All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
jamesanthony: The Belmont transmitter has just started switchover, and therefore the BBC service. It's possible you would have had the issue even if you hadn't retuned.
If you want to use the Nottingham transmitter, the aerial should be set up with its elements going up-and-down rather than side-to-side. There should be enough cross-polar rejection to ignore the signals from Belmont. For 'X' type aerials you want the Xs to be standing up rather than laying on their sides - narrow rather than tall.
Your box might be one which won't discard channels it thinks it already has - you will have to delete the BBC channels so that it actually stores the manual-tuning results. If your box doesn't have a delete feature you'll have to unplug the aerial, do a full reset, then plug in after the automatic scan has completed. Then do a manual scan on the frequencies above - note that right now, SDN is on C67, ArqA on C63 and ArqB on C49, at low power.
On 24 August, ArqB moves from C39 to C48 so you will need to retune. Then on 27 September, SDN and ArqA move to their final channels.
Belmont's version on BBC A is on C22, so you can't just plug the aerial in after the scan gets to a certain point.
You haven't said exactly where you are so it's not possible to say whether you'd get better results from a different transmitter.
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Mike Myers: The software here treats the groups as a hard cut-off. In fact, aerials still have some gain just above the official edge of the group and also just below - Ofcom have used this as part of their planning, though do note that the official edges of the aerial groups have already changed a bit, to accommodate Channel 5, so an older Group B might not work as well up at C56 than a newer one.
You can see some aerial gain graphs at ATV Stock Aerial Tests , which indicate that at least some aerials have as much gain at C56 as they do at C40.
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clive: Group C/D aerials still have some gain below the official group - there isn't a sharp cut-off as there is above the aerial group. See ATV Stock Aerial Tests for a comparison.
Whether or not you need a filter for 4G will depend on where you are and where the 4G transmitters using the 800MHz band are. As Tacolneston uses horizontal polarization, and 4G base stations are expected to use vertical polarization, your aerial is likely to pick up rather less 4G signals than if it were vertically-polarized.
Please check Digital UK's prediction to figure out if you really are in a weak signal area - if you provide a full postcode I can check this. Post-switchover digital signals will be substantially stronger than before. The coverage area could even be larger than analogue, because while the new digital maximum power levels are lower than analogue, they are relatively higher than at most other transmitters. Other sites use a power level one-fifth of analogue, Tacolneston will be two-fifths.
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Peter K: I recently disassembled one that I used to use (before finding articles on why boosters are useless). The active part of it - the bit that wasn't just converting AC power down to DC - was one transistor.
A transistor amplifier basically works by a small flow of current between two of the three pins on the transistor producing a much larger flow between two other pins. The larger flow is related to the small flow, but not linearly - there is an exponential relationship.
The current can also only flow in one direction, while an RF signal flows in both directions. To correct this, the signal is 'biased' with a voltage about half that of the power supply voltage, which means it never has to flow backwards; the amplified bias is then stripped off the output with a capacitor.
The bias is set so that the expected size of the input results in the right amount of gain, in an approximately linear region of the amplifier's transfer function.
Digital TV requires a very linear amplifier, it cannot handle much distortion at all. If the input is larger than expected, the response of the transistor amplifier is no longer linear. If the input is particularly large, the amplified signal can reach the voltage of the power supply at the top or the minimum voltage that the transistor will pass, which causes clipping - a severe form of distortion.
The distortion caused by this, when you have multiple frequencies in the input, is known as 'intermodulation'. The effect is to cause errors in other carriers in the same multiplex, and/or carriers in other multiplexes, which - if there are too many - causes failure to decode, either intermittent or total.
You can also get problems if the power supply starts to fail, particularly if the capacitors in it fail and the voltage to the amplifier circuit starts to 'ripple', as the power supply voltage - therefore the maximum output - will be periodically getting closer to the peaks of the output, and clipping them periodically.
Any electronic circuit also adds thermal noise, caused by the random movement of atoms when hot (random movement of atoms *is* being hot). What's important is the signal quality, the ratio between the signal level and the level of noise. All else being equal, adding an amplifier adds noise, reducing signal quality. Only do it if the signal is too low for the box to decode, although if this is the case, you're either too far away, the aerial doesn't have enough gain, or have some other problem in your aerial system. A booster might give an improvement if the receiver is particularly noisy, but usually the receiver front-end is better built than your average booster.
It is usually necessary to amplify if distributing the signal more than a couple of ways, as the aerial gain just isn't enough to offset the loss in the splitter and the extra cable length - in this case, amplify as close to the aerial as possible, just enough to offset losses in the splitter and cables, and ensure that the input to the amplifier is lower than the maximum input it can handle (without distorting).
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Bill Procter: I assume that you are trying to use the Sheffield transmitter rather than Emley Moor, on the basis that you mentioned a retune event this week. If you are, the aerial should have rods going up-and-down rather than side-to-side.
If using Sheffield, you can't use the aerial-out trick, because Belmont BBC A is on C22 and Sheffield on C27 - the other PSB multiplexes will be interleaved.
For now, use these frequencies:
BBC A - C27
Mux 2 - C63
Mux A - C57
Mux C - C45
Mux D - C42
Mux B - C60
You can ignore Mux B if you don't have Sky Sports on BT Vision or Top-Up TV, because all the BBC channels are already on BBC A.
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Jean Timms: The responsibility for providing a good aerial and connections to it lies with the building manager. See PARAS - Professional Aerial Riggers Against The Sharks for some thoughts on who to contact. Some part of the building maintenance fees, or rent, should be going to maintaining the communal aerial system - insist that it is maintained.
The Digital UK predictor does show that there are a number of possibilities for the local transmitter, though the most likely is Sutton Coldfield. If the system is not channelised - only amplifying selected channels - your box could decide to tune in signals from a different transmitter rather than the best one that the aerial is pointing at, if it encounters that transmitter earlier in the scan. Try doing a manual retune using the frequencies at the top of the page. If that works, check whether your box automatically retunes itself - some do. If so, turn that feature off.
If a manual retune doesn't correct the problem, ask the building management to get the system properly set up, so that there is sufficient signal at the socket(s) in your flat. If the analogue pictures have also deteriorated, it's likely to be a system problem, and if it's affecting all residents, it probably is the aerial, or an amplifier, or an amplifier's power supply.
If the system *is* channelised, it will need to be retuned at switchover on 21 September. The levels might need to be corrected after the BBC high-power multiplex starts on 7 September.
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Steve T: The commercial multiplex operators don't want to pay the extra cost of transmitting from the other 1,000+ sites in addition to the 81 sites they used before switchover. Ofcom have decided that their powers don't allow them to require the operators to do it (I disagree).
Ofcom are currently running a consultation on renewing the Multiplex C and D (Arqiva A and B) licences: see Ofcom | Television Multiplex Licence Renewals . Multiplex A was already renewed last year, so that ship has sailed for about the next 10 years.
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You will *not* be able to get high-power BBC digital from the Derby transmitter from the 17th. This transmitter is an oddity, it relays BBC One East Midlands and ITV1 Central East from Waltham to an area that is otherwise perfectly served by Sutton Coldfield. It is therefore horizontally-polarized. Because it doesn't transmit BBC Two, and the available frequencies are so tight in this area, there will be no changes here on the 17th.
Instead, on the 31st, BBC A replaces BBC One on its current channel (its final location), and D3&4 replaces ITV1 on its current channel. HD will not start up yet.
A retune event is scheduled for 5 October, when the HD multiplex will start up and D3&4 will move to its final location.
ITV1 analogue takes over BBC Two's slot, at all relays except Derby, so that the BBC A multiplex can start up on ITV1's old frequency. This means that ITV1 will appear on button number 2 on most analogue TVs. This has no effect on the TV's digital tuner or set-top box.
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timonthenet: Best bet for you is Postcode Checker - Trade View .
This shows that the *only* change at Waltham, for a digital tuner, is Mux 1 closing (from C49) and being replaced with BBC A on C61. No offset is given so this will be 794000 kHz. The new mode for BBC A will be 64QAM, FEC 2/3, 8K carriers, 1/32 guard interval.
Mux 1 carries: BBC One, BBC Two, BBC Three, CBBC Channel, BBC News, BBC Red Button (105) and 301.
BBC A will carry all of the above, plus: BBC Four, CBeebies, BBC Parliament, and the radio channels, BBC Radio 1, BBC R1X, BBC Radio 2, BBC Radio 3, BBC Radio 4, BBC R5L, BBC R5SX, BBC 6 Music, BBC Radio 4 Ex, BBC Asian Net., BBC World Sv.
For the two weeks between the 17th and 31st of August, those channels new to BBC A will continue to broadcast on Mux B as well.
The HD channels will start up on the new BBC B multiplex on the 31st, replacing Mux B which closes. All four, including the ITV1 and Channel 4 HD channels, are on the same multiplex. This broadcasts using DVB-T2, 256QAM FEC 2/3, 32K carriers, 1/128 guard interval. You may need a new tuner card or stick to get them. They will also be compressed with MPEG-4 AVC, aka H.264, not MPEG-2.
On the 31st, Mux 2 closes and is replaced by D3&4 on C54- , 738 MHz with -167kHz offset so 737833 kHz. Mux C closes and is replaced by Arqiva A on C56, 754000 kHz. Mux D closes and is replaced by Arqiva B on C57, 762000 kHz. All of these will use 64QAM 2/3 8K mode with 1/32 guard interval.
Sky Sports 1 and 2 move from Mux B to Arqiva B (former Mux D) at switchover. 303 closes.
Mux A stays on its current frequency and mode setting until 12 October, when it changes to 'SDN', switching to 8K mode to match the other four SD multiplexes. It will remain on the same frequency.
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Friday 12 August 2011 2:12PM
65 MPEG-2 streams on satellite would require 5 transponders if you crammed them in as tightly as the Freeview SDN multiplex is (11 streams in 24Mbit/s, 33.8Mbit/s available from a satellite transponder gives about 15 streams).
Unless, again, you're going to insist that potential viewers upgrade to DVB-S2, but I note that DVB-T was specified for the DTT service.
I suspect you'll be lucky to find 5 spare transponders
Local stations had better not want to use any music; PRS want a minimum of £16,500 for broadcast rights:
UK channels without a BARB rating
. You also have to pay PPL but I can't find a price on their website. That's only if broadcast in the UK, so those transponders are going to have to be found on Astra 2D or 1N UK beam - you could blow your whole budget on music licences if the channel ends up on a Europe-wide beam.