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Author Topic: New BT Bell Wire Filter  (Read 22571 times)


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Re: New BT Bell Wire Filter
« Reply #30 on: January 31, 2008, 07:21:43 PM »

Thanks, sorry for the confusion, I was meaning 2 way filtering in both directions of the telephone side of things as opposed to the 2 seperate sides of adsl and telephone. So the filters on the NTE5 telephone side filter out the adsl signal going onwards, and also filters out the adsl interference coming back.

                                                 >>>>>>>> filtered telephone signal >>> telephone
1) Telephone line >>>>>NTE5 filter     
                                                 >>>>>>>> unfiltered adsl side >> adsl modem

                                                                       blocked <<<< intereference on extensions on same frequency as adsl <<< telephone
2) Telephone line <<<<< NTE5  filtered faceplate
                                                                       <<<<<<<<  adsl modem



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Re: New BT Bell Wire Filter
« Reply #31 on: February 01, 2008, 12:01:02 AM »

This is a very good question and I certainly do not know the answer.

Given the component is a choke so to speak, one would think the component itself would work the same in either direction. Unfortunately I do not know myself exactly how it all works in practice.

I get rather confused as for example I originally thought the ring wire only carried current when the line was rung and the voltage increased and so the capacitor generated the ring signal??

Then again the I guess the ring wire would have to be somehow "powered" to act as an antenna?

If I can I will endeavour to ask someone who knows something about electronics (unlike myself)  ::)


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Re: New BT Bell Wire Filter
« Reply #32 on: February 01, 2008, 07:16:33 AM »

The point about the ring wire is that the 2-wire input is essentially balanced as far as the ADSL signal is concerned. The two wires are also twisted together. So any interference which is present is picked up equally by both wires, so there's no noise voltage difference across the pair, and therefore the interference has no effect on the signal.

But the ring wire is a single wire connected via a capacitor to one side of the signal pair. So any interference which is picked up by the ring wire is fed via the capacitor to one side only of the pair, and therefore causes a noise voltage difference across the pair. In some situations this can have a considerable effect on the quality of the ADSL signal.


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Re: New BT Bell Wire Filter
« Reply #33 on: February 02, 2008, 10:26:21 AM »

Hi Soms, yes I am a CSE, been on the broadband side (on top of "proper" faults )for 3 1/2 years now

If there's an issue with the internal wiring from the NTE about half the time it's due to the bell wire on it's own. When you look at the basic circuit diagram for a radio, it has a good resemblance in part to an nte, particuarly with the bell wire.

The choked plate tries to get around this problem, as I mentioned before you can reconise one either by the flap at the front being a slightly off white-grey tint. unless it already came with an nte with the openreach logo. other wise look for a small (I think I mistyped it as smell before, soz) cylinder behind the plate.

If the problem is caused by the other pair of wires (terminated in ports 2 & 5) then you would look at the dsl/ssfp front plate. The one with both a telephone & dsl socket on the front much like a micro filter.

The micro filters let the dsl signal go straight through them, the filters are there in case any equipment such as phones faxes, sky boxes,etc act like a radio reciver (by radio I mean any electrical noise, not bbc broadcasts & the like) & throw some of this noise on the phone line. If it's some thing the broadband can hear then it affects the quality of your connection to the net.
Also some times your equipment might hear the dsl signal & convert it to something you can hear so the microfilter prevents you trying to have a normal phone conversation over some horrible squealing & shushing noises.
This can come through either the bell or voice (other 2) wires.

As Roseway has said, some phones will respond to the ringing down the line without a bell wire. if you have the bell wire disconnected for any reason then microfilters have a capacitor within them which will still allow any bit of equipment to ring on an incomming call regardless if the equipment needs a bell wire or not.

Using CAT3,5,or 6 cable means you're less likely to pick up interference via the cable, it's really intended for digital data signals, the extra twists on each pair of wires mean one cable is less likely to overhear a nearby similar digital signal. Hence the're common in commercial networks where there are lots of data cables in close proximity.

As to wether old internal wiring or pristine new wiring would give you a better result for broadband, It's mainly luck. If the wiring is in tune/harmony with some noise source. You'll have an issue.

Don't know if this makes things clearer, ho hum..........


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Re: New BT Bell Wire Filter
« Reply #34 on: February 02, 2008, 05:19:49 PM »

...... Don't know if this makes things clearer, ho hum..........

Thanks for that excellent explanation ...  certainly makes things clearer as to how it all works.  :)
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