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All posts by Mike Dimmick

Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.

Ian: I wish Brian would take down Mux 7 and 8. They were only hypothetical, possible assignments in an Ofcom study on 'interleaved spectrum'. Some sites had two possible frequencies, some had one.

They're not licensed to anyone, Ofcom haven't even set a date for auction of any frequencies, so we're a long way from knowing if there will even be a service and what frequencies it will be on.

At some sites, these two erroneous bits of data are actually hiding real allocations, although usually temporary ones.

Since the study was done, European countries have collectively decided to flog off channels 61 and 62 (about 800MHz) for mobile phones, which means the whole thing has to be replanned anyway.

PSB3, or BBCB, will be the high-definition multiplex after switchover. The BBC's other services all move to BBCA, which gains 6Mbit/s, one-third, extra capacity over Mux 1. This isn't really enough for all their services, but the BBC have closed most of the services that wouldn't fit.

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illiad: looks fine to me in IE9 Beta, assuming you're talking about this page, titled "Freeview on the Crystal Palace transmitter". What browser are you using?

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Tay Bridge (Fife, Scotland) Freeview Light transmitter
Tuesday 18 January 2011 5:50PM

jimgordon: They're not public service broadcasters in the Broadcasting Act sense of the term.

The BBC were granted the BBCA multiplex as part of their Royal Charter extension in the mid '90s. They're only allowed to host BBC channels on it.

The Broadcasting Act 1996 requires that the second multiplex is run by the Channel 3 companies collectively in partnership with Channel 4 Corporation (a statutory corporation), and must run the local Channel 3 franchise (STV North) and Channel 4. The 2003 Communications Act mandates that they also host S4C in Wales and Channel 5. Apart from the parts reserved for S4C and C5, and the public teletext licence holder, all the remaining bandwidth is only allowed to be used by channels run by Channel 3 companies and C4C, unless prior permissions is sought from and granted by Ofcom.

Multiplex B was auctioned, originally operated by onDigital, which became ITV Digital, then collapsed in 2002. The BBC won it in the subsequent re-auction, but it's licensed under the same terms as the other multiplexes as if the BBC were a commercial organization. Ofcom then required that it was converted to HD mode and carries any of Channel 3, Channel 4 and Channel 5 that want to provide an HD service.

The other three multiplexes: Mux A was originally required to be auctioned to one of S4C, Channel 5, or any joint venture of the two. S4C applied, Channel 5 didn't - SDN stood originally for S4C Digital Networks. S4C subsequently sold the multiplex to ITV plc in 2005. Ofcom then mandated Digital 3 & 4 Limited (see above) to carry both channels. Mux C and D were originally onDigital and re-auctioned after ITV Digital collapsed, being won by Crown Castle International (the BBC's former, privatised, transmission department). Through a series of takeovers and mergers they've ended up with Arqiva.

Ofcom requires that public service broadcasters reach the same proportion of the population as analogue was estimated to do. When ITV plc and Arqiva asked, in planning for switchover, if they intended to serve any more transmitters, they said no, citing high costs and low benefits. Presumably their customers, the channels on their multiplexes, were consulted first.

Russia Today and Al-Jazeera weren't running at the time. Even if they had been, they might have been out-voted by the other customers, or decided it wasn't worth it. And they're not allowed to move to a multiplex that is broadcasting from Tay Bridge.

Before their merger into one company, the two transmitter site owners NGW and Arqiva were held to have significant market power (a duopoly), and were forced to each publish a Reference Offer for transmission services. Arqiva were going to charge £12.5m to PSB multiplex operators and £5.5m to commercial multiplex operators. NGW quoted £12.25m to PSBs and £4.8m to COMs. A 40% increase in costs is pretty significant to many of these channels.

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Lark Stoke is currently a 6.3 kW transmitter according to Ofcom, and also Wolfbane's list of transmitters. I don't know where you got 25W from. It gets a reduction to one-fifth at switchover like the vast majority of sites.

25W would be very powerful for a compact fluorescent lightbulb, but very weak for a conventional incandescent. It's actually a good indication of how electromagnetic wave transmission drops off with increasing frequencies, as light is just an extremely high frequency radio wave (500 or so terahertz, one million times faster than TV signals). 25W at UHF TV frequencies can carry quite a long way - by the ITU-R estimation formula, with clear line of sight and assuming no interference or fading, you'd still get enough signal for digital TV more than 70km from the transmitter. For perfect analogue you'd have to be within 5 km as it requires so much greater signal-to-noise ratio.

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Brian: Stuart was clearly talking about analogue, not digital, services. I missed that you were talking about the current low power digital service. The Wrekin tests clash with BBC Two analogue at Lark Stoke, on C26, and ITV1 analogue, on C23 - both 6.3 kW. They don't clash with any current digital services from Lark Stoke.

Unless they're *actually* testing the Single Frequency Networks that will form between The Wrekin, Lark Stoke, and Bromsgrove, in which case I would expect LS to be temporarily broadcasting the same test signals as The Wrekin, at 1.26 kW.

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Hannington (Hampshire, England) transmitter
Thursday 20 January 2011 11:56PM

Ann - sorry to come back two weeks later! It's actually Mux 2 and A which use the 64QAM 2/3 mode. This mode requires *more* signal, relative to noise, than the 16QAM 3/4 mode used by multiplexes 1, B, C and D.

ITV and S4C didn't change modes on their multiplexes in 2002, after ITV Digital collapsed and the BBC-led Freeview consortium took over their previous multiplexes. The BBC, operating Mux 1 and B, and Crown Castle, who got Mux C and D, were encouraged to change mode to improve coverage and reliability, at the cost of losing one-quarter of the capacity.

For most people, multiplexes 2 and A therefore are the least reliable, but it does depend on the clashes between channels. As you say, you're very close to Horndean. While it uses a different polarization - vertical rather than horizontal - the aerial does still pick up some signal from the other polarization. The spec sheet may refer to 'cross-polar rejection', which is the difference between the amount of signal picked up when the aerial matches the transmission, and when it doesn't. Horndean's not very powerful but you're only half a mile away, whereas you're nearly 15 miles from Midhurst and there are two hills in the way, completely blocking line-of-sight. Horndean frequencies clash with Midhurst multiplex 1, A and C. After switchover Horndean multiplexes will clash with Midhurst multiplex A and D.

Digital UK's predictor algorithm allows for 16 dB of cross-polar rejection, and 16 dB of rejection of signals from another direction, but oddly caps the total contribution from both sources at 16 dB. Most real aerials have at least 20 dB of cross-polar rejection and also have good directional response, so DUK's prediction may turn out to be too pessimistic.

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billy: no, I'm afraid that Sky require you to keep paying to get access to anything you have previously recorded. For the SD Sky+ box you have to pay the minimum £10.25 to keep recording and playback functionality, I'm not sure if the Sky+HD box requires you to keep paying the extra £10 HD fee.

You also lose the ability to pause live TV, as I understand it.

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Hannington (Hampshire, England) transmitter
Sunday 23 January 2011 11:28PM

GStrickland: looking at your location, I'd say the problem is that the line-of-sight signal path from the transmitter is very, very close to the hills. It may be obstructed at a couple of points. There's a strong chance of getting reflections that weaken the signal.

However, Digital UK's predictor does predict very good results both now and after switchover, so there could be another issue, some form of electrical interference from streetlighting or neighbouring houses. You're pretty close to the transmitter so even a reduced signal should be relatively strong.

There is absolutely nothing you can do if the issue is that the signal is being attenuated too much by the terrain and/or by reflections from terrain or structures. If it's nearby structures you could try raising the aerial. The estimated coverage level of terrestrial transmission is 98.5% of the population; the other 1.5% have to make their own arrangements. As I say, though, DUK (which is a front for the broadcasters) reckon that you are covered.

The man from Ofcom should have investigated other possible sources of interference. I believe they can require the owners of any interfering equipment to get it fixed.

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All Banbury residents having problems with Bretch Hill: go to BBC - Help receiving TV and radio and enter your postcode under 'Diagnose your problem'. Follow the wizard through, selecting the appropriate options: I picked 'Lost BBC Channels', 'Television', 'Analogue'. Once it lists the transmitters (and tells you there are no problems), select 'No' next to 'Did this answer your question', and it will take you to another page, where you can report the problem.

It could be that they've made a mistake in setting up the new equipment. This site and some others say that all channels are 87W but Ofcom's analogue transmitter page says it's supposed to be 348W for BBC Two, four times stronger (or 6dB). This could be to allow for the fact that channel 48 is used for a digital multiplex at the Oxford transmitter.

I don't know why a power cut would have corrected the problem temporarily, though, unless it cut off some other source of interference. (RG47SH)

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GB flag
Upgrading from Sky to Freesat | Freesat
Monday 24 January 2011 3:14PM

Pete: to get the full range of channels on the Freesat box, you will need to run at least one new cable from the dish to the box. You should use a separate output on the LNB - the box on the end of the dish arm - rather than splitting any existing cables.

Some Sky boxes have a 'dish output' connector on the box. This usually allows another box connected to it to have full control of the dish when the Sky box is turned off, but only allows the other box to select the same range of channels as the one the Sky box has selected when it's switched on. It's better to run separate cable to the dish.

Sky normally install a 'Quad LNB', with four outputs, and connect two of them to the Sky+ or Sky+HD box. The other two are to allow for easier multi-room installation. If you already have multi-room you might need to replace the LNB with an Octo-LNB, 8 outputs.

Technical detail: the satellites broadcast channels in both Horizontal and Vertical polarization, and in a Low and a High frequency range. Signals from the box, carried on the input cable, tell the dish whether to select Horizontal or Vertical, Low or High, for that cable. If two boxes are connected to the same cable, or two inputs on the same box are, the LNB gets confused about what it's being asked to do. The result is that channels stop working on one or other box, seemingly at random, depending on what channel the other box is tuned to.

If you use the 'dish output' on one box to connect to the other, usually it blocks the H/V and Hi/Lo signals from the other box when it's on, but allows them through when it's off. This means again that the channels that work on the second box depend on which channel is selected on the first box. It's better than nothing if you can't run another cable, but another connection is preferred.

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