All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Paul: First, forget about averages, they're meaningless. The coverage on one set of channels is not altered by the power level on another set of channels, and what you care about is whether it's strong enough on the channels you do want.
On digital, TV channels are carried in a 'multiplex', a digital transmission that carries lots of channels in the space that one analogue channel used to take. This is done because there's much more capacity available that one compressed TV channel takes (three to four times typical DVD rates, TV is more heavily compressed than a DVD), and there's a certain amount of space wasted if you tried to divide up the broadcast channel into smaller blocks.
All that's happened so far is that the high-power BBC A multiplex has replaced the old analogue BBC Two and the low-power Multiplex 1, which carried most of the BBC channels before yesterday. Because BBC Two's analogue channel is being freed for 4G mobile services, the digital transmission has actually taken over BBC One's frequency and BBC One has temporarily moved to BBC Two's old location.
That new BBC A multiplex is transmitting at a maximum of twice the maximum power of the former analogue transmitter on that frequency. It's 20 times the power of the old low-power Multiplex 1.
Where there are factors of millions of times between the highest and lowest levels accepted, engineers usually use a logarithmic scale - decibels. A multiplication of the original by a factor turns into an addition of the logarithms of the original value and the factor. Multiplying power by two is adding (about) 3 dB and multiplying by ten is adding 10 dB. It's a useful scale if, like human hearing and vision, there is a logarithmic response, or if, like TV transmission, the source is diffused in all directions, but we largely 'see' just the straight-line path between transmitter and receiver.
On the 17th, the remaining analogue and low-power digital transmissions will be switched off. The replacement high-power transmissions will start up at their allocated power levels - 2000W for the three multiplexes that were gifted or licensed to the public service broadcasters (BBC A, D3&4, BBC B/HD) and 1000W for the three multiplexes licensed to commercial operators (SDN, Arqiva A, Arqiva B).
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Alan Parker: It depends where you are.
Switchovers for the following regions aren't scheduled until 2012:
Meridian (excluding Oxford)
London
Tyne Tees (no dates announced)
Ulster (no dates announced)
Switchovers for some transmitters in these regions will require changes after the publicised 'switchover' dates, in 2012:
Anglia (Sandy Heath, Sudbury)
Border (Caldbeck)
Central (Oxford)
STV Central (Darvel)
West (Mendip)
West Country (Stockland Hill, Huntshaw Cross)
The interlocking nature of the frequencies used at sites around the country means that some changes have to be delayed so that as few people as possible lose any service they already had.
Where I say 'channel' below, I mean the radio frequency that the service or multiplex is transmitted on.
At almost all sites, the three public service multiplexes - carrying the BBC services, ITV1, ITV1+1, ITV2, Channel 4, C4+1, E4 (except Wales), More4, Channel 5, and the four HD services - start up on their final channel at switchover and at full power. This is because they are directly replacing the analogue channels, which don't clash with any pre-switchover services at other sites.
At some relays, the post-switchover channels are different from those used before switchover, and are not available until changes happen at other sites. Where this happens, a temporary channel is used, but at full power. In some cases, if a relay only transmitted two services - cases where the 'wrong' region was transmitted from the best main transmitter - the HD multiplex is postponed until later (e.g. Derby).
The three 'commercial' multiplexes, which carry the rest of the services, are harder. The analogue system was planned for four channels at each site, meaning at least two more frequencies have to be found for sites carrying the commercial multiplexes (only the 81 main sites that broadcast digital before switchover). Two chunks of space aren't available, either, with C31-C38 being released for some undefined future use and C61-C68 released for 4G mobiles, which often rules out at least one of the old analogue channels. They have to be co-ordinated with overseas broadcasters too. So there are often clashes between what has been allocated, and what is available at the time of switchover. Those clashes might be with a relay that will move to new channels at switchover, or with a low-power digital service elsewhere that is yet to switch.
Where this happens, the best option is usually to leave the low-power versions of the commercial multiplexes alone for the time being - on their original channels and original power levels - until the other site switches over and releases the required channel. Where possible, a (relatively) small increase in power is used, and/or a retune to use one of the frequencies used by a multiplex that has moved to its final location.
However, some services have started up in the BBC's spare capacity before switchover, and they must move to a different multiplex at switchover because this capacity is removed (for HD services). To do this requires the Mux C/ArqA and Mux D/ArqB multiplexes to change mode, and this mode needs more power. Many sites get a small power boost, but this may not be enough for all viewers to keep the services in the interim period before the final channels and/or power levels are available.
A few sites have to swap their interim channels for different temporary channels, before getting their final locations, to work around more than one clashing site.
If you provide a full postcode I can give an indication of when you will need to retune and whether any interim arrangements apply.
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Mark A.: Just like the broadcasters, we use decibels, because the signal strength reduces logarithmically with distance, not linearly.
One-fifth is actually not a very large drop in power - especially compared to the difference in signal-to-noise ratio required for PAL, which was 43 dB, and that for the post-DSO digital mode, about 20 dB. The other 16 dB is a margin to allow for the normal variation of levels - on analogue you'd see increased snow, annoying but not totally obscuring the picture, but digital would start cutting out.
Your box likely has a very wide range of gain available. Mine claims to handle 35 to 95 dBuV, a range of 60 dB or one million times the power. (1000 times the voltage, because power = voltage squared divided by resistance.)
At some sites, the pre-DSO digital power can be a tiny fraction of analogue power - e.g. Sutton Coldfield analogue is 125x more than low-power digital, or Sudbury analogue was 227x more powerful than Mux D. Using your method, the bar would be incredibly short, giving quite a false impression.
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jason: Because the channels that will eventually be used are still required at other sites. Both the allocated channels and the temporary ones clash with other sites. Moving to full power now would damage other people's reception, either of analogue or of low-power digital services.
In order of what's going to happen:
On 31 August, Mux A will move from C43 to C31, at the same power level, to free C43 for Sutton Coldfield. C31 is released by Waltham on that date.
On 14 September, ArqB moves from C67 to C48, its final frequency, at full final power level. C48 is in use at Oxford before this date. Mux C changes mode, becoming ArqA, and takes over C67 from ArqB, at the same power level.
On 23 November, ArqA moves from C67 to C52, which is currently used by Tacolneston, at full power. This is its final channel.
On 9 May 2012, Mux A changes mode, becoming SDN, and moves to C51 at full power - this is its final location.
On 27 June 2012, the vertical beam tilt - the angle of the main part of the transmission - will be raised. It is currently restricted to the south-west, to protect services at Rowridge. Effectively this will increase power levels for viewers further away from the transmitter.
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Briantist: I'd been meaning to mention that. I'm certain they will not shut down ITV1 analogue early.
There will be some clashing with Belmont C60 but that has been handled by delaying power-up - it will double to 8 kW at DSO2. Hemel Hempstead will release C60 for Oxford on 14 September; this is a PSB channel at Oxford, launching at full power (100kW). It's already an analogue frequency there, so there might be a small net reduction in interference at TAC for the last couple of months.
There's a note in the 'Supplementary License Documents for DSO' version 3 that says, for Mux 2, "until 17 August 2010 services will be provided on channel 60". If we assume that 2010 should read 2011, it does match where Mux 2 is now, and the date that Mux B moves from C58 to C66, and Mux C from C61 to C68. I have to assume that this note is just complete rubbish.
The document does not mention the Mux B and C retunes at all. It does seem to indicate a power increase to 10kW on Mux A at some point between now and DSO2, or that could just be more rubbish.
There is a version 4 document now at http://licensing.ofcom.org.uk/binaries/tv/sup_dso/digital-stations-v4.pdf which looks like it has the same notes, but for Mux 2 the rows of the spreadsheet are too short and only the first line is visible.
Ofcom are really, really failing with switchover. They're letting the broadcasters do whatever they want and not keeping up with the paperwork.
The best source of information seems to be the 'Trade Region' section of DUK's postcode checker at Postcode Checker - Trade View , though that doesn't include power levels.
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snowflake: You may be in a region where the local ITV1 service, and the Channel 5 ad region, are encrypted. The viewing card is required to decrypt it. Also, the postcode that the card was registered with is used to determine which version to store. Make sure the viewing card is still in the box and that it's making good contact.
I believe Briantist intended to link to What can I do when my Sky Digibox says 'No Signal' or 'Technical fault'? | ukfree.tv - independent free digital TV advice which goes into more detail.
If that process doesn't work, you may need to buy a new viewing card from Sky.
Alternatively you can use Other Channels to tune into any ITV1 regional variant or the unencrypted version of Channel 5. See Astra 2A / Astra 2B / Astra 2D / Eurobird 1 (28.2°E) - All transmissions - frequencies - KingOfSat or Eurobird 1 & Astra 2A/2B/2D at 28.2°E - LyngSat for the tuning details.
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Peter Henderson: We simply don't know yet. Typically the dates are announced with about a year to go. For example, London (4 April 2012) was announced on 7 April 2011, Meridian West (lead transmitter Hannington: 8 Feb 2012) on 15 Feb 2011, and Meridian East (lead Heathfield, 30 May 2012) on 10 May 2011. The later the announcement comes, the later the switchover is likely to be. I think the shortest period between announcement and switchover was for the Channel Islands, about 10 months in advance.
Every region that Ulster TV borders has already switched over, so I don't really see any reason why it can't all happen on the same day. The Sutton Coldfield/Emley Moor switchover next month affects many more relay transmitters and people than all of the Ulster region. Fenton switches on the same dates so that's three 'Full Freeview' transmitters on the same date. The most is five main transmitters with some work to do on the same day - Oxford DSO2, Mendip, Bromsgrove, Lark Stoke and The Wrekin retune and power up all on 28 September, Nottingham and Sheffield have a retune and power up scheduled for the day before.
Some co-ordination may still be required with the Republic, I'm not sure if they've started up Saorview on final channels or not. Because of the pull-out of the commercial provider, they seem to have only launched two multiplexes so far, and one of them just contains placeholder content according to Wikipedia, so they may not have as tight frequency requirements as we do, right now.
Looking at the Geneva 2006 frequency plan, it appears Ireland asked for 4 frequencies at each major site, in addition to their existing analogue frequencies, so the co-ordination may already have been done. (They still use Band III VHF for RTE1 and 2 at many sites, so only six UHF frequencies total are required at those sites, before analogue switch-off.).
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dan preston: It is likely that the 9200T stores the first version found rather than the best quality, although you'd have to ask why it wouldn't pick all services from Sandy Heath.
Your best bet is the Hemel Hempstead transmitter, which requires a vertically-polarized aerial pointing east-south-east. You might alternatively be aiming at the Crystal Palace transmitter, which is horizontally polarized, and due south-east.
If the box was just storing the channels based on the first found, you would expect it to store Crystal Palace C25 C22 C28 C32 C34 C29 (1-2-A-B-C-D) in preference to Sandy Heath C27 C24 (C21) C43 C40 C67 (C21 carries HD signals which the PVR9200T won't tune in). Hemel Hempstead is up at C48 C55 C59 C60 C62 C65.
Check which way your aerial's elements are running - up-and-down or side-to-side - and if up-and-down, try tuning into the Hemel Hempstead frequencies, otherwise, try Crystal Palace. However, the predicted reliability from Crystal Palace is variable, up until switchover.
If you are using Hemel Hempstead, be aware that you will need to retune on the 16th of September, as Mux 1 and A move to temporary frequencies to allow Oxford and Sandy Heath to use their current frequencies.
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Richard Evans: Some TVs or boxes can get confused if they're retuned from one site to another. It might be necessary to do a full reset/factory default/first-time installation on the TV.
If you get really stuck, try doing a full reset with the aerial unplugged, in order to delete all channels, then manually tune the frequencies you actually want.
Do also be aware that the Humax PVR9200T has an RF output, that is, it puts the currently-tuned picture and sound on the RF cable going to the TV, as if it was an analogue TV station. Check that this doesn't clash directly with any multiplex you want to use, isn't immediately above or below one, and that it isn't 5 or 9 channels away either.
My PVR-9200T seems to create a lot of noise on and around C36 no matter what I set the RF output channel to, when it's in standby.
The Humax also has a small amount of gain and noise, so if reception is slightly marginal, you may find that it works better or worse when the box is in standby.
There will be a retune at Bromsgrove on 28 September, which will move the COM muxes to their final channels and power levels.
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Wednesday 3 August 2011 5:40PM
Michael: Yes, as I thought, the BBC A and D3&4 multiplexes only at present.
Digital UK have failed to properly communicate that Sudbury can't go to its full, final configuration yet, giving the impression that everything is done. The commercial multiplexes actually remain at lower power and on busy channels. There will be another retune on 16 November, to free up a channel for Tacolneston, which isn't expected to make any improvement on your reception but may help some people. The final step, with all multiplexes going to their final channels and power levels, doesn't happen until 27 June 2012, when the Dover transmitter switches over.