Strange BBC Freeview

[leevis]

Hi All
On one of my televisions Freeview is playing up. I haven't delved into it too much yet but it appears to affect just BBC1 and BBC2 and probably all the others on that MUX. I have two televisions connected to a power amplifier and then on to the aerial, normally i would say its the aerial but the aerial is fine and so is (strangely) the other television. Any ideas? Only happened in the past day or two.
L

 

[Lucid]

I answered a similar question a few days ago. Here's a copy of the relevant text...

I have a similar problem with a Samsung TV that I haven't been able to solve, although it is the BBC SD channels that I have trouble with.

........

So I can only assume in my case at least that it is the TV that has a signal strength/quality problem, but of course I can get nothing out of Samsung!

TV tuners range in sensitivity within an allowable range, so you might find one TV has better or worse reception than another. However, all should give a picture so long as the aerial signal is also within acceptable limits.

Problems tend to arise if the tuner or TV aerial socket is broken, or if the aerial system has issues such as being split too many times or over amplified. Where the aerial on the roof and its down lead are in good working order and appropriate for the area, then in cases such as yours excessive splitting and over amplification are possibly the most common causes of problems.

From your description I'd say there's another clue too. You're having problems with the BBC SD channels. These are part of the core range of channels that most households should be able to receive as part of the Public Service Broadcasting (PSB) remit. They are broadcast in the transmitters higher power muxes. In short, these are the signals you should be able to receive in all but the worst conditions.

In the absence of other information, the fact that the Samsung picks up the lower power channels (I presume) without issue points to there being too much signal via your aerial system rather than too little. The signal for that mux could be swamping the tuner. This will result in problems that look very similar to low signal quality on digital, and it often goes misdiagnosed as exactly that.

The answer in most cases is to fit an attenuator to reduce the signal level for that particular TV. It's a very useful and very inexpensive diagnosis tool. This one is useful because there's a level control on it to allow the signal level to be adjusted: MA1620 Adjustable 0-20dB attenuator, coaxial connector version

When making the adjustment, use the TV's Signal Quality bar when tuned to the BBC SD channels as a guide. When there's too little or too much signal this will go down. When the level is getting towards the correct range then the bar will show higher quality. Stop adjusting when the bar indicates the highest quality. Check the other stations to make sure you still have good reception there too.

Let us know how you get on.

 

[ellal]

And I can vouch that Lucid's solution was spot on.

I haven't had a BBC SD channel problem since using an attenuator.

 

[leevis]

I will check this out later when I get home.

One thing though, my aerial goes from the roof and into a cupboard in the loft where it is split using a powered amplifier. There are multiple outputs on this unit with different db levels so would I be right in thinking that this is similar to an attenuator. What I could do (in theory) would be the change the output to a different db level and see what happens.

Not too sure what it is connected to at the moment (will have to check)

If I do go the attenuator route (I think I have one of these already in the house somewhere), whereabouts does it go? in between the splitter and the tv I guess?

 

[Sam Gangee]

An attenuator should go close to the aerial input socket on the equipment (TV or Freeview receiver). Don't put it directly into the socket because any excessive strain will damage the socket, requiring an expensive repair! Best to use a short length of double-screened coaxial cable such as WF100.

Note, however, that while an attenuator is often the answer it is not always the answer. For example, see
http://www.satcure.co.uk/tech/kinked_cable.htm

and http://www.satcure.co.uk/tech/freeview_problems.htm

 

[Lucid]

If you have the option of running on a lower powered leg of the DA then yes, definitely try that first. Anything you can do to narrow down the cause of the problem or eliminate possibilities is good.

If you have a variable attenuator then it gives the opportunity to adjust the amount of signal reduction - to fine tune it if you like - at the TV end while watching the Signal Quality meter display. The attenuator connects to the aerial socket on the rear of the TV. The fixed steps of attenuation from the DA won't give you that fine tuning ability, but at least you'll be able to see if it's too much signal at present that's causing the issue.

 

[leevis]

OK this gets weirder

I have two televisions, same make (although cheap). Lets call them TV A and TV B for now.

Both of them are connected to a powered amplifier/splitter box which is a stepped amplifier of 8db, 12db and 16 db. This box is connected to the aerial (on the roof). Each TV is fed from this box and at the end has an adapter to turn coax into the feed for the tv where there is another cable which goes into the each television.

Anyway, TV A does not receive channels on the BBC Mux whereas TV B does.

I decided to swap the outputs from the box so that the feed that went to TV A goes to TV B to see if that made a difference. When i did this I got exactly the same symptoms, TV A did not work for BBC but TV B did. I tried all the different db levels on the box and none of them got me the BBC channels.

At this stage i thought that it might be TV A that had the problem, I swapped the TV's around and still got the same results.

To me, this means that its either the cable from the amplifier/splitter OR the connectors on either end OR its the adapter OR its the cable which goes from the adapter to the TV.

I can test each of these although will be a bit time consuming. Don't quite understand whats going on with this one. Still unsure of how this only affects the BBC channels.

Getting there slowly

 

[Lucid]

Try the simple stuff first. Check your plug end connections for loose or stray filaments of the shield braid touching the centre core connection.

Also try bypassing the aerial amp completely and connect the aerial feed directly directly to the downlead that goes to troublesome TV point A.

More esoteric causes: If coax cable is folded, bent or crushed through bad handling or poor routing or just stuffing it away to "look neat" then is can create a notch filter. This traps certain frequencies but lets others through. If the trapped frequencies coincide with the frequency of your BBC mux then that's why both TVs have a problem.

The solution is to replace the cable where it is damaged. This can be either a part replacement or in whole.

 

[leevis]

Thanks for the tips, I will back on it when I get home tonight. Will certainly check the ends and see if it works direct from the aerial. Will report back.

 

[Sam Gangee]

To me, this means that its either the cable from the amplifier/splitter OR the connectors on either end OR its the adapter OR its the cable which goes from the adapter to the TV.
Don't quite understand whats going on with this one. Still unsure of how this only affects the BBC channels.

But I already gave you a link to the answer.

 

[leevis]

Appreciate the responses here chaps.

Another question, my coaxial cable goes from the amplifier to a surface socket and then a fly lead into the tv. From reading the links it could be here that the problem lies. The question is, can you go direct from the amplifier into the tv? Is there an adapter that I can put on the end of the coax which will go into the tv or is there an adapter that goes onto the end?
If there is, whats it called

 

[Lucid]

Adapters that cover various options for converting the screw on F plugs to normal TV coax plug and vice versa http://www.satcure.co.uk/accs/page7.htm#fconn Scroll down the page to about 2/3rds and look at 090-4333 TV male to F adapter


General advice Ditch the moulded coax fly leads. Buy some WF100 and coax plugs. Make up your own. It's not difficult nor is it expensive and you're virtually guaranteed to remove a weak link for interference at the same time. It's a far far better quality solution. There's good information and everything you need here: http://www.satcure.co.uk/tech/tvplugs.htm

 

[leevis]

Fixed it
It was the surface socket, its unshielded and think it must have had some dust on it or something.
I disconnected it with the idea of putting a male adapter on it but the cable was too short to go into the television so i aborted it. I re-connected the cable to the surface socket and its now back to working again good times
Many thanks for all the help i have had on here.
Next on the shopping list will be a shielded surface socket me thinks.
L

 

[winston1]

I'm glad you fixed it.
method is here:

http://www.megalithia.com/elect/bellinglee/index.html

 

[leevis]

That's the method I used (almost - without the solder) to connect up to the TV but the coax cable would not reach. I think I will try and work out how to extend it (I think that there is enough cable just need to feed it through the hole in the wall).