All posts by Steve Donaldson
Below are all of Steve Donaldson's postings, with the most recent are at the bottom of the page.
Nick : I have made further observations in respect of the Chalford relay.
At 700MHz Clearance, on 5 June 2019, the Chalford relay doubled ERP from 25W to 50W. At the same time, the positive offset on C21 (PSB3) was removed. All three channels, 21 (PSB3), 24 (PSB1) and 27 (PSB2), currently have no offset. At the same time, the ERP of Chalford Vale transmitter increased from 3W to 5W. I guess these power increases occurred to improve coverage rather than to counter possible CCI.
Ridge Hill has positive offset on C21 (COM4), while C24 (COM5) and C27 (COM6) are centre-channel. There was no change there. It has been like this since switchover.
If a receiver is presented with another, weaker signal co-channel with the wanted one, will one having offset and the other not (or one having positive offset and the other negative offset) mean that it will have more difficulty making sense of the signal than were they both to have the same offset, or both be centre channel? Does the fact that they aren't exactly overlayed potentially present more difficulty interpreting the signal?
The same question may be asked for signal mode, and in terms of DVB-T with DVB-T2 interferer and vice versa.
Why do some transmitters have offsets and some don't? What is the purpose of them?
If Chalford is a source of interference here, then it may be visible on the spectrum analyser below the frequency of the C21+ signal of Ridge Hill. The Ridge Hill C21+ signal is +0.167MHz with respect to Chalford's C21.
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Nick: The bandwidth the signals of Ridge Hill C21+ and Chalford C21 are not exactly overlayed, one on top of the other. I was wondering whether might be visible on the spectrum analyser.
The bandwidth of a DVB broadcast is 8MHz. The centre of channel 21 is 474MHz. Thus, a signal centred on the channel (as with Chalford) will begin at 470.000MHz and end at 478.000MHz.
The Ridge Hill C21+ has positive offset of 0.167MHz, hence it starts at 470.167MHz and ends at 478.167MHz.
I wondered whether the Chalford signal was visible between 470.000MHz and 470.167MHz.
In addition to the questions I posed in my previous comment, questioning the reason for offsets, will a C21+ signal not interfere with one on C22? Presumably it will, hence why Ridge Hill C22 (PSB3) also has a positive offset.
Another question is: could it be that the positive offset of Ridge Hill is the problem? What if Ridge Hill had been negative offset and Chalford centre, or Ridge Hill centre and Chalford positive offset (i.e. Ridge Hill comes first in order of ascending frequency)? Might the situation have been different in terms of the receiver being able to make sense of the Ridge Hill signal, even in the presence of the Chalford one?
Or, has the offset got nothing to do with it and it just so happens that the ratio of the strength of interfering Chalford to the wanted Ridge Hill at the receiving aerial is greater for C21/C21+ than for C24 and C27? Or is it to do with the fact that Chalford C21 is DVB-T2?
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Michael: Both Freeview and the BBC are reporting no faults or engineering with Belmont, although this isn't a BBC multiplex, so the BBC is not likely to report on it.
The most obvious thing I can think of, other than a transmitter issue, is that some vegetation has been moved by the storm, vegetation such as a tree branch, which is somewhere in front of your aerial and therefore casting a shadow with respect to the TV broadcasts from Belmont.
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Nick : I know, you've gone much deeper into it than most viewers.
If you refer to the Chalford page on this website you will see the radiation pattern for it. There are two lobes, one roughly east north east, which gets the full 50W, and; the other is roughly west north west which is narrower and 9dB down, and which is pointing in your direction.
Chalford is on a bearing of 307deg out from your road.
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Felixstowe PSB1 is listed as having positive offset, C33+, above. The 700MHz Clearance spreadsheet from Ofcom, the 700MHz Clearance Events sheet for the Anglia region from Digital UK and the Freeview Detailed Coverage Checker all show it does not.
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nick : As per my comment on the Felixstowe page, I believe the positive offset on C33 (PSB1) is a mistake. The Freeview Detailed Coverage Checker does not show any offset on any of the three channels of Felixstowe.
Additionally, the 700MHz Clearance spreadsheet from Ofcom and the 700MHz Clearance Events sheet for the Anglia region published by Digital UK does not show offset.
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Alan Clerk: BBC Reception [bbc.co.uk/reception] carries reports on the two BBC multiplexes, PSB1 (BBC A) and PSB3 (BBC B). At the present time, for Eyemouth it has these four entries:
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Timestamp: From 4:00pm to 4:13pm on 27th Oct 2023
Multiplex: BBC A
Status: Off the air due to a fault
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Timestamp: From 10:15am to 4:00pm on 27th Oct 2023
Multiplex: BBC A
Status: Off the air due to essential engineering
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Timestamp: From 4:00pm to 4:13pm on 27th Oct 2023
Multiplex: BBC B HD
Status: Off the air due to a fault
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Timestamp: From 10:15am to 4:00pm on 27th Oct 2023
Multiplex: BBC B HD
Status: Off the air due to essential engineering
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According to this, it was off air for 13 minutes from exactly 4 o'clock this afternoon. It does say that this was "due to a fault", but judging by the fact that there are also reports of "essential engineering" it suggests there are engineers on site and we might speculate that they needed to turn it off for the short period.
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Tony1234: Yes, Wenvoe is broadcasting TalkingPictures TV (which is on logical channel number 82).
"Logical channel number" or "LCN" is the number you dial on the remote to bring up a programme channel. It being a "logical" channel number to distinguish from the UHF channel number, which is the broadcast channel, in this instance 39 from Wenvoe, or 36 from Mendip.
Freeview has published a list of programme channels here, and this identifies which multiplex each is on:
Channel listings for Industry Professionals | Freeview
Here is the list of the six main multiplex designations and their respective name (which you will see on the Freeview page I linked to):
PSB1 = BBC A
PSB2 = D3&4
PSB3 = BBC B
COM4 = SDN
COM5 = Arq A
COM6 = Arq B
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nick : No. A multiplex is either minus offset, centre-channel or plus offset. Most transmitters are centre-channel, meaning no offset.
See the entry "Offset" under Glossary here:
Digital television transmitter details - Ofcom
This is what it says (I have added the double quotes because I think it makes it clearer):
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Offset: A "+" or "-" indicates that the frequency is offset by +0.167MHz or -0.167MHz with respect to the channel centre.
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Take C33 as an example. The centre of the channel is 570.000MHz. The bandwidth of a DVB broadcast is 8MHz. Therefore:
- a signal which is centre channel (has no offset) spans from 566.000MHz to 574.000MHz;
- a signal which is negative offset spans from 565.833MHz to 573.833MHz, and;
- a signal which is positive offset spans from 566.167MHz to 574.167MHz.
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Saturday 21 October 2023 6:50PM
Pete Howell: The Kings Weston transmitter does not transmit BBC and ITV Wales, which means if you are receiving these then your TV is tuned to another transmitter, probably Wenvoe. It is worth checking that all the channels are tuned correctly, even those which do not vary by region, in order that you have reception from the designated transmitter, and not another which your aerial might not face and which (in the case of Wenvoe) it is at opposite polarity (your aerial is vertical for Kings Weston whereas for Wenvoe it should be horizontal).
The channels for Kings Weston are:
PSB1 - BBC A - BBC One: C43
PSB2 - D3&4 - ITV: C40
PSB3 - BBC B - BBC One HD: C46
COM4 - SDN - QVC: C22
COM5 - Arq A - Sky Mix: C25
COM6 - Arq B - Quest: C28
Check the UHF channel number on each of these six programme channels to see that they are tuned to Kings Weston.
The six channels for Wenvoe (in the order of multiplex, as above) are 41, 44, 47, 42, 45, 39.