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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 by 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 www.ukfree.tv link icon Freeview modes.

This is a particular problem for people using www.ukfree.tv link icon Loft aerials and www.ukfree.tv link icon  Indoor aerials .

Non-wideband aerial in some areas

In some places you will need a www.ukfree.tv link icon wideband aerial to receive all the Freeview multiplexes. To find out if you need an aerial upgrade, see www.ukfree.tv link icon My Freeview reception.





Your comments: most recent posts are at the bottom

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Your comments are always welcome. Please use the form below to add your thoughts or questions to this page. We will get back to you as soon as we can.

Ed
Sunday 24 July 2011 3:04PM
OK done a bit more investigation to my problem. Layout = roof aerial, feeding into amp in loft - feeding to two televisions. One TV uses external freeview, other is built in and has its own amp in the set I believe. The tv with built in freeview was seeing channel 22 yesterday but weak, freeview box nothing, no signal displayed. I disconnected the aerial lead, and fed it straight into the co-ax going to the freeview box. BINGO, signal time. Only getting 98/58 - 98/62 but its there. BBC is 100/68. The tv with built in freeview doesn't get 22 at all now, but this one did yesterday, very weak tho. So at the moment have aerial running into 2 TV's directly and the co-ax are direct connections, no components splitting them etc. Anyway, Does a tv with built in amp overload the loft amp? damaging it. Got to buy a new amp, but don't want it to burn out again. This one has lasted a year.

Many many thanks
jb38
Sunday 24 July 2011 6:54PM
Ed: the fact that the TV with built in Freeview was giving a picture on Ch22 but the separate Freeview box wasn't only means that the TV is more sensitive at picking up signals, and not that any problems exists with the Freeview box, the only reason for it (Freeview box) working when the aerial lead was fed directly into it being because the signal was higher going into it whereby it took it above the cut off threshold level of the box.

However that raises another point, as "if" when you said you fed the aerial lead straight into the Freeview boxes co-ax you are meaning the lead directly from the aerial itself? as if you are then that's inclined to indicate that the loft amp is faulty, as the signal should always be better through it than direct from the aerial, the only exception to that being where a signal is too strong, something I don't think is likely to apply at your distance from the transmitter.

The other thing is that TV's don't have a built in amplifier that can damage anything, but what I did wonder was whether or not your loft distribution amplifier is of the self powered type? and if it is make sure its not a type that is able to power a mast head amp, as some can! and if these type are used directly into a normal aerial they will be running into a short circuit.

I still think though its just a case of you being in a bad location for that particular Mux channel, and as aforementioned in previous reply the fluctuating signal levels you receive on Ch22 is out with your control, although sometimes experimenting with different aerial mounting positions can help no end, like gutter mounts being moved from one end of a roof to the other, as well as a signal sometimes being better at that level than high up on a chimney, as the usual rule of "the higher the better" doesn't apply in most difficult reception area's.

The latter being said though fully appreciating that aerial installers cannot really be expected to cater for this type of experimenting because of the time factor Vs financial costs involved.
Ed
Sunday 24 July 2011 8:24PM
Thanks. Yes the aerial lead was fed directly from the aerial into the freeview box.
The amp is a mains powered box in the loft with an aerial-in port and 4 output ports, but just two are used. I don't think it is too strong because it has an adjustable pot, and turning it down weakens the signal to the no-signal point, whereas right up gives around 100/80 on channel 22, with no fluctuations or interference. I have a philex 48 element on the roof which I hope is adequate, it's the biggest in the street. No mast amp but says it has a 15db gain.
jb38
Sunday 24 July 2011 9:24PM
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.
Ed
Monday 25 July 2011 1:06PM
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?
Mike Dimmick
Monday 25 July 2011 1:47PM
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.
Ed
Tuesday 26 July 2011 3:42PM
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.
Briantist
Wednesday 27 July 2011 7:06AM
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.
Richard
Saturday 27 August 2011 8:41PM 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..
Briantist
Sunday 28 August 2011 7:32PM
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.
Richard
Monday 29 August 2011 5:49PM 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
Richard
Monday 29 August 2011 6:05PM Harrow
Haven't lived here long, didn't even realise we still had analogue :s

Solves the rugby problem at least...
jen
Sunday 11 September 2011 12:12PM Beverley
ah i dont get any from C or D =[ and a few missing from A
Mike Dimmick
Sunday 11 September 2011 12:45PM
jen: Multiplex C and D - after switchover we usually call them ArqA and ArqB - are still on relatively low power from Belmont. If you didn't have digital before switchover you might still have a Group A aerial, and these multiplexes are on much higher frequencies that the aerial doesn't pick up. You really need a wideband for all services.

I'm surprised anything is missing from Multiplex A/SDN, usually you get all of a multiplex or none of it. Do the channels appear anywhere else in the list, maybe around 800? If so, your box might have trouble with the size of the Network Information Table. A list of some boxes known to have trouble is at www.digitaluk.co.uk link icon http://www.digitaluk.co.u…tnit .
John W
Wednesday 30 November 2011 8:00PM Harrow
Hi,

Any idea why I have lost all Mux 2 completely and have seriously reduced signals on Mux 1?

My transmitter is Crystal Palace and this has worked fine last week when these channels on these 2 Muxs just vanished. It's occurred on all the Freeview boxes and the Freeview TV.

We do have a loft arial, but as I say its worked for years with no problems till last week. I'm around 30K from the transmitter. HA1 postcode.
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