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Single frequency interference

What to do when several channels are missing, but they are all on the same 'multiplex'

What to do when several channels are missing, but they are all
published on UK Free TV

There are several different type of interference that cause problems. This page deals with the situation where:

There are two main causes of this type of intermitted interference: The current Freeview mulitplexes are shown in this diagram. If all of your missing channels are on a single row, you have "single frequency interference":

Local analogue interference

Local analogue interference is caused by household video devices that you can attach to a television such as:
  • Sky Digibox;
  • Video cassette recorder (VCR);
  • Personal digital video recorder (PVR);
  • Games console;
If you have any of these devices, you will need to work out which of these is blocking the reception of a Freeview multiplex by turning off the suspect device and then rescanning for channels on the Freeview box.

If powering off the device restores the Freeview channels from the blocked multiplex, you will need to either stop using the problem device's RF output by disconnecting the RF output cable permanently, or by changing the RF channel number that that device uses. See, for example, How do I change the RF output channel on a Sky Digibox?

You can find a list of the frequencies that are not in use at your location by putting your postcode into the My Freeview reception and looking under the "Free RF channels" section.

Non-wideband aerial in some areas

In some places you will need a wideband aerial to receive all the Freeview multiplexes. For example in Central Birmingham, the Sutton Coldfield has multiplex D on an 'out of group', so you cannot view Film4 or ITV2+1 without an aerial upgrade.

To find out if you need an aerial upgrade, see My Freeview reception.

Help with Freeview, aerials?
How do I get a test card with Freeview1
I would like to know if it is possible to receive UK terrestrial Freeview servic2
I have been told I would receive too much singal from my Freeview tansmitter as 3
Can my Freeview box receive more than one BBC and ITV region?4
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In this section
Official aerial installers guide to the TV spectrum future1
Which free digital TV system will give me the most reliable reception?2
High pressure causing channel loss through "Inversion"3
Digital Region Overlap4
Freeview reception has changed?5
Two frequency interference 6

Friday, 27 April 2012
2:42 PM

Dave Lindsay: Thanks for your reply - I will try some of the alternatives you suggest, but I'm confused if it is a signal overload judging from my signal information - two of the multiplexes have excellent signal quality, and the three that all have channels with static scrambled images all have poor quality. Would the signal vary for each multiplex coming from the same transmitter? If I were being overloaded wouldn't every multiplex be scrambled?

My aerial is a set top aerial - or at least an old-school silver hoop one that came with my TV. I've tried a even smaller one that came with my stereo and the result is the same - clear for the BBC/Channel 4 multiplexes, nothing for the others.

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Dominic's 3 posts GB flag
Dominic's: mapD's Freeview map terrainD's terrain plot wavesD's frequency data D's Freeview Detailed Coverage
Dave Lindsay

3:01 PM

Dominic: There are a whole variety of reasons why it may differ.

The Commercial (COM) multiplexes use a less robust (less fragile) transmission mode, so if all five standard definition multiplexes have equal distortion due to signal overloading problems, then you may find that it is too much for the Commercial ones to be useable, but not the Public Service ones.

Different frequencies get bounced around and refracted to different degrees when passing through walls and other objects. Thus the affect and end result may be different for all.

Personally I wouldn't ever use a loop aerial for TV reception. All signals come from one direction (from one transmitter) and are polarised (horizontal or vertical). A loop antenna has little or no directivity or polarisation.

I have one of these Labgear aerials that I use where a fixed aerial isn't available:

Set Top Aerial Labgear | eBay

Of course, it's not forced to be too much signal. It could be too many reflections, your aerial not really having any directivity means that it may be more susceptible to picking up these. Again, if the signal received by your aerial results in all five channels being poor (battered around with reflections etc), the Public Service ones may be useable when the COMs aren't.

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Dave Lindsay's 5,724 posts GB flag
Mike Dimmick

5:44 PM

Terry Ward: The frequency offsets mean that the centre of the signal is moved about 2% above or below the proper centre frequency (167 kHz in a channel bandwidth of 8,000 kHz). If one transmitter has a - offset and another a + offset on the same channel, there's still 96% of the signal overlapping (actually, more, because the transmissions don't go quite up to the edge of the channel bandwidth).

That means that the services on C23, C24, C26, C27 and C29 will all be knocked out. You'd be left with the HD services from Caldbeck (English and Scottish, though ITV1 Granada/Border HD, not STV HD), BBC England SD and ITV1 Border SD, and UTV from Divis (plus the other channels on those multiplexes). After switchover, C23, C26 and C29 will carry the commercial multiplexes from both transmitters, but because they're so far apart, they won't be synchronized and will cancel each other out rather than reinforcing. You cannot get most of these channels from Freesat.

The only way you could do this is if you had selective filters that only picked the services you want from each transmitter. Single-channel filters are quite expensive! They're usually only used in distribution systems for large buildings, e.g. tower blocks.

STV HD is available free-to-air on satellite, it's just not currently in the Freesat EPG. This was due to a conflict with ITV plc, I believe. Your box should have an 'Other Channels' or 'non-freesat channels' option. You can find the frequency and other tuning information at…html - it's currently on 10936 MHz, V polarization, symbol rate 22000 baud, FEC 5/6.

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Mike Dimmick's 2,486 posts GB flag
Thursday, 3 May 2012
12:31 AM

problem instantly solved by your advice on analogue interference- turn off video, re-tune, all the channels (including, of course, rubbish shopping and pay-for-porn) but 93 to 114 is an instant result. Thanks!

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Riddlywalker's 1 post GB flag
Monday, 7 May 2012
Ian Robinson
9:47 PM
High Wycombe

I seem to only be able to receive BBCB, SDN signals & intermittently D3+4 on my Samsung smart TV in the lounge. All other signals are pixelated. I live at HP10 9NN. The transmitter I select is Crystal palace. I live in a flat with a communal aerial. The TV & radio signals come down a single co-ax into the double socket (that splits TV & radio signals) in the lounge & a second co-ax goes out from the sockets to another flat. from the socket there is a splitter/booster that sends one signal to the TV in the lounge & one to the bedroom. The Warfdale TV in the bedroom can show all channels perfectly from the same feed. I have swapped the cables & removed the splitter, the outcome is the same. The signal checker on the Samsung TV shows the signal is strong but the bit rate error level is fluctuating. I suspect the Samsung is more sensitive than the Warfdale. Any advice you can give would be greatly appreciated.

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Ian Robinson's 4 posts US flag
Ian's: mapI's Freeview map terrainI's terrain plot wavesI's frequency data I's Freeview Detailed Coverage

11:37 PM

Ian Robinson: If your main socket is wired as said then its not a particularly good arrangement, as the splitter / booster should be located where you are on one output and with the 2nd flat on the other, but the fact of you stating that the Wharfedale (a Vestel chassis device) works perfectly OK on its feed is rather strange as these chassis aren't the best for sensitivity, and so I would be interested to know what the result would be if you tried the Wharfedale on the feed used by Samsung.

I will say though that the signal level predictor (trade) doesn't really give glowing indications for your particular area, but if the Wharfedale works OK that's the main thing, although what you should do is select a programme channel on the Wharfedale that you know is poor on the Samsung and go into the Wharfedales signal check screen and note the mux channel number that's seen associated with the reading, then select that same programme channel on the Samsung and go into its signal check screen and see if the the same mux channel number as on the Wharfedale is indicated, the purpose of this being to ensure that the Samsung hasn't tuned to an alternative transmitter.

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jb38's 7,179 posts GB flag
Tuesday, 8 May 2012
Ian Robinson
12:08 PM

Thank you jb38, i need to just clarify a few points. The splitter/booster is what i have added to split the TV signal, coming from the socket, to the respective rooms. The Warfdale is the furthest from the socket. The original set up behind the socket is one co-ax in, carrying the TV & radio signals & one out. I presume this is to another flat as the co-ax cables are connected together via the back of the socket. The two co-ax come in opposite each other, with the centre cables going to the same screw & the outer braiding have a clip each. The socket filters seperate the TV & radio signals into their respective sockets (TV out & Radio out). I have tried fitting a new single socket; a double socket; i have swapped the feeds to the different rooms; i have run the cable direct to the Samsung (without & without the splitter/booster). the result is always the same, the Samsung struggles & the Warfdale works. At the end of the auto retune on the Samsung i have to select secondary region (London or Southern). The channels affected differ between the region selected. Whichever region i select, i get pixelated channels, even if the channels affected differ. I will check the mux number & if they differ, does that mean i have to manually tune to the better signal? Also could i manually tune to receive one channel from London & another from Southern, depending which is best? If the cable set up at the socket does supply other flats, could one or all of them be causing the problem, by either having interfering equiptment connected or just too many TV's being watched at one time? The pixelation is worse at peak times.

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Ian Robinson's 4 posts GB flag

6:13 PM

Ian Robinson: Yes, but its the set up behind the socket that I was referring to, as if one of the two coax cables does loop through behind your socket and its not been done for feeding to another socket in your own property then it has to be for another flat, meaning that whoever wired the flats has done it more in an electrical fashion and not RF, like a communal ring mains circuit, not the best policy!

The reason for this being, that if someone in another flat plugs something of an electrically undesirable nature into their socket it can affect your reception, this by either pulling your signal down or interfering with it in some other way, this being why individual feeds are always the best as a person in one flat cannot interfere with reception in another.

However I was just making a point, because if the reception on the Wharfedale is basically satisfactory then your signal must be reasonably OK, and so albeit that its a bit of a long winded operation, but you will have to carry out a test on a few EPG listed programmes on the Samsung to ensure that they are being picked up from the same mux transmitters as when they are picked up on the Wharfedale, this done by comparing the muxes shown on each devices signal test screen whilst on the same EPG programme.

If on comparing the results from both devices you find that they are indeed tuned to the same mux channels then the problem has to be caused by something very localised to within your installation, or the equipment used, and I would therefore like to know the model of Samsung TV that you use, as Samsung TV's are not exactly absent from the lists published by Digital UK of the various brand / models that can experience problems with the new transmission modes in use. My other reason for requesting the model being to ensure that its not one of the types that has a powered aerial supply facility, as if it does then this should be switched off.

Regarding manually tuning each of the muxes required, it might end up that you will have to do that.

By the way so that you don't waste time by duplicating muxes on the same transmitter use the following programmes for a test, the mux channels mentioned (in brackets) being from Crystal Palace:- BBC1 (Ch23) / ITV1 (Ch26) / ITV3 (Ch25) / Dave (Ch22) / Yesterday (Ch28)

Your alternative station is Hannington, albeit the three commercials being shown as being variable reception, the channels being :- BBC1 (Ch45) / ITV1 (Ch42) / ITV3 (Ch41) / Dave (Ch44) / Yesterday (Ch47)

(HD being left out of the equation)

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jb38's 7,179 posts GB flag
Wednesday, 9 May 2012
Ian Robinson
10:08 AM

Ian Robinson: jb38,
My Samsung model is UE32D6530.
when i go into manual tuning, it shows the signals from both the Meridian & Crystal Palace transmitters (see below).

Channel Frequency Bandwidth
23 490000Khz 8MHz Rescan
42 642000KHz 8MHz Rescan
25 506000KHz 8MHz Rescan
22 482000KHz 8MHz Rescan
28 530000KHz 8MHz Rescan
30 546000KHz 8MHz Rescan
45 666000KHz 8MHz Rescan
61 794000KHz 8MHz Rescan
26 514000KHz 8MHz Rescan

However in the programme guide list there are not, for instance, two BBC1's, one from each region. Otherwise i could just select the strongest signal BBC1 at any given time.
I only had time last night to check BBC1 on the Warfdale & that was from Crystal Palace. If i remember rightly, the last time i did an auto tune on the Warfdale, all the channels showed from Crystal Palace.
I don't know if it is relevant, but the Wooburn repeater is dead in line between my flat & crystal Palace?

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Ian Robinson's 4 posts GB flag
3:55 PM

I don't understand why I can get nearly all the freeview channels upstairs, but downstairs can only get itv3, itv2 +1 and other random, not much watched channels, I can't even get BBC1,2, ITV or Channel 4 downstairs. Both Tv's are about 12 months old and have in built freeview. I would be greatful for comments

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Natalie's 1 post GB flag
Natalie's: mapN's Freeview map terrainN's terrain plot wavesN's frequency data N's Freeview Detailed Coverage
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