we can check if you provide your full postcode.
I have no idea about whether all the channels will decide that they no longer need to be 4:3 safe. Some viewers are likely to be sticking with 4:3 sets converted using an external box (particularly for second TVs). It's really up to the channel. Of course when showing content that was recorded 4:3 (e.g. archive content predating widescreen) they are likely to show it 4:3 and put the logo in the 4:3 area.
The logo - usually called a DOG, for Digitally-Originated Graphic, though that's reallly a term from the analogue era - is directly encoded in the picture so it cannot be removed, or therefore moved.
I assume that the logo - which is animated on some channels - is generated by the playout equipment, and changing its location isn't that difficult to do. It probably depends on the exact equipment and how old that equipment is.
I would hope that BBC One, BBC Two and ITV1 continue to be DOG-free, though BBC One HD has gained a DOG. I can't remember right now if Channel 4 has a DOG.
HD transmissions are required to be in 16:9, so it's likely that the DOGs will move as channels change from targetting SD to targetting HD equipment. Richard Abbott: I'm afraid we need a full postcode to see exactly what the situation is for you.
Signal propogation does change over time, and that can lead to increased interference or increased or decreased signal levels, any of which might be the cause of the problem. Nightly changes are usually down to Tropospheric Enhancement, when a cool/warm air boundary develops in the upper atmosphere around/after dusk. Signals can refract off this boundary when they would normally travel through.
If signal levels are fairly high, your problem could be caused by the signal you do want being reinforced by enhancement, resulting in too much signal overall. If you have a booster or amplifier, try it without, or turn it down. If that doesn't help, try adding an attenuator.
If signal levels are low, you would get best results by moving or replacing the aerial, or replacing the cable so that it loses less signal. If it's down to enhancement of a distant transmitter, rotating the aerial slightly to point further away from that distant source, or replacing with a more selective aerial, is all that you can do.
Other sources of interference causing problems at specific times would be street lights, thermostats or central heating pumps. Chris: Yes, the broadcasters use these dots to indicate changes. As far as they're concerned, the box should be ignoring these dots. Contact TVonics for suggestions - 
TVonics Store - Product Support David C, jb38: Considering that the signal levels from Darvel have increased greatly in the last couple of weeks, I suspect that the nightly variation is causing you to have, overall, too much signal at night.
I realise that sounds like my default answer to everything, but it's not a commonly-understood problem!
If you have a booster or amplifier, remove it or turn it down. If not, or that doesn't help, try adding an attenuator.
Signal levels as reported by your TV or box could be substantially lower than 100%, and yet have too much signal, for two reasons. Firstly, the box's strength meter could be calibrated for test lab conditions rather than real-world conditions. Secondly, some boxes are known to display a low signal level when they actually have a high one. I conjecture that this is because they're reading off the gain setting from the automatic gain control, but the AGC is automatically turned up when it can't decode the signal, so it's assuming lots of gain = low signal. edin: You have to have a "Freeview HD"-branded box or TV to receive HD channels. Just an 'HD Ready' TV is not enough.
HD Ready indicates that the TV can accept and display HD signals from another source. It might have an integrated digital tuner, but it won't implement all the standards necessary for HD TV in the UK. These are:
- DVB-T2 (not just DVB-T) for the multiplexed signal
- MPEG-4 AVC/H.264 for the more efficient compressed picture
- The BBC's special Huffman tables to decode the EPG data
If you're using a PC with a TV tuner card or USB tuner that claims to be able to do HD, it may be able to do it overseas, but not here. Some other countries use the older MPEG-2 video compression on DVB-T, and some use MPEG-4 AVC on DVB-T, but here DVB-T2 is required. Stuart O.: If a set-top aerial works but an outdoor aerial doesn't, it's most likely to be down to getting too much signal from the outdoor type.
Go for a fairly small aerial, I'd try a 25 or even 18-element log periodic, and fit an attenuator to reduce the signal levels. Obviously don't use the amplifier!
Amplifiers - including the automatic gain control and the mixer in the receiver - can only handle so much signal before distorting. This distortion essentially creates frequency-shifted copies of the signal (intermodulation products) which interfere with the original signals. It's the overall signal level, the level from each multiplex added together, from all transmitters received, that is important. (Filtering is usually done after mixing, to put the selected channel on a specific intermediate frequency, as it's easy to tune the mixer but hard to tune the filter.)
The individual carriers making up one multiplex are 'orthogonal', deliberately spaced so this problem doesn't happen within one multiplex. Unfortunately the multiplexes are not orthogonal to one another!
The 'outdoor' aerial should still have better directional properties than the set-top, so you're still likely to get better results overall.
Theoretical signal levels can be calculated with 
Field Strength Calculator . For an aerial with 7 dB of gain, I get 70.4 dBuV of output from Winter Hill (100kW on C61 at a distance of 71.6km), which is more than the CAI's recommended level of 65 dBuV. The Wrekin (20kW on C23, 55.8km away) gives 69.7 dBuV. You lose some signal through tiles and rafters, of course.
Sutton Coldfield is 97.3km away but you do have line of sight, power levels are currently low at 8kW on C41 giving a theoretical 58.6 dBuV - that's still plenty for the box to decode. Post-switchover level will be 200kW on C43, giving 72.4 dBuV. steven wallace: Postcode?
A 'new digital aerial' is often over-specified for the job, leading to excessive signal levels. Jack Cannon: Most likely you have too much signal. If you have a booster or amplifier, you should remove it. If that doesn't help, add an attenuator.
Since you were able to receive the BBC channels before last night, your box must be able to handle 8K mode. Jack Cannon: though at that postcode, Digital UK reckon Tay Bridge, Angus or Craigkelly are better options than the Kinross relay (which the predictor doesn't even list). Can you confirm which transmitter you're using?
Angus is your best bet for a six-multiplex service. Craigkelly will provide a service from late 2012 (possibly after Tyne Tees switches over?) but it's expected to be poor on two of the commercial multiplexes and variable on the other. Black Hill is expected to provide a good PSB service from next week, but no prediction is shown for the commercial multiplexes. Gaz: If the flat uses a communal aerial, the system may need adjusting for the new channels and power levels. Check with your landlord or agent - see 
PARAS - Professional Aerial Riggers Against The Sharks for some more advice.
Looking at the prediction for that postcode - which shows 100% probability of reliable reception across the board - it's probable that you have too much signal. If the landlord won't sort it out, try fitting an attenuator, although if the distortion is occurring at the master distribution amplifier, it won't help you.
Check if your neighbours are affected too. Gaz: Sorry, I've just noticed that you posted at 2am, when all services were off. They should have been restored by 6am, so if you haven't retuned after that, try doing that first. Dave: The Rosehearty relay only transmits the three public service multiplexes. The commercial multiplex operators - which carry the other channels - don't want to spend the money on having equipment installed and maintained at all the small relay sites. They were quoted about three times as much as their current running costs, about £12m more, for all the relays.
You would likely need a new aerial for Rumster Forest, as the existing aerial is probably Group B, while Rumster Forest transmits in Group A and C/D (i.e. below and above the range that your existing aerial is designed to receive). Gary: Make sure you're doing a full reset? New TVs should come with a leaflet specifically telling you how to do a retune on the switchover dates.
Apart from that, my next best hypothesis is that you now have too much signal, for which the solution is to reduce signal levels. If you have a booster or amplifier, remove it or turn it down. If not, or that doesn't help, try adding an attenuator (a device that reduces signal levels).
If you previously had interference on BBC Two analogue, it could still be there and interfering with the BBC digital multiplex that replaced BBC Two last week. See Single Frequency Interference for suggestions. Green Gordon: You can find a map of Scottish Gaelic speakers at 
File:ScotlandGaelicSpeakers2001.gif - Wikipedia, the free encyclopedia . The highest percentages are in north and west Scotland, particularly the Western Isles.
Radio nan Gaidheal is available on FM. It is also available on the Central Scotland regional DAB ensemble, and on a number of local ensembles, but it is not on the BBC National Radio ensemble. The ensembles have to be identical on all transmitters that carry them, and BBC National Radio (12B) is carried across the entire country, from Dover to Eitshal on Lewis.
The local ensembles:
Aberdeen, run by Switchdigital Scotland (UTV). Broadcasts from Durris, Meldrum and Mormond Hill.
Ayr, run by NOW Digital (Arqiva). Broadcasts from Darvel, Brown Carrick Hill and West Kilbride.
Perth & Dundee, run by Score Digital (Bauer). Broadcasts from Angus, Purin Hill, Kirkton Mailer, Faire Mhor, and possibly Kinross.
Inverness, run by Score Digital (Bauer). Broadcasts from Mount Eagle.
You can get an idea of the Freeview coverage from the *main* transmitters at 
mb21 - Transmitter Information - Analogue Television Map , though this is really an analogue TV map. It doesn't show the coverage of the relays, which - in the difficult terrain of north-west Scotland - are numerous. Nick: HD services start at switchover, apart from the few transmitters providing an early, temporary, HD service. These are large population centres where the HD signal could be crammed in, usually at lower power than the other multiplexes. The temporary service is replaced by the permanent service at switchover.
Early HD services are available from:
Lichfield, for Birmingham
Crystal Palace, for London
Black Hill, for Central Scotland
Pontop Pike, for Newcastle
Emley Moor, for Leeds
Winter Hill (for Manchester and the north-west) gained HD services at switchover, as have all transmitters which switched over from March 2010 onwards. Those that switched over before this were gradually upgraded between March and November last year.
No more early HD services are planned. HD services from Sudbury, and its relays, will start on 20 July, a little over a month away. Ian: Channels can choose to be 'free-to-view', though, only requiring the one-off purchase of a viewing card ('Freesat from Sky'). Dave is a subscription-only channel on satellite, you must subscribe to the appropriate Sky channel package and keep paying every month to keep the channel.
Sky distribute a (small) proportion of subscription revenue to the channels in the package, and the cost of carriage in the EPG is believed to be lower. Sky may also have offered to 'help' the channel with transponder space or rental costs. Speug: Brian went to a lot of effort to add those and is now being stubborn about removing them.
In short, they are wrong. They only ever appeared in an appendix to an Ofcom consultation about what to do with 'interleaved' spectrum, possible frequencies that could possibly made available, to cover a limited part of the transmitter's coverage area.
There has been no auction to licence these frequencies, and there is no date set for any auction. They might form part of the Culture Secretary's pet Local TV project, but that seems to be mired in disagreements over the scope. Even if local TV does get off the ground, it might not use the frequencies shown, as it may be better to use other frequencies for the job (for example, the Channel M multiplex in Manchester was actually allocated C56 where 'NEW7' was shown as C57).
To sum up, you're not missing anything now and you're not that likely to get anything in future. Ann Tait: Is there only one aerial on the building, or is there an aerial for each flat?
Your mother's situation may be like mine, that there is one aerial, the aerial cable comes in to one flat and is then split via an active splitter between the TV in that flat, and a cable running to the other flat. (I rent a converted terrace.) If that's the case, ask the upstairs neighbour to check that the powered splitter is still plugged in, and that it's working.
Some powered splitter equipment automatically reverts to bypassing on the first socket if the power supply fails or is unplugged. If this equipment does this, the neighbour may still have excellent reception.
You're only 5km away from the transmitter and it's fairly powerful for a relay, so signal strengths could still be high enough to use a passive, unpowered splitter instead. They could be high enough to get a distinguishable, though not perfect, picture without the aerial.
If there is one aerial per flat, or the splitter is passive or a powered splitter is definitely working, check that all cables are properly connected and that the cable is making proper contact with the connector. If there is a separate aerial, ensure it is pointing in roughly the same direction as the other and that it appears to be intact. steven wallace: At 20km from Winter Hill, with clear line of sight, too much signal is the most likely explanation. If there is a booster or amplifier, try removing it.
If that doesn't help, and this aerial was fitted by an installer, get them to come back and reduce signal levels, or otherwise do what's necessary to sort it out. If it was DIY, try adding an attenuator, and/or swap for a smaller aerial such as a log-periodic type, e.g. 
Online TV FM DAB Aerial sales . Lisa: PSB, Public Service Broadcasting multiplexes. PSB1/BBC A carries all the BBC's SD channels, interactive services and radio stations. PSB3/BBC B/HD carries BBC One HD, BBC HD, ITV1 HD and Channel 4 HD.
PSB2/D3&4 carries ITV1, Channel 4, Channel 5, and whatever else the ITV companies and Channel 4 decide to carry on their respective halves of the remaining capacity. At present that's ITV2, C4+1, More 4, E4, and ITV1 +1.
Digital UK's predictor shows you have a good chance of reliable reception on all six multiplexes from both Sudbury and from Crystal Palace after their respective switchovers are fully complete. You aren't expected to get reliable service from Sudbury COM5 and COM6 (Arqiva A and B) until the final channel change in June 2012. Nick: Challenge is carried on Mux A/SDN. At switchover step 1, 6 July, this takes over the current main BBC multiplex's channel. It is due a power increase to double the current level, 14kW, but it's not clear whether this happens at step 1 or step 2 - Digital UK's predictor for Lisa's postcode shows the same (poor) prediction for 6 July as it does now, increasing to good at step 2 on 20 July. 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. 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%. 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. 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). 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. 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. 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. 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 . 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%. 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. Chris: Assuming your postcode is RH5 5BH which you've put in before, you have very high probability of reliable reception, including Mux D.
You've previously indicated having problems with C66. If this happens when it's raining, the problem could be water ingress into junction boxes or cables. Higher frequencies are affected to a greater extent than lower ones. Cables are damaged by exposure to the sun over time, which can allow water to penetrate, or, if not well secured, the cable can rub against brickwork or tiles and eventually the insulation wears through.
Mux D also has the lowest signal levels of all, which is another reason it can be worse affected than other multiplexes. Dave: Because they're not actually comparing like with like.
It's stated that analogue transmissions require 43 dB of signal-to-noise ratio - about 20,000 times the power from the signal compared to the interference - for perfect reception.
Simulations show that error-free digital reception only requires 17.3 dB signal-to-noise ratio - about 0.27% of the analogue difference. However, this is the absolute minimum required: any lower, and the picture and sound start to break up. The broadcasters' statistics show that, to provide reliable results, the signal needs to be about 12.8 dB higher. They've also added about 6 dB of fudge factor (i.e. nearly 4x power levels), making for 36 dB in all.
This 36 dB of signal-to-noise ratio is 7 dB less than the required analogue level, so in general, levels have been set 7 dB lower than the analogue levels, or one-fifth of analogue power. This is the same right across the country, except that the south-east coast transmitters have been set higher to provide extra resilience to signals from continental Europe, or perhaps reflecting better aerial systems that send relatively less signal overseas.
Experience seems to be showing that the 6 dB fudge factor is unnecessary and levels could have been set lower. In effect, the digital coverage areas are actually larger than the analogue ones. Ray Reed: I'll say at the outset that I'm a software engineer rather than an electronics or RF engineer, though I did cover some electronics at university (started out studying electronics but couldn't handle the maths for Semiconductors and Opto-Electronics).
You could have too high a signal level.
Digital TV requires that all amplifiers in the signal chain are linear, or as close to linear as possible. Distortion introduced by non-linearities causes intermodulation of the individual channel transmissions making up the service as a whole. At Crystal Palace there are currently 11 services, four analogue and seven digital (six SD, 1 HD), plus one from Croydon (analogue Channel 5). The intermodulation changes the amplitude and phase of the signal from what was intended.
The digital transmissions carry redundant information from which the original data can be recovered. They also carry a further checksum which can indicate when the data recovered is incorrect. This 'outer code' can detect up to 16 bytes in error and correct 8 of them. The effect of uncorrected errors depends on where the errors fall and could cause colour errors, synchronization errors, picture and sound drop-outs. The MPEG 2 video compression scheme relies heavily on encoding only changes from frame to frame, so errors in one frame can persist for several frames.
The Confederation of Aerial Installers recommend that digital signal levels are set to 45 to 65 dBuV (some sources have 60 dBuV) for each 8 MHz channel. Ensure that your meter is measuring the power across the whole channel - for analogue, most of the power is concentrated in the carrier (especially in sync pulses) so older meters may measure a narrower bandwidth. Analogue levels should be set to 60 to 80 dBuV. At Crystal Palace, digital transmissions are currently 17 dB down on analogue, which means that setting analogue correctly should give roughly correct digital levels and vice versa. The overall signal level should be less than the maximum marked on the distribution amplifier.
I wouldn't rely on a cheap meter, they are often poorly calibrated - both in the designed range and in quality control - or don't even measure the right thing.
Intermodulation between two analogue signals tended to only cause a 'herringbone' pattern, due to the channel spacing. Digital signals had to be packed in anywhere they could be fit in, but to an analogue receiver, a superimposed digital transmission just looks like random noise - this is a deliberate property of digital transmissions, which are fed through pseudo-random processes to make them look random.
Your postcode is only 12km away from Crystal Palace. If the aerial were outside, you could theoretically get 100 dBuV at the aerial terminals for analogue channels, and 83.5 dBuV for digital. http://www.megalithia.com/elect/fieldstr.html
Neighbouring buildings can block signals, so you often get 10-20 dB less than predicted, and the tiles or walls will reduce that further. Still, I think it's more likely that your problems are down to excess signal than insufficient.
To fix the problem, I'd go back to your old aerial. You didn't need to change it. If that still gives excess levels even without the amplifier, add an attenuator. Ensure that the amplifier is only adding as much gain as you lose through the splitter - this should be marked on it, typically a four-way splitter drops 8 dB.
I suspect the change was down to the switchover at Sandy Heath in April. Digital transmissions cause more harm to each other than an analogue signal does to a digital one. While Sandy Heath is off to the side, aerials do still pick up some signal from neighbouring transmitters, and refractions and reflections within the loft space will amplify this. ITV 3Tuesday 21 June 2011 3:50PM
James: You are in a very strong overlap area between the Craigkelly and Black Hill transmitters, so there is a chance that your box could have decided to store a weak off-beam signal from the other transmitter when you retuned.
Check which transmitter your aerial points to - south-west for Black Hill, north-east for Craigkelly - and see Digital Region Overlap for some suggestions.
It's also worth checking that the signal levels aren't too high. If you have a booster or amplifier, you should remove it. Signal levels should be more than adequate without.
If you're using Craigkelly, and you never had a wideband aerial fitted, you will need one now as Multiplex A/SDN has moved from within the analogue group to outside it. This would apply if you couldn't get Multiplex C or D, or analogue Channel 5, before switchover.
A few boxes have problems if they pick up some multiplexes in 2k mode and others in 8k mode. Right now, all signals from Craigkelly are 8K mode, while all except the main BBC multiplex from Black Hill are in 2K mode. If you don't have BBC One, your box may not support 8K mode and, if so, will not pick anything up after Black Hill switches over tomorrow morning. As I say, some behave oddly with mixed modes so you could just try retuning after 6am on Wednesday, when everything will be in the same mode. You can find a list of some equipment that is known not to work at http://www.digitaluk.co.uk/2kequipment .
A few boxes are still around that don't support the larger Network Information Table introduced in 2008, but this is only a problem after they are retuned. If you hadn't retuned it since then, before this switchover began, you might not have encountered this problem before. You can find a list at http://www.digitaluk.co.uk/splitnit .
There are also some TVs and boxes that can't handle the number of channels now transmitted. I don't know of a list of these, unfortunately.
Craigkelly did not use the new frequency for Mux A/SDN before, so if you're using a communal shared aerial you might need to get the landlord to get the system adjusted. You might also have to look for any equipment that has an RF modulator, such as an old VCR, games console or Sky box, and check it isn't using C45. ITV 3Tuesday 21 June 2011 3:53PM
Ken Clark: In Aberdeen, she is likely to be using the Durris transmitter, where the second stage only resulted in the Public Service multiplexes moving to their final channel. The Commercial multiplexes had to stay on their old channels at reduced power. With Craigkelly having completed switchover, the commercial channels at Durris could move onto their final frequencies and power levels, which happened last Wednesday morning (15 June).
To get ITV3, which is carried on a commercial multiplex, back, you will need to retune the equipment in the same way as the other switchover stages.ITV 3Tuesday 21 June 2011 4:13PM
James McEwan: See my earlier reply to James from Falkirk! I can't do a detailed prediction, because you haven't provided a full postcode.
There is a relay transmitter at Canongate, to the north-west of Holyrood Park, which may indicate that signals from Craigkelly are poor in this area. Signals are aimed to the north-east of the transmitter, i.e. north of the park, on horizontal polarization, and to the west and the south on vertical polarization. Craigkelly is the other side of the Firth, above Burntisland.
Sometimes signal levels from the main transmitters are improved further than you would expect from the change in published levels, because the new main aerial is that much higher or has a different radiation pattern from the old digital aerial, so check for high levels. That said, both the old aerials - main and digital - were replaced by October 2010.Simon: Because the operators of any multiplex have to agree to any change to their licences, such as to add new transmitters, and the commercial multiplex operators have refused. They say it is not cost-effective.
All you can do is complain to ITV plc and Arqiva, who operate these multiplexes, or to the channel you want.
If they did extend their services, it would likely be only to the high-power relay stations, though Conway would probably be counted as one of these as the pre-switchover analogue power was more than 1kW.
The Public Service Broadcasters were quoted £24.7m for broadcasting from all 1,100 relay stations in 2007, while the commercial broadcasters were quoted £10.39m for broadcasting from only the 81 sites they used before switchover. Some of the costs for the PSBs would be shared if the commercial broadcasters extended their services.
It is much cheaper for the channels to be carried on satellite or cable, particularly with Sky subsidising carriage or providing subscription revenue, for subscription channels.
Because they refused the option early in the planning process, no frequencies were left free, and it may now be difficult to find any to provide the service. The coverage area for any additional services may not match the PSB multiplexes.
Depending on your location, you may find that it is possible to receive from a main transmitter - in this area probably Llanddona. If you provide your full postcode, I can check, or you can check for yourself at Digital UK - Home . Ron Wagg: Check the power supplies for all equipment in the vicinity and ensure that power cables are kept well away from the signal cables.
The scrolling line is likely to be interference from the near-50Hz power on the near-50Hz SCART connection from the box to the TV. If both were exactly the same frequency the line would be static, in the same place all the time.
The intermittent interference is more likely to be mobile phone interference on the satellite cable. Keep your phone well away, or replace the cables with better-screened ones. Satellite connections should always use 'satellite-grade' cable with copper braid over copper foil screening - traditional 'low-loss' coax is not up to scratch and will pick up more noise.
If that doesn't seem to help, check whether the interference is related to some motor or other electrical equipment switching on or off, for example the thermostat or central heating pump. If so, get that equipment checked. Nick: Well, I can only assume that the set overscans a bit more on one than on the other. No idea why it should do this, if the Sky box is sending the sync pulses at the same time on SCART as it does on the RF modulator.
My TV has zoom modes to reduce the black bars (e.g. 14:9 sent as 4:3, 16:9 sent as 4:3, which happens often on VIVA), any chance that you have a zoom feature selected when SCART is selected? Mine uses the same zoom mode for all inputs, but perhaps yours remembers the zoom setting separately for each input?
Otherwise you're probably down to hunting for 'timing' configuration options on the TV! Dave Dickinson: A freesat box should work with any dish pointed to the Astra 2 cluster, though it may need to be larger than usual to ensure that adequate signal is received from the 2D satellite. The 2D satellite carries most of the Freesat service (the same transmissions are included in the Sky service, they are not duplicated) and it is designed to only cover the British Isles with normally-sized dishes. The closer you are to the UK, the smaller a dish you're likely to need.
French-language transmissions come from a different cluster of satellites - looks like Astra 1 at 19.2°E. Some dishes do allow more than one LNB to be carried. You then have to set the dish up so the signals from one cluster are deflected to one LNB while the signals from the other, go to the other. It's possible, but the dish has to be adjusted more precisely. Separate dishes may be easier! Mike: Different equipment has different limits on what it will accept, and behaves differently if more than one transmitter is received.
Digital UK's predictor suggests a good probability of reliable reception from three transmitters: Craigkelly, Durris and Angus. They're all in kind of the same direction - Craigkelly slightly west of due north, 341°, Angus slightly east, 6°, and Durris a bit further clockwise, 20°. Aerials do have a fairly wide acceptance angle - the larger the aerial, typically the smaller the angle - so the TVs could be picking up one of the further transmitters. It's likely to be Durris, as that has the lowest frequencies, though I would have expected it to pick PSBs from Craigkelly and commercial channels from Durris.
The Durris commercial multiplexes only moved to this location on 15 June, their temporary locations were on much higher frequencies.
If the TV offers a choice of region, ensure you select Central Scotland rather than Highlands and Islands. If you can find a status screen, check that the UHF channels match those from Craigkelly, rather than one of the others. See Digital Region Overlap for other thoughts.
Check that you do need the amplifier, and if you're adding an attenuator, add it before the amplifier. Amplifiers should normally only have just enough gain to offset the losses in the splitter and cabling, if the basic level from the aerial is correct.
On pure distance alone, I would expect the signal levels at your location to be very high, but you're in a fairly deep valley, which means that you're relying on refraction over the hills. Still, excessive amplification can still cause problems. Bill Kett: The prediction for your postcode isn't great at the moment. It's possible that the level of interference has been steadily climbing as the switchover programme has progressed, both here and abroad, and is now too high for reliable reception.
Still, I'd normally expect break-up rather than complete failure. Check that the aerial is pointing in the correct direction and that all cables are still intact.
You say that the aerial is new. How new? Was there a guarantee? I'd recommend calling out the installer and getting them to remedy it. David Jackson: Presumably this was a diplexer, combining a Group A aerial pointing at Crystal Palace with a Group B or E pointing to Sudbury? There's no problem diplexing aerials together for digital services. Nothing in the chain is specifically 'digital' or 'analogue' until you get down to the receiver, and usually only the decoder stage rather than the front-end tuner.
Some boxes do get confused with a mixture of 2K and 8K mode transmissions, in the interim period between Sudbury and Crystal Palace switchovers, and it can be difficult to get some boxes to store the services you want at the preferred numbers. Most would likely store Crystal Palace services at the published channel numbers as they are on the lowest frequencies.
CP may be stronger now, but Sudbury shows a slightly higher probability of reliable reception once the switchover programme is completed. Both are ultimately good, though (except Crystal Palace Mux C/Arqiva A drops to 'variable' in 2013, possibly due to changes overseas as the UK programme should be complete by the end of 2012.)
However, you have to wait almost another year for Crystal Palace to switch over, while Sudbury starts in a couple of weeks. That said, only the PSB multiplexes change to final channels and power levels. The COM multiplexes (A/SDN, C/Arqiva A and D/Arqiva B) stay at low power from Sudbury until after Dover switches over in a year's time. 
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http://www.wrightsaerials.tv/articles/whatsat-short-reflector-syndrome.pdf 
Ofcom - Digital Radio Coverage Map (DAB Primary Protected Areas) . 
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