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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
Is it true that my 87 year old mother is entitled to a FREE upgrade when the ana5
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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

Comments
Sunday, 21 August 2011
R
Rhys brooks
12:46 AM

Hi I have moved into a new house and am using the previous owners aerials.

The living room tv picks up every channel fine however my bedroom and kitchen TV's are not picking up a number of channels. We are only picking up the itv channels and some of the channel 4 channels.

Why can I not get all the channels on my other TV's? And how do I fix it?

I live in Cardiff and we have a full digital switch over with no analogue signal

Thanks in advance

Rhys

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Rhys brooks's 1 post GB
J
jb38
sentiment_very_satisfiedPlatinum

11:07 AM

Rhys brooks: This can be simply because no two brands of receivers are the same as far as sensitivity is concerned. It could also be made worse by the method used to provide the extra sockets, as they shouldn't really be looped in the same way ring mains power sockets are done unless in a strong signal area, even at that its much better for each socket to have exactly the same signal, meaning that they should be fed as individual outputs from a powered splitter.

Just to test if its is a sensitivity issue, try one of the problem sets on the living room socket, if its OK there then what I mentioned about a powered splitter is the only answer.

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jb38's 7,179 posts GB
Thursday, 25 August 2011
J
jb38
sentiment_very_satisfiedPlatinum

1:20 PM

Stephen F: As your BT vision box is first in line from the aerial and as such receiving the maximum signal, then the symptoms you are getting could possibly be caused by an over strong signal that's swamping the tuner, especially if you are using a roof aerial, or worse if a booster is also involved, if this applies then it should be removed, but if the signal is too strong then a simple attenuator in line with the aerial socket should cure that.

Purely for a test, if you have access to a set top aerial give that a try, as although the picture (should one be there!) might well be poor it would be interesting to know if the type of interference seen was still in evidence or not.

Regarding the TV being OK but the box not, no two tuners are the same in sensitivity nor their ability in coping with over strong signals.

(Is the BT vision box a grey or black type?).

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jb38's 7,179 posts GB
J
jb38
sentiment_very_satisfiedPlatinum

1:28 PM

Stephen F: Meant also to say, try leaving the co-ax link from the BT box to your TV disconnected.

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jb38's 7,179 posts GB
S
Stephen F
4:02 PM
Colchester

jb38: Thanks for the reply, I tried a set top aerial and leaving the coax to the tv disconnected but no success. The BT Vision box is grey/silver, is the black one a newer type?

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Stephen F's 10 posts GB
J
jb38
sentiment_very_satisfiedPlatinum

5:39 PM

Stephen F: Yes the black one is a later type, the only reason I asked about the colour was that its rumoured (unable to check for myself) is that the older type you have is fitted with an RF modulator whereas the later black ones doesn't, not of course that this would be liable to cause the problem you are experiencing, as it would be the TV that would suffer.

However, if you have now tried a set top aerial and the problem still exists, then there is only about one thing left that I suspect could be responsible, that of the BT vision box not being fully compatible with the new 8k transmission mode that is now in use.

I have had a look at the spec and it does state that its 2K/8K compatible, though experience has proven that this is not always the case in various pieces of equipment. A test you could try for this being, blanking out the channels stored on your BT box by scanning without the aerial being connected, then to manually tune in Mux Ch49, as MuxA/SDN is the only channel that's shown as still being on 2K operation, so it would be interesting to know if this one works OK, if you can receive it that is, as its shown on the predictor as being variable. (Ch49 as) 

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jb38's 7,179 posts GB
S
Stephen F
6:01 PM
Colchester

jb38: I will give that a go, but how do I manually tune in Mux Ch49 on the BT Vision box the only option I can find is to scan for channels

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Stephen F's 10 posts GB
J
jb38
sentiment_very_satisfiedPlatinum

6:17 PM

Stephen F: "If" it has that facility then it should be seen within the tuning menu selections, but not being acquaint with that particular box I appreciate that it might not offer this facility, if it doesn't then you will have to carry out the normal re-tuning then try tests on ITV3 or QVC, as these are on Mux Ch49.

A thing you might be able to do though is, if during re-tuning you can see the tuning bar running up the screen then leave the aerial unconnected until about Mux Ch45 then quickly plug it back in again, as that will miss most of the higher powered channels with the exception of the HD one, which shouldn't register anyway.

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jb38's 7,179 posts GB
S
Stephen F
6:54 PM
Colchester

Hmmm itv3 and QVC dont get the interference problems anyway so not sure how this is getting me any closer to the problem.

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Stephen F's 10 posts GB
S
Stephen F
8:49 PM
Colchester

jb38: I have tried an attenuator to reduce signal strength but inference did not seem to reduce/disappear. But i have discovered a labgear msa262 distribution amp in the loft near the aerial so is this likely to be what is causing all my bt vision problems?

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