All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
andy: The Wokingham area suffers from there being a big difference between the analogue and digital signal levels from Hannington. This is a deliberate restriction on the digital output, to protect the analogue services at the Guildford transmitter, and means you can have completely clear analogue pictures and yet unreliable digital.
This flaw was designed into the plan in 1998 and was never lifted despite much investigation of the actual effects on Guildford reception around 2001/2. It will be lifted for the PSBs at switchover (8 and 22 February 2012) and for the commercial multiplexes after Guildford switches (4 and 18 April).
You may get better results from the Crystal Palace transmitter, now and after switchover.
I don't have an explanation why it should have got worse recently, unless a new antenna at Guildford has a different radiation pattern from before (causing an increase in interference) - I can't see any other transmitter that's recently switched over which clashes.
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Howard: Can you confirm which transmitter you're using? If you're trying to use Hannington (aerial would point south-west), you may get better results from Crystal Palace both now and after switchover. The aerial would need to point east-ish for CP, and you'd probably need to replace it as an old aerial for Hannington would be Group E, designed to *reject* signals in the Group A range that Crystal Palace uses.
There are a couple of hills blocking line-of-sight from your aerial to Hannington, which is probably the cause of the problem.
It is worth checking that your box is tuned to the transmitter the aerial is pointing at - see Digital Region Overlap for advice on this.
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mike Roberts: As FM radio is on a lower frequency, it bends more over terrain and so can travel further than UHF television.
The radiation patterns for radio transmitters *are* published by Ofcom, and you can see the one for Emley Moor at FM, MW and LW radio broadcasting | ukfree.tv - independent free digital TV advice . However, The Pulse broadcasts from Vicars Lot, a site several miles to the west and omnidirectional: Vicars Lot analogue radio transmitter | ukfree.tv - independent free digital TV advice .
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Dave Lindsay: The antenna would have been designed to minimize the differences between channels, but it's an inevitable consequence of having multiple channels share one aerial, that the radiation pattern of each channel is a bit of a compromise.
The picture is somewhat exaggerated as engineers normally measure differences in decibels (dB) while those graphs are shown in relative volts. To convert, take the base-10 logarithm of the voltage ratio ('log' on most calculators, LOG10 in Excel) and multiply by 20.
Giving each channel its own antenna at the same site would still be a compromise, as the coverage area is also affected by the aerial's height. Each antenna could be shorter - fewer tiers - to reduce the differences in height, but that would reduce the gain of the aerial and increase the electrical power needed to produce the same output.
In the US, it seems to be common to erect a tower with quite a wide platform that the various antennas then are mounted on - where they do share sites at all. The Empire State Building is festooned with different aerials.
As it is, the commercial multiplexes at Emley Moor are on an aerial slightly further down the mast (about 11.5 metres) than the PSB multiplexes, presumably so that each can be optimized better for the frequencies it's carrying, though it will also offer greater redundancy. The other issue is that the commercial muxes clash more with transmissions from other sites (e.g. Nottingham also uses 48/51/52) and with PSB transmissions from the relay transmitters - both co-channel, using the same channel at both sites, and adjacent-channel interference, due to the imperfect nature of the transmitter - some power leaks into adjacent channels. It is filtered, but filters with a less sharp roll-off are permitted at lower-power sites, therefore more leakage. This only affects the residents near to that site, but it still ends up reducing coverage.
There's a possibility that Emley Moor ArqB might currently be restricted to half power to avoid damage to the low-power Mux 1 at Pontop Pike - there is provision for that in Ofcom's licence documents.
Signal propogation is slightly frequency-dependent, and higher frequencies don't travel quite as well as lower ones, nor carry along cables quite as well. I also mentioned multi-path effects before.
All this put together means you may see different levels across the various multiplexes, and those differences may change over time, despite the published maximum ERPs being identical.
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Mazbar: Channel moves have to be negotiated with neighbouring countries, for the highest-output transmitters. The Geneva 2006 frequency plan for Europe, Africa and the Middle East shows an interaction with Ireland, for Winter Hill.
I'd guess that they will be moved to C49 and C50, on the basis that those channels have been cleared from the Sutton Coldfield and Emley Moor plans. These are still in Group C/D. If those aren't possible, I'm sure great efforts will be taken to keep in Group C/D if at all possible, as Winter Hill has the second-largest population coverage in the country, after Crystal Palace, and just beating Sutton Coldfield.
However, the plans for C31 to C37 are still not finalized; Arqiva have been asked to provide Reference Offers for Network Access for up to three new multiplexes in this space. A wideband aerial would be needed for best results on these frequencies. A Group E might be good enough down at C31, which I see is a possible allocation at Winter Hill. International clearance would also be required for these channels, though, so don't treat those as definite allocations.
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Kate: Ofcom regulates the charges that Sky can charge other providers to carry their sports and movie channels (wholesale charges). It does not regulate the charges that Sky is making for carrying channels in their EPG. That's why we're in this mess.
Encryption is a slightly complicated subject. Encryption can be used for two reasons:
1. To gain access to direct subscription revenue
2. To avoid rights issues that would involve paying more for programmes
The footprints of the earliest satellites in the cluster that carry the UK channels actually cover most of western Europe. You can see them at
ASTRA 2A - SES.com
and
ASTRA 2B - SES.com
(the pages are currently broken, hopefully they'll be back soon). The programme-makers - particularly films and acquisitions from the US - want more money if the broadcasts can be received anywhere in Europe. Encryption segregates the market, only allowing those with a UK viewing card to watch a channel intended for the UK, avoiding this problem.
Astra, with influence from the BBC, launched a new satellite Astra 2D in 2000 with a footprint only covering the UK and Ireland. This allowed the BBC to stop paying Sky to encrypt their channels and broadcast free-to-air. The EXACT SAME UNENCRYPTED transmissions are viewed by Freesat, Sky, and non-branded free-to-air boxes. There is NO encrypted copy of BBC One, Two, Three or Four on this satellite cluster. Later, Channel 4 moved all of their non-subscription channels here and ITV moved most of theirs, and there's also one version of Channel 5. Unfortunately, it's run out of capacity, so the other channels cannot move yet.
Even if you do encrypt, you may not get any revenue. There are 'free-to-view' channels such as 5* and 5 USA, where the operator has decided that ANY current or former Sky subscriber can watch the channel. It's encrypted, and you need a current-generation viewing card and a Sky box (because Sky's encryption method is proprietary), but you don't need an ongoing subscription. Sky call these 'Included Channels' on their website and indicate those that require the viewing card.
Of the traditional PSB channels, only a few regional variants of ITV1 and Channel 5 are encrypted, and those are 'included channels'.
On another page, Briantist claimed that transponder rental is about £4m per year. One transponder carries seven to nine SD channels or variants, or three to four HD channels, or of course a mixture, but on average maybe £500k for an SD channel. That said, there are 18 SD variants of BBC One, £9m.
Sky charge the BBC £5.6m for BBC One being on the platform at all, £25,000 for the EPG slot, and a very complicated charge I haven't fully worked out for regionalization, could be as much as £500k. But that is on top of the broadcasting charges that have already been incurred, for ZERO additional work on Sky's part.
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I should add that there is a new satellite, Astra 1N, designed to provide more capacity for the UK and Ireland channels. One of its broadcast dishes has a footprint covering only the British Isles. It should have arrived at the cluster site in the last day or so, or if it hasn't, it will do soon. The rumour is that some of the 'free-to-view' channels will move across and become free-to-air, no encryption.
Channel 4 HD recently stopped using encryption and moved to the Eurobird 1 satellite which is a little further away, but still close enough to provide coverage. This is Europe-wide. They have decided it's better to pay the extra for content than to continue to pay for encryption. It may well move to Astra 1N once that goes into service, as Channel 4 have agreed to rent space on that satellite.
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Channel 5 have definitely applied. For some inexplicable reason Ofcom's web team have put the application under the Third Invitation to Apply section, rather than creating a top-level Applications Received (Third ITA) section as for the previous two openings.
Ofcom | Third Statutory Notice of Invitation to Apply for DTT Multiplex B Capacity
As expected this is a simulcast of Channel 5 in HD.
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Dave Lindsay: It's helpful to cross-reference with Arqiva's Reference Offer for Network Access from 2005.
Useful documentation
Emley Moor was originally an IBA transmitter so you'll find it under the Arqiva offer, rather than the ex-NGW (former BBC transmitters).
If you look at Appendix 3, List of Main Stations, you'll see that they intended to construct one antenna for the PSBs, and one for the COMs. The reserve antenna can act for the PSBs or for the COMs but not both at once.
My belief is that the two main antennas, PSB and COM, simply sit on top of one another with the PSBs on top, giving the appearance of forming a single antenna. The difference of 11.5 metres is the distance between the centre points of the two antennas (Ofcom use average height). Each antenna is constructed in halves to allow half-antenna running. Each panel is 1.09m in height - if they're using Radio Freqency Solutions' PHP panels - which would suggest 10 tiers, five tiers in each half antenna.
Sandy Heath's data is odd - Ofcom are listing SDN at the same height as the PSB muxes, which doesn't sound right (and the difference between the two is only 2.7m). It would certainly make sense to put 48/51/52 on a different antenna from 21/24/27. Arqiva did project a Class 2 antenna system, each antenna able to offer half-working (at 6 dB down) so I assume that the top 6 tiers are one antenna for PSBs (in 2 x 3 tiers) and the bottom 6 for the COMs.
The reason I say we don't know exactly what's in there is that at other sites, e.g. Winter Hill, the main antenna is a wrap-around of the main lattice structure - I assume RFS PHP12S which have 12 panels per tier - and at others, e.g. Hannington and Dover, the structure has five faces, e.g. RFS PHP5S. At Dover, only three of the five faces have panels fitted, to reduce the amount of power sent into France.
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Thursday 20 October 2011 5:24PM
C: Sky require a minimum £10 per month payment or they'll disable the recording functions on the boxes. It's quite difficult to find information about paying this fee, Sky would obviously prefer to extract the £20 basic subscription.
Ending your subscription also means you won't be able to time-shift, and you won't be able to play back anything you previously recorded either.
To get full recording features back without paying a subscription, you can buy a Freesat+ box as a drop-in replacement for the Sky box.
Channels that are broadcast without encryption don't require the viewing card, but some regional variants of ITV1, ITV1 +1, ITV1 HD and Channel 5 are encrypted - if you're in one of these regions, that channel will stop working if you remove the card. (Channel 5 has different advertising regions, the programmes are the same.) The region selection is based on the postcode that the viewing card was sent to. There are a couple of other channels that require a viewing card - see Compare Freesat and Freesat-from-Sky TV | ukfree.tv - independent free digital TV advice for the differences in the line-up.