Incoming Cable(ntl) signal overloading PC via soundcard connection to radio?

Emerson · 17-08-2004 08:35PM #1

Hey all,
Hope someone can help me on this one.
I've got a stand alone high quality FM tuner (from the 80s, picks up everything). I hooked it up to the ntl cable for it's radio selection.
Output sounds perfect when put into an amp but when the audio output is put into the PC sound card (Videologic - SonicFury) a fairly loud hum comes across the speakers (even when the audio is muted in Windows) and the LCD monitor experiences slight interference.

I think it's something to do with the cables resistance or it's overloading the pc via proxy (the radio). I've put a few splitters and switched off (and removed) the signal booster for some attenuation but no joy.
The problem imediately disappears when I plug out the cable in the back of the radio or replace it with an antenna and I can record from the tuner onto the PC.

Love to get to the route of this

Emerson · 18-08-2004 09:37AM

ntl coax > standalone tuner > phono cable > line in on soundcard

Would this connection be causing a drain on the PC's power supply? Doesn't seem to be the radio itself as when there is just an antenna plugged in, there is no problem.

I can't plug the coax straight into the pc as it's a sound card in there, not a tuner!

John R · 18-08-2004 09:27PM

it sounds like an earthing problem. The ntl cable carries quite a large current, it is probably being transferred to the PC causing interference.

bazzer · 19-08-2004 12:20PM

I agree with John R - sounds very much like an earthing problem.

Emerson, if you don't have a proper Cablelink/NTL TV-FM outlet box, ring them up to have one installed (it's free), or failing that buy yourself an in-line co-ax isolator from Peats or somewhere similar.

Emerson · 19-08-2004 12:53PM

That reminds me, I don't know what happened to our NTL box. There used to be one there (builder must've given it the snip). It's just a cable coming in now which is split.
There should be a ground coming in though, shouldn't there?

Will an ntl box not just bridge the cables grounding outer sheath (or lack of) as before or does it have some sort of internal grounding ciruitry?

I'm intrigued, what will an isolator do? Reduce the current?

bazzer · 19-08-2004 05:27PM

An isolator will make sure that no voltage passes through to your equipment, except of course that of the TV signal itself. The voltages used on NTL's cable lines can be up to around 60 Volts. Sometimes it actually makes its way down the drop cable into the house, but would normally be blocked by an outlet box.

If not, it may sometimes manifest itself as what's known as 'hum bars', horizontal bands moving up and down the TV screen. Or in your case, as a humming sound on your audio equipment.

Isolators were (I don't know if they still are) used on some of Cablelink's MMDS installations in lieu of an Outlet Box to provide protection against surges into the decoder from the aerial, especially during lightning storms when the outside air would be subject to an increase in static.

They way you have described your situation, it seems as if you may be missing an Outlet Box. These boxes (and isolators) keeps the grounding part (the surrounding braid of co-ax) connected at all times, but the centre core is only allowed to pass through the frequencies of the TV signal - and not the 50Hz frequency of voltage supplies.

Emerson · 19-08-2004 09:10PM

Great info.. thanks!

I see a phono isolator is Maplins(http://www.maplin.co.uk/products/module.asp?CartID=040819205835943&moduleno=33172), this'd probably stop it getting through but I should ultimately place one along the coax before it gets into any electrical equipment.

Getting a TV card for the PC next week and I'd say I'm going to have the same problem with it. Can't see a UHF/VHF Isolator in Maplins. Will be in town tomorrow, if anyone can tell me where else in Dublin I'd get one, I'll pick it up...

thanks again for the help!

Emerson · 20-08-2004 10:04AM

Will try Peats..

Emerson · 20-08-2004 08:19PM

Will do cableguy,

Peats don't sell them any more and Maplin.. oh dear god are they understaffed.. they had no idea, nor interest

Incoming Cable(ntl) signal overloading PC via soundcard connection to radio?

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