Interference on certain LCD TVs with NTL analogue?

dcGT · 24-09-2007 11:36am #1

Hi All,
A bit of a strange one this. Please move this post if in the wrong location. I'm using an NTL analogue feed on a 21" LG LCD TV in the kitchen. However, I noticed that some of the stations were not coming in very clear after endless fine tuning. I then plugged an old 15 CRT in to the same location and same cabling, and the stations are crystal clear for the most part. I thought this was very odd. After some messing around, I realised that the location of the co-ax cable going into the back of the LCD TV affected the interference I was experiencing! (Moving the cable around like rabbit's ears) If I moved the co-ax a certain way, I could eliminate the interference. It seems the NTL signal is fine, but there is some other interference involved. I have the exact same problem with the other TV's in the house. Are LCDs more sensitive to interference than analogue TVs? Has anyone any suggestions on what this could be?

Any help would be great.

Cheers,

DC.

DingDong · 24-09-2007 12:07pm

Have you got a proper outlet box or one of the 1950's style ones. If it the older one I have seen that cause the problem. LCD's do seem to be more sensitive.

dcGT · 24-09-2007 12:10pm

Yes, outlet boxes are all brand new.

Cheers,

DC.

DingDong · 24-09-2007 3:54pm

I have seen some LCD that make analogue/DTV kinda look like grease proof paper is over the image. This put me off LCD for awhile but most of the newer ones look great. But from what your saying its only some of the channels. Which channels are the problem?. Is it a new cable into the house and the screw on outlet box.

conax · 24-09-2007 4:53pm

LCD tV's generate loads and loads of noise, I assume that the interference you are viewing is a wavey lines or grainy picture with dots.
moving the fly lead around is moving the cable in and out of the field generated by the TV. This interference only affect signals on two points.
1 if the signal level is below a certain threshold and 2 if the coaxial fly lead is substandard.
solutions, make a new lead with at least CT-100 keep the lead as short as possible (although some times making it longer helps as well) in some cases depending on the screening factor of the TV outlet plate this may have to be changed or just join the cables with F connectors. move the TV slightly away from the outlet plate and finally install a small set back amp behind the TV (or even better where the signal is split )to bring the signal levels above threshold of the noise.

but start by replacing the fly lead.

dcGT · 25-09-2007 7:23am

Thanks for the replies so far folks.

conax wrote:

LCD tV's generate loads and loads of noise, I assume that the interference you are viewing is a wavey lines or grainy picture with dots.

Yes, that sounds about right.

conax wrote:

moving the fly lead around is moving the cable in and out of the field generated by the TV. This interference only affect signals on two points.
1 if the signal level is below a certain threshold and 2 if the coaxial fly lead is substandard.

Why then does the normal CRT TV not have the same problem and has a clear reception? Does this still mean that the signal level could be below a certain threshold? Also, the fly leads are brand new (bought in power city), but perhaps they aren't good enough?

conax wrote:

solutions, make a new lead with at least CT-100 keep the lead as short as possible (although some times making it longer helps as well) in some cases depending on the screening factor of the TV outlet plate this may have to be changed or just join the cables with F connectors. move the TV slightly away from the outlet plate and finally install a small set back amp behind the TV (or even better where the signal is split )to bring the signal levels above threshold of the noise.

I will try some of these. I did try putting a booster in (where the co-ax enters the house), but it didn't seem to help. Is a 'set back amp' different? I also tried moving the TVs away from the outlet plate but that didn't make a difference. The only thing that seemed to make a difference is moving the fly lead around

Thanks again,

DC.

dcGT · 25-09-2007 7:29am

DingDong wrote:

I have seen some LCD that make analogue/DTV kinda look like grease proof paper is over the image. This put me off LCD for awhile but most of the newer ones look great. But from what your saying its only some of the channels. Which channels are the problem?. Is it a new cable into the house and the screw on outlet box.

Yes, it's only some channels but not any particular channels. What seems to happen is: if I 'fix' a channel by moving the fly lead around, it doesn't necessarily 'fix' it for all other channels and it could make some of the others worse. There doesn't seem to be a pattern to it or a 'one fly lead position fixes all' possibility. Yes it's a new cable into the house and the input signal has been checked by NTL. Yes the outlet box is mounted on the wall beside the TV and it screw-on.

Cheers,

DC

Antenna · 25-09-2007 8:42am

A 'screened' type outlet plate would be strongly recommended, if not there already:
http://www.maplin.co.uk/module.aspx?ModuleNo=30772&doy=25m9

Also the fly lead may be poor

failing that

try complaining to the retailer - that an old CRT gave a perfect picture at the same location.

They may give you a clip on ferrite to go on the cable, which may make a difference, these things:

http://www.maplin.co.uk/module.aspx?TabID=1&criteria=usb&ModuleNo=32799&doy=search&C=SO&U=Strat15

dcGT · 26-09-2007 7:39am

Thanks Antenna. I'll give those a try. Is there a way I can tell if the existing outlet plate is 'screened' before I buy another?

Cheers,

DC

DingDong · 26-09-2007 8:19pm

If its an NTL outlet box it is dcGT

byte · 26-09-2007 8:51pm

Did you replace the flylead as suggested by Conax? Many of those premade leads are cheap thin cable, and usually with dire screeening and connections. Being new doesn't mean it's perfect.

A lenght of CT100 with connecters on either end would be my first port of call (as already mentioned).

Bazzy · 26-09-2007 9:06pm

There is probably more strain on the lead than normal. The crt tv's the lead goes on the back.

Whereas with the lcd's / plasmas the input is normally underneath of on the side when you bend the lead this will add strain on the stinger (copper piece) inside the cable and cause bad picture

dcGT · 28-09-2007 8:08am

Thanks for all the suggestions folks. Can anyone recommend a good place to buy a better fly cable? Maplins perhaps? Should I buy the components there a make a fly cable?

@Bazzy, there's no strain on the cable and it goes directly into the back of the TV (the more common way), not at the side. Good point though.

Thanks again,

DC.

Antenna · 28-09-2007 8:20am

dcGT wrote:

Thanks Antenna. I'll give those a try. Is there a way I can tell if the existing outlet plate is 'screened' before I buy another?

Cheers,

DC

unscrew the outlet plate and look at the back of it. If screened, the connection is all enclosed in a metal enclosure on the back of the outlet plate. If not screened, the 'inner' of the coax is 'uncovered' and acts as an aerial and can picks up interference from things nearby.

byte · 28-09-2007 3:08pm

dcGT wrote:

Thanks for all the suggestions folks. Can anyone recommend a good place to buy a better fly cable? Maplins perhaps? Should I buy the components there a make a fly cable?

@Bazzy, there's no strain on the cable and it goes directly into the back of the TV (the more common way), not at the side. Good point though.

Thanks again,

DC.

Best to buy the components and make it yourself.

Interference on certain LCD TVs with NTL analogue?

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