All posts by Dave Lindsay
Below are all of Dave Lindsay's postings, with the most recent are at the bottom of the page.
Mike O'Pray: TV is broadcast on (UHF) channel numbers and these go from 21 to 69. They equate to frequencies. So 21 is a lower frequency than 41, for example.
The channel numbers are divided up into three Groups "A", "B" and "C/D" which are roughly the first third, middle third and top third of the band used for TV. For a table of which channel ranges relate to which Group, see here:
Aerials, TV Aerial and Digital Aerial
Back in the day of four-channel analogue, each transmitter broadcast on four channels that were all within the same Group. Sandy Heath used four channels in Group A (21, 24, 27, 31) and Waltham used four channels in Group C/D (54, 58, 61, 64).
In order to join the two leads from your aerials together a diplexer was needed; they can't just be connected together. A diplexer filters the feed from each aerial. See here for some examples:
Online TV Splitters, Amps & Diplexers sales
Suppose that your diplexer "splits" at channel 36 (C36); therefore it allows C21 to C35 through from your Sandy aerial and C37 to C69 from your Waltham aerial. This worked fine with the former analogue channels mentioned above.
Due to the channel allocations of Sandy Heath and Waltham now, such a setup will mean that some are unreceivable.
Sandy Heath now uses channels 27, 24, 21, 51, 52, 48
Waltham now uses channels 61, 54, 58, 29, 56, 57
I'll come on to which carries what in a moment. But hopefully you can now appreciate that if your diplexer is as suggested, you will be unable to receive 51, 52 and 48 from Sandy and 29 from Waltham. All the other channels would be expected to come through.
For some transmitters, including these two, "out of group" channels have had to be used for Freeview. That is, some channels are outside that of the Group used by the former analogue. This is because there aren't enough spare ones, now that six are needed instead of four.
Digital channels carry a number of services whereas previously one analogue service occupied one UHF channel. See the graphic under the heading "After switchover configuration" on this page (you may need to refresh by pressing F5 to get the graphic to show):
Freeview multiplexes | ukfree.tv - independent free digital TV advice
You will see that there are six groups or multiplexes and each occupies one UHF channel:
PSB1 = "BBCA" (BBC One, BBC Two, BBC Three etc)
PSB2 = "D3&4" (ITV1, ITV2, Channel 4, Channel 5 etc)
PSB3 = "BBCB" (BBC One HD, BBC HD, ITV1 HD, Channel 4 HD)
COM4 = "SDN" (ITV3, ITV2+1, 5* etc)
COM5 = "ArqA" (Pick TV, Dave, Really etc)
COM6 = "ArqB" (Yesterday, Film 4, 4Music etc)
The six UHF channel numbers that I gave above for Sandy and Waltham those which these six multiplexes broadcast on in that respective order.
If your aerial diplexer is as I suggested, then you could theoretically receive the three PSBs from Sandy and the three PSBs from Waltham. You could only receive COM5 and COM6 from Waltham, but you won't be able to receive COM4 from either because they are outside of the range of channels that the diplexer allows through for each respective aerial.
On the 9th May there was a change to the channel used by one multiplex at Sandy Heath and that was COM4 which was on C31 up to that date. This was within the channel range allowed from your Sandy aerial which probably explains why you could pick it up then.
Receivers usually give the UHF channel number that they are tuned to on the signal strength screen. You may be able to ascertain what transmitter(s) the Panasonic is tuned to for which multiplexes. I suggest that you focus on the first service from each multiplex, i.e. BBC One, ITV1, BBC One HD (if applicable), ITV3, Pick TV and Yesterday.
My guess is that Pick TV and Yesterday will be coming in on 56 and 57 respectively, which means that you are receiving them from Waltham.
If there are duplicates, then you will find them in the 800s. So you will probably have duplicate PSBx services. The signal strength screen should allow you to identify which is which.
Perhaps you are receiving the PSBs from Sandy and COM5 and COM6 from Waltham (with Waltham's PSBs being in the 800s). Therefore your regional programming will be BBC East/ITV Anglia.
As for why the Goodmans hasn't tuned-in as many services, perhaps it doesn't cope too well with signals from different transmitters or perhaps during its automatic tuning sequence it stores channels from only one transmitter. This is speculation on my part and a clearer picture of what's happened may be gained through some investigation. If the Goodmans gives the tuned UHF channel number on the signal strength screen you will be able to piece together what it is picking up. Perhaps it only has the two (standard definition) PSBs from Sandy tuned.
If it has manual tuning, you may be able to add missing channels. However, before you do this, I suggest that you use the Panasonic to find out what it is that you are receiving; are COM5 and COM6 coming from Waltham? If they are and the Goodmans has a manual tune function, then you need to try and manually tune C56 and C57.
I suggested in an earlier posting that a possible solution may be to turn your Waltham aerial to face Sandy Heath. Obviously you will no longer be able to receive BBC East Midlands/ITV Central from Waltham, but the UHF channel spacings (if you wish to receive COM4) have probably put paid to that anyway.
My reason for making this suggestion is down to the likelihood that your aerials will be "Group" ones rather than widebands. That is, they will be designed with maximum sensitivity on channels within their respective Groups only. I think that we can safely say that this is so as they are 20 years old.
This is why I think that using one or the other may not work satisfactorily. For example, if removed the diplexer (and therefore Waltham aerial) from the equation by connecting the Sandy aerial directly to your downlead, then being a Group A aerial, it may not be sensitive enough for the high Group C/D channels that Sandy's COMs operate at.
There will be many people who will only have a single Group A aerial and who will now have to replace it with a wideband one in order to receive the COMs.
However, as you have another aerial that works at the desired channels (48, 51 and 52), I suggest that the cheapest solution may be to turn the C/D (Waltham) aerial to face Sandy.
Whether this will work hinges on what channel the diplexer splits at. If it is as I suggested above (36), then it should work. If, however, it splits at C51 (for example), then obviously it's not going to work. It needs to split below C48.
It is also to be hoped that the securing bolts can be undone!
I respect jb38's comments as he's a professional and I'm not. However, I suggest this as being a possibility, primarily because it seems to be the cheapest option.
Whilst there is signal loss with using the diplexer (and I couldn't say to what degree), it would appear that you picked up COM4 (via your diplexer) prior to 9th May when it was on C31 and radiated at a transmission power of 20kW. Now it is at 170kW, so you have a much stronger signal to play with.
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Harri Ackone: The most likely and most ideal solution is probably to switch your aerial from horizontal to vertical.
See the postings here from and in response to Brian Smith:
ITV 3 | ukfree.tv - independent free digital TV advice
As for why one might be unable to cope with the difference in signal levels where others will can, not all are the same. There are differing factors:
1. The sensitivity/tolerance of the components in the tuner.
2. The loss in the cable and connectors from the point that the signals are distributed.
You could try swapping your receivers from different aerial points, as a test. Before you move any, observe the signal strength (where they give such information) and see if the signal is shown as being stronger at another outlet.
If the Sony fairs better in another room, then maybe a bit of attenuation will lower signal levels and provide a more stable result.
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Gareth: According to mb21, it is located at TQ374776.
This resolves to the junction with Creek Road and Deptford Church Street. Other than that, I haven't found any information online as to what channels and polarisation it uses.
Perhaps Ofcom could advise you.
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Beverley Hampton: See Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice
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Harri Ackone: I would like to clarify my last sentence of my earlier posting. There is no guarantee that attenuating will work. It is a try it and see sort of thing.
An attenuator reduces signal levels and an amplifier increases them. If you have an amplifier which splits the feed from your aerial to different rooms and it has a variable control on it and it is accessible, then you should try reducing the level of amplification before introducing an attenuator.
An attenuator connects in line with the aerial lead. Variable ones are available for £3 or £4 online from souces such as eBay and Amazon. One such example is here:
1-20dB variable digital tv aerial signal attenuator | eBay
The problem is that the signals that you are having difficulty with are weaker horizontally than the Public Service channels (BBC, ITV1, C4, C5 etc). Vertically they are the same. So you have a marked difference in strength.
Resolution of a picture with digital signals works or it doesn't work. There is a threshold known as the "cliff edge". So a slight change in signal (which happens all the time) has meant that the services are viewable again. Therefore, I wouldn't be surprised if it goes again.
Therefore, for reliable reception you need sufficient signal so that the natural slight changes do not leave you without enough (at some times).
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John: Have you confirmed that it is Waltham that you are tuned to for the BBC services? The signal strength screen should tell you what UHF channel you are tuned to and for BBC One etc it should be C61.
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Jim Bell: Don't read too much into coverage predictors; use them as a guide only.
The fact that you have had two different results suggests that the method of calculation is different or has changed.
Digital UK Tradeview predictor puts Gartley Moor a little behind Durris.
However, Durris is a full service transmitter and Gartley only carries Public Service (PSB) multiplexes/channels, so you will probably wish to receive from Durris, even "if" relatively speaking, it does not provide as good a reception as Gartley at your location.
Durris uses Group A channels and Gartley uses C/D ones, so if you have a Group aerial, then it will be less sensitive for one.
The three Commercial (COM) channels from Durris (those that Gartley doesn't transmit) are at half transmission power to its PSBs. The COMs also use a less robust transmission mode (so that they can fit in more services). What this means is that if you are going to have difficulty picking up any channels from Durris, then it is likely to be the COMs.
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Jim Bell: This plot to Durris shows that your line of sight is blocked roughly between 5 and 6 miles away:
Terrain between ( m a.g.l.) and (antenna m a.g.l.) - Optimising UK DTT Freeview and Radio aerial location
Why are you asking about re-aligning your aerial anyway?
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Jon D: I posted previously to you about checking that the receiver is tuned to Sandy Heath channels rather than Crystal Palace (or any other transmitter):
Freeview on Sandy Heath TV transmitter | ukfree.tv - independent free digital TV advice
You should confirm that you are tuned to Sandy Heath by bringing up the signal strength screen on the following and checking that they tuned correctly:
BBC One = C27
ITV1 = C24
BBC One HD = C21
ITV3 = C51
Pick TV = C52
Yesterday = C48
If they all show 100% quality and they remain so, then you have nothing to worrry about.
For digital reception, so long as the signal strength is above the threshold needed to resolve the picture, and it remains so, then the picture quality is the best that it can be.
Not all the strength scales (shown on the screen) are the same so I wouldn't go comparing expecting them to be the same.
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Thursday 17 May 2012 11:02AM
Andrew Lees: Yes but you will obviously need a terrestrial aerial for the Freeview receiver; the satellite dish won't work for it.