All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.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.
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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.
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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.
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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.
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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.
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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 .
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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.
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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!
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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!
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Monday 20 June 2011 2:41PM
Dorking
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.