No BBC1 or BBC2

[opps]

Hi all.

A friend recently asked me to look at the co-ax cable in the house where he rents a room. He sill had a signal on his TV but the rest of the house didn't.

His coax was plugged into a wall plate with a TV and a FM female socket (actually his coax was plugged in to the FM socket but works fine).

Next to the socket was an amplified splitter with a total of 4 ports. Although there was power running to the amplifier, I guessed that it had died. I picked up some F-type screw on plugs and a 4 way passive splitter.

I then re-tuned the 3 TVs that had previously lost their signals. None of them now have BBC1 or BBC2 (although they do have the hi-def BBC1/2). My friend's TV does though.

I cannot work that one out. There are only 2 aerials on the roof, both on the same mast, a Yagi and an FM aerial. Might it be the case that those big round FM aerials are good enough for digital TV (location- Ealing, west London)?

Is it worthwhile picking up a new amplifier for the TVs down stairs? If yes can anyone recommend one from either toolstation or screwfix?

 

[Lucid]

When dealing with a bit of a mish-mash of possible issues, I think it's useful to go back to basics and then build up the system in stages. That way you can tell when something is added that causes the system to fall over.

In your friend's case there are a couple of things that don't quite add up. The wall plate is a filter. A mixed radio and TV signal goes in, and then the frequencies are split into the TV and the FM bands. These are quite separate from each other.

FM is 88 to 108 MHz. The UHF TV bands don't start until 470MHz. That's a big gap to bridge, so even if the (VHF) FM aerial is picking up UHF signals, I'm left wondering how they make it through all the filtering and combining/decombining stages so that his TV picks up signals from the FM radio socket.

Do you think it might be possible that the cabling itself is acting like an aerial? According to Wolfbane, postcodes in the W13 area need nothing more than a basic set top aerial to get reception from Crystal Palace.


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[winston1]

First of all plug the lead into the correct socket. If the TV socket is the "other sex" get a coupler (barrel connector). This will probably solve the problem.

As Lucid implied you probably don't need the amplifier but you can't assume it s dead when being incorrectly fed.

Those "big round FM aerials" are called halos and are defiantly not good for UHF TV, in fact they are pretty useless at FM having a negative gain compared to a dipole.

 

[opps]

In your friend's case there are a couple of things that don't quite add up. The wall plate is a filter. A mixed radio and TV signal goes in, and then the frequencies are split into the TV and the FM bands. These are quite separate from each other.

Given that there are two aerials, wouldn't there be two cables going into the wall plate (ie without the need for a filter)?

When dealing with a bit of a mish-mash of possible issues, I think it's useful to go back to basics and then build up the system in stages. That way you can tell when something is added that causes the system to fall over.

Makes sense, thanks. I guess that I could take one of the ground floor TVs up stairs and plug my friend's fly lead in and see what happens.

The whole set up is a bit Frankenstein-esque (as is the rest of the house). In an area with decent reception how many TVs can you expect to run off one roof top aerial? Is number X amounts of TVs or X amount of TVs that are actually turned on at any time?

Once again, thanks for your reply.

 

[opps]

First of all plug the lead into the correct socket. If the TV socket is the "other sex" get a coupler (barrel connector). This will probably solve the problem.

The weird thing is that the only telly getting BBC1/2 is not the one plugged into the splitter but into the port that says FM on the wall socket. The old amplifier is about 6" away from the wall plate with the cables popping up from the floor. I guess I could try plugging a lead from the socket to the input of the splitter (ignoring the cable that I previously identified as being the supply).

Those "big round FM aerials" are called halos and are defiantly not good for UHF TV, in fact they are pretty useless at FM having a negative gain compared to a dipole.

I had to Google dipole. Thanks for the heads up.

 

[Lucid]

Given that there are two aerials, wouldn't there be two cables going into the wall plate (ie without the need for a filter)?

This is one of the difficulties about being remote from a system and diagnosing through a 3rd party. I know how it should be done, but whether it has been done right is another question entirely.

Running two wires to each aerial point is a colossal PITA. How it's done properly is to have a device that combines the two aerial signals in to one. This is called a diplexer. They take various different forms: For example, sometimes it's a box up on the aerial mast. Other times it might be the aerial distribution amp itself which has an input for UHF (the TV aerial) and one for VHF which is the radio aerial. However it's done, the result is the same; the FM and the TV signal go down the same cable(s) to the various TV points.

The wall plate with the splitter is more of a convenience thing, but it does help reduce the risk of crosstalk between the FM and TV signals. It shouldn't really be an issue, but the FM aerials will pick up TV signals, albeit very weakly. TV aerials will also pick up FM, again, at weaker strengths than TV reception, but some TV aerials will tolerably well as an FM aerial particularly if the FM and TV transmissions come from roughly the same direction. Combining two aerial signals where they're both picking up the same transmissions can result in some odd effects. Certain signals can be reduced. That's called destructive interference. To get around this problem, the diplexer responsible for combining the signals should have adequate filtering to isolate the VHF and the UHF ranges. However, we can't rule out the possibility that something isn't quite right at the head-end or in the signal cable run down to each room, so having the filtering at the wall plate does make sense.

Your system should be the same if whoever installed it knew what they were doing. That's a big If though.

The only way to know for sure is to take the wall plate off and have a look behind.

Makes sense, thanks. I guess that I could take one of the ground floor TVs up stairs and plug my friend's fly lead in and see what happens.

The whole set up is a bit Frankenstein-esque (as is the rest of the house). In an area with decent reception how many TVs can you expect to run off one roof top aerial? Is number X amounts of TVs or X amount of TVs that are actually turned on at any time?

Once again, thanks for your reply.

Somewhere with really strong signal, and a decent aerial that matches the signal profile of the local transmitter, then it wouldn't be unreasonable to run an 8-way passive splitter.

TVs being on or off makes no difference.


If this or any other reply was helpful to you, then please do the decent thing and click the T-H-A-N-K-S button. It appears when you hover the mouse pointer near the Quote Multi-quote buttons. This is the proper way to show your thanks for the time and help someone gave you.

 

[ericmark]

If there is a mast head amplifier it needs power, it can get power from a amplifier, power supply, or the TV, but some way it needs power, it is not normal to use a mast head amplifier for radio, so it is possible the two aerials are not combined, then split, as the units that combine could also stop DC which is maybe required for the TV mast head amplifier?

So plugging into the VHF aerial if that is not got a mast head amplifier the TV may work, also if there is a mast head amplifier and one TV has the output to mast head turned on, it may be only when that TV is used will all TV's work.

My old LG TV could be set to give 5 volt to mast head amplifier but don't think the rest of TV's in house have that option. I had to use a 12 volt power supply between TV and aerial socket to power the mast head amp.

Most of the TV/Radio face plates have a filter which will not allow DC through, but not all, I used years ago the Sky eye so I could swap channels in other rooms, and I had to hunt for plates without filters so DC would pass through.

There was a free booklet that explained the filters, but it went out of publication when digital TV came out, although I have scanned the pages the pictures are BMP so can't show on these pages.