All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Richard: It's likely that you have too much signal. The Philips box's automatic gain control probably can't handle as much signal as the other devices, and is therefore distorting - making the later decoder stages unable to interpret the results.
16 dB is too much gain unless you're splitting the signal more than 8 ways. The estimated strength on that channel at that distance is 75 dBuV from the aerial before amplification, though you should expect less than that if you don't have clear line-of-sight. The CAI recommended 45-65 dBuV before switchover and 60-80 dBuV for analogue. The upper limit is there to avoid distortion - digital signals cannot cope with any distortion, while the artifacts on analogue (herringboning) are not all that noticeable.
If you're not getting that level before amplification, you need to try to find a different location for the aerial where it does.
The signal strength meter on most boxes is poorly calibrated, and there is no standard (as far as I know) in the DTG D-Book specification for what signal level means what. In some cases where distortion is occurring, the box reports an incorrect level.
It's best to set up your system so that there is just enough gain from any amplifier to offset the loss through splitters. Generally you're looking at 4 dB loss for a two-way split, then another 4 dB for each doubling of the number of connections, though check the splitter itself to see what it says, if you're using one - it's not clear that you are.
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Peter: Milton Keynes is a bit of a special case. As a new town, it was cabled throughout by the GPO, therefore there was no need for a relay transmitter for the parts that Sandy Heath didn't reach. As BT was privatised the service fell into their hands, but they were forced to lease it, which apparently they did in such a way that makes it an anomaly in Virgin Media's network, an analogue cable system that can't be digitised.
The terrestrial broadcasters haven't done anything to compensate for this. Their only requirement for DSO is for the predicted DSO coverage to match the predicted analogue coverage percentage.
However, as I said before, the prediction for *your* postcode is for a high probability of reliable service. The most likely explanation is that the monkey who installed the aerial for the developer used too big an aerial, setting the levels too high in the first place, and it's now overloading. Try adding an attenuator.
If it's a communal system, sharing an aerial among multiple houses (some developers do this), it may need to be adjusted, again so that the levels are correct. It could be a channelized system, and might need retuning, although the PSB multiplexes and ArqB all use frequencies that were previously used before DSO.
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Peter: I should add that for developer jobs, it's common for the fitter to use the cheapest possible aerial (contract type) and cables, and sometimes the developer gets the regular electrician to put the RF cables into the walls, to reduce the labour charges. RF cables should be treated very carefully, with a limited bending radius, because they are waveguides. If you kink the cable, or just bend it too tightly, it stops working properly. Power cables can be bent at much tighter angles, so electricians aren't used to taking this degree of care over routing.
You may end up having to have the whole lot ripped out and replaced by someone who actually knows what they're doing.
If the cable from the aerial terminates at a wall socket, make sure it's a properly screened type, e.g. Online Satellite/TV/FM/DAB Cable, Leads and Connectors sales. , and use a decent fly lead such as Online Satellite/TV/FM/DAB Cable, Leads and Connectors sales. .
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James Tobin: See BBC HD to switch to DVB-S2 on 6th June 2011 | ukfree.tv - independent free digital TV advice .
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Ian: No, the dish remains the same whichever UK satellite service you use. The only reason to change it is if you want to get non-UK services broadcast from different satellites.
A freesat-branded box will automatically find all the services for you and provide a full electronic programme guide. Other free-to-air boxes are available, but you have to manually tune in every service that you want to watch. Some may support automatic scanning but won't necessarily put the channels in a consistent order in the channel list.
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Bill Marshall, Kate Mooney: Check that your box or TV is not one of those that does not support the 8K mode. TVs and boxes that do not support the 8k-mode | ukfree.tv - independent free digital TV advice . Make sure that you're doing a full reset (also called full retune, first time installation or default setting) because some boxes won't store channels that they think they already know about.
Kate Mooney: If you had reliable Freeview before this started, it could also be that the signal levels are now too high and overloading the equipment. If you have a booster, try without it. If that doesn't help, or you don't have one, try adding an attenuator.
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Hugh: What's your full postcode?
I'm guessing that you're on a relay transmitter that started switchover today. So far, only the BBC multiplex - which does only carry 9 TV channels - has been switched on. The STV, ITV, C4 and C5 services will start in two weeks at the second stage, when analogue STV and C4 are switched off.
If you want to use a Freeview recorder as well, the aerial cable should go to the recorder first, then link the RF out connector on the recorder to the TV's aerial socket. You should still link the recorder to the TV using a SCART cable, because very few Freeview boxes add their output to the RF cable, and in any case the picture quality is better.
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Richard Wiseman: The BBC's interactive stream on Freeview (301) moved from Mux B to Mux 1 about a month ago. If you haven't retuned then it won't work.
I'm afraid I can't find any support documents for this box. Check for a DTV Setup button on the remote?
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Stewart McKellar: BBC Two is switched off first at all sites, but in some cases the BBC A multiplex has been allocated a different channel, so whatever service was using that channel moves to BBC Two's old frequency.
In general there are only two steps: in step 1, BBC A replaces BBC Two (though, as I said above, not necessarily directly), while in step 2, all other analogue services are shut down and all the replacement digital services start up.
At main transmitters there can be more steps, because they have to fit six services in where there used to be only four, and the commercial multiplexes can't always start up on their final channels at 'Step 2'. At a very few relays, there are subsequent retunes where there's a clash with some other interim service.
Finally, channels 61 and 62 have to be cleared for 800 MHz mobile phones, which might require subsequent retunes at sites using those frequencies, or a frequency that another site has to move to (typically C39, C40, C49 or C50). This doesn't affect you as all three multiplexes are at the bottom end of the spectrum.
There will not be any subsequent step to add the commercial multiplexes to this relay. The broadcasters don't want to serve any more sites, for reasons of cost, and the regulator has decided not to force the issue (it's debatable whether they can legally do so).
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Wednesday 8 June 2011 12:22PM
Lauren: The most likely explanation is a broken or disconnected cable, or that the aerial has rotated.
If there is a booster or amplifier in the system, it's also possible that the power supply has failed.
The aerial should point more or less due south to use Emley Moor, which is the most likely transmitter. There is a local relay at Beecroft Hill which does not yet transmit digital signals - after switchover in September it will be a 'Freeview Lite' transmitter, only transmitting the PSB services.
You have a very high probability of reliable service already on everything but the early HD service, so boosters should not be necessary if the aerial and cables are in good working order.