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Two frequency interference

What to do when Freeview reception is perfect, but several channels are missing, and they are all on the just two 'multiplexes'.

What to do when Freeview reception is perfect, but several chan
published on UK Free TV

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

  • Freeview reception is perfect or near-perfect most of the time
  • Several channels are missing, and they are all on the just two 'multiplexes'
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 two rows, you have "two frequency interference":



Transmission mode problem

If you are unable to receive the channels on BOTH multiplex 2 and multiplex A, this will be down to the transmission mode. See Freeview modes.

This is a particular problem for people using Loft aerials and Indoor aerials .

Non-wideband aerial in some areas

In some places you will need a wideband aerial to receive all the Freeview multiplexes. 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
Single frequency interference6

Comments
Sunday, 24 July 2011
J
jb38
sentiment_very_satisfiedPlatinum

9:24 PM

Ed: Regarding your aerial system, I suspect that it could possibly prove as being more than adequate come next April, this to the extent that the adjuster on the loft amp could well have to be kept permanently at a much lower level to avoid causing signal overloading problems on your receivers, although I would be inclined to keep it just about fully advanced for the time being.

On the subject of the amp having a variable gain control, this likely explains why trying aerial directly into the box was better than through the amp, simply because the adjuster was turned down slightly making the signal through it be less than the direct one, so there obviously isn't any problem with your amp to be worrying about.

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jb38's 7,179 posts GB
Monday, 25 July 2011
E
Ed
1:06 PM

Briantist
"Ed: You wouldn't need to do the switchover if the signals were already strong"

I thought the switchover was analogue to digital? What I'm saying is, everyone isn't going to do the magic switch from A to D on a particular day, it would be a real mess. It's better to have a time span to iron out problems etc which is what's happening. But why not have a nice powerful signal now? There are people out there buying huge amps, boosters, and paying experts when it's due to the signal being lower than it will be in April. I wasn't aware of this either, so you can guarantee millions of others aren't. Is it simply to save money on the power output?

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Ed's 8 posts GB
M
Mike Dimmick
sentiment_very_satisfiedPlatinum

1:47 PM

Ed: The analogue television service was planned carefully to allow four complete networks to be transmitted from every site in the country. (There are a very few one- or two-channel fill-in relays where a region has later been split, to allow an area to get its 'correct' local news.) The original digital signals were crammed into this plan, at whatever level would cause minimum interference to analogue services, and from very few sites: the 'main' transmitters (fed directly from studios) and a few relay sites that served a large chunk of the population, or parts that the advertisers considered important. There simply wasn't room for any more power increases.

The frequency plan wasn't just for this country, but also to allow for the transmissions from Ireland, France, the Netherlands and Belgium (being most affected by our transmissions).

There were various changes over the years since digital services started in 1998, particularly the equalization programme of 2000, bringing the coverage of each multiplex from a particular site to a similar level (originally the plans might have huge coverage on Mux 1 but very low coverage on Mux D), the mode change of late 2002 (effectively increasing coverage with no power increase, on the four multiplexes that changed mode), and power doubling of many multiplexes at many sites in 2003/4. These changes could be made as interference wasn't as bad as first thought, or few people were actually using theoretical coverage from one transmitter: some changes were required to analogue relays to prevent people from losing reception. There were diminishing returns on these changes, though, and analogue switch-off is really the only way to improve it further.

The switchover is to turn off the analogue signals, so that the digital signals (at least for the public-service broadcasters) can take over their frequencies and, largely, power levels. But it can't happen until analogue is switched off, because it will cause too much interference at other sites.

You get two weeks to sort out problems - BBC Two is switched off first, replaced by high-power digital BBC services. Then, two weeks later, the other four analogue channels are switched off and the remaining high-power digital services start. If you had digital before, you need to retune on both days, as the signals move to the much better frequencies vacated by the analogue transmissions.

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Mike Dimmick's 2,486 posts GB
Tuesday, 26 July 2011
E
Ed
3:42 PM

I can't wait for the switch over. Having said that, I have done a LOT of investigation on my system today and installed a new amp. The new amp actually got the freeview box working MUCH better, but the set with built in freeview still didn't work :( However, I pulled the aerial out the back of the tv, and inserted a freeview box into the circuit. Still no signal at all on channel 22. This eliminated the TV and only one possibility remained, the wire itself. I unplugged the co-ax from the tv, AND from the amp and did a continuity test across the co-ax plug in the loft. It was shorting. I took the co-ax plug off and the short disappeared. Everytime I put the plug back on, the short re-appeared. Then it hit me like a ton of bricks. There is a tiny screw in the centre plastic section to hold the copper core steady. This was rubbing on the plugs outer metal case and shorting. I removed the screw and the short was gone. The signal on the built in freeview is now great and the signal on the freeview box has also increased. I must have been getting other channels just by having the wire as an aerial. Anyway, headache over, such simple faults are nightmares.

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Ed's 8 posts GB
Wednesday, 27 July 2011
Briantist
sentiment_very_satisfiedOwner

7:06 AM

Ed: Yes, indeed, it always annoying when a problem turns out to be a wire with a short circuit.

I'm pleased you have found the problem and fixed it.

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Briantist's 38,844 posts GB
Saturday, 27 August 2011
R
Richard
8:41 PM
Harrow

This describes my problem exactly - I don't have any channels from multiplexes 2 and A, so it is a transmission mode problem. But what are the fixes for this problem?

Have had the same problem for ages, but becoming urgent with the impending rugby world cup on ITV..

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Richard's 3 posts GB
Sunday, 28 August 2011
Briantist
sentiment_very_satisfiedOwner

7:32 PM

Richard: You probably need your aerial looking at, or possibly just the cable from the aerial to your set.

You live in a very good signal area, and if you are missing the two multiplexes, then some part of your equipment is faulty.

I would see what your analogue signal looks like (for the moment) as a guide.

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Briantist's 38,844 posts GB
Monday, 29 August 2011
R
Richard
5:49 PM
Harrow

Briantist:

Thanks for the advice. Have just managed to pick up multiplex A for the first time in forever, but still without 2. I will look into aerial/cable problems

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Richard's 3 posts GB
R
Richard
6:05 PM
Harrow

Haven't lived here long, didn't even realise we still had analogue :s

Solves the rugby problem at least...

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Richard's 3 posts GB
Sunday, 11 September 2011
J
jen
12:12 PM
Beverley

ah i dont get any from C or D =[ and a few missing from A

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jen's 2 posts GB
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