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Topic # 18026 23-Dec-2007 09:51
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Merry Christmas fellow Geekzoners.

I've had a problem with my ADSL connection for a couple of years now, and only thought about posting for a solution today.

We listen to NewstalkZB (am radio) once the TV goes off, and whilst the reception was never great in Wellington, we could still make out the conversations etc...

Over the last 12 months especially the interference on the radio seems to have gotten 10 times worse.  After some investigation late at night we've tracked the static down to the ADSL modem - switching it off fixes our static problem.

My question is; can we purchase some sort of filter to remove this static (other than the standard ADSL line filters we already use)?

Cheers.

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  Reply # 101497 23-Dec-2007 10:00
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The problem is that ADSL uses the same frequency band as AM radio (and an awful lot more besides).

You may be able to reduce the problem by shifting your ADSL modem to a jackpoint as far away as possible from the AM radio.  However if you have a central splitter installed, there will only be one jackpoint which is available for ADSL.

Actually, if you don't already have a central splitter installed, the best solution may be to get one put in at the Point-of-Entry to your house.  This will keep the ADSL frequencies out of your house wiring, and may prevent them from radiating at sufficient levels to interfere with AM radio reception inside the house.

Another simpler alternative is to listen to Newstalk ZB on FM:

Wellington/Upper HutNewstalk ZB89.4 FMNews/Talk

This information was found here:  http://www.clearchannel.com/IntRadio/New_Zealand.aspx

Is there some reason why you aren't using FM already?

The quality on AM is pretty awful compared to what you get on FM -- low bandwidth, mono, poor S/N ratio, generally YUK Frown

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Reply # 101499 23-Dec-2007 10:06
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You can even stream it live through the Yahoo!Xtra website (which goes through NewsTalkZBs website). Or through their own website.





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  Reply # 101514 23-Dec-2007 12:04
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Haha wow, you really do learn something new every day - I never knew ADSL used similar freq's to AM...

Should the signal be leaking out though? Surely it should be relatively contained to the wiring and not getting out to be able to cause interference?

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  Reply # 101517 23-Dec-2007 12:10
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willnz: Haha wow, you really do learn something new every day - I never knew ADSL used similar freq's to AM...

Should the signal be leaking out though? Surely it should be relatively contained to the wiring and not getting out to be able to cause interference?

You will find that most low cost hardware is built to a price, not a specification, and as long as they meet FCC guidelines etc, to hell with everything else!







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  Reply # 101522 23-Dec-2007 12:21
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willnz: Should the signal be leaking out though? Surely it should be relatively contained to the wiring and not getting out to be able to cause interference?

It's the primitive nature of phone wiring that is to blame here...

Remember, the POTS system was designed to carry AUDIO signals in the range of 30 to 3400Hz.  At these frequencies, the twisted pair arrangement works well and not much signal radiates out of the wiring.

But at AM frequencies of 500-1600kHz, it is a whole different ballgame given that this is well into the RF region.  In order to contain RF signals, coaxial cable should be used.  But it's not, because phone wiring was never intended to carry RF signals Tongue out

Eventually, this problem will be solved when we all have FTH, but who knows when that will happen.

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  Reply # 101529 23-Dec-2007 12:51
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Would it be best to use shielded twisted pair for all internal phone wiring then?

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  Reply # 101533 23-Dec-2007 13:09
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willnz: Would it be best to use shielded twisted pair for all internal phone wiring then?

Yes, that would certainly help to reduce interference so long as you grounded the shield.  However, in doing so, it might reduce the ADSL bandwidth slightly.  It would be a matter of trying it to see if the potential improvement in interference to AM radio was worth the slight degradation of ADSL speed.

It's a lot of hassle to rip out all your phone wiring and replace it though Yell

A better idea is not to listen to AM radio Cool

There are FM alternatives for pretty much all stations now, and as CM pointed out above, you can also stream many stations via the net nowadays.

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  Reply # 101549 23-Dec-2007 14:17
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As Grant and others have mentioned DSL shares spectra with the AM band, ADSL1 operates in the 25kHz to 1.1MHz region, AM radio in the 500kHz to 1.6MHz region, so plenty of space to cause problems.

Normally if the phone line is buried or at least of good twisted pair then most radiation is contained, however the biggest problem is non twisted pair cabling and house wiring that has 3rd bell wire that none of the house phones are using, this acts as an antenna and reradiates the DSL energy.

Personally I would giveup on AM radio, broadcasters are heading on dumping it, so be in before the rush.

On a more serious note, I would attempt to examine your house wiring, attempt to place the modem at the first socket entering the house, or add one there if you must, cable the ethernet with new cat5e cabling to the PC's or use WiFi. Add a central filter and place all phone lines down stream of the that via the filter.

This way all the house wiring will be post the DSL filter, so most DSL energy will be absent. Replace the lead from the phone jack to the DSL modem with as short as possible cat3 or higher cable, normal cables supplied with the modem are not twisted but flat pair and radiate quite a lot. You may need to get specialist cables made up for this as cat3 or 5 cables with RJ11s on the end cannot be readily purchased.

Finally change the house wiring to 2wire, this eliminates the bell wire reradiation. This is quite a problem, the two signal wires are terminated at voice and DSL frequencies so normally dont radiate too much, however the bell wire is connected to one of the signal legs via the ring capacitor and in most homes is not terminated as all the phones are newer types that dont touch the bell wire so it ends up unterminated and dragged around the house acting as a big beverage antenna.

Cyril

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  Reply # 101550 23-Dec-2007 14:18
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grant_k: ... it might reduce the ADSL bandwidth slightly
...
It's a lot of hassle to rip out all your phone wiring and replace it though


Why would it cause a degredation in speed rather than an improvement (ie, less interference getting IN to the cable as well as out) ? Plus the wiring at my place is due for replacement anyway, it's a pretty crappy job at the moment. I've always wondered what the cause of Mum's constant complaining of AM radio not working was! Tongue out

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  Reply # 101551 23-Dec-2007 14:22
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If you live in high signal areas (ie close to Txs like Henderson in Auckland) then you can couple quite a bit of energy back into the cable and upset the DSL service.

In the UK where there are quite alot of hi power AM Txs this has been a real issue, especially since BT never went down the same route as Telecom and dumping the three wire system some years ago.

Cyril

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  Reply # 101553 23-Dec-2007 14:26
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cyril7: If you live in high signal areas (ie close to Txs like Henderson in Auckland) then you can couple quite a bit of energy back into the cable and upset the DSL service. In the UK where there are quite alot of hi power AM Txs this has been a real issue, especially since BT never went down the same route as Telecom and dumping the three wire system some years ago.


Sorry Cyril I meant why would STP cable cause a degredation in speed - forgot to quote Grant's post :)

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  Reply # 101554 23-Dec-2007 14:32
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Its quite possible that in grounding or more importantly where grounded STP may or may not improve things. Personally I would have thought that STP would not effect the DSL performance, if you were to use it I would earth it at one end only and ideally the end that the modem is at, leave the earth at the other end isolated.

Cyril

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  Reply # 101559 23-Dec-2007 15:04
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willnz:
grant_k: ... it might reduce the ADSL bandwidth slightly
...
It's a lot of hassle to rip out all your phone wiring and replace it though


Why would it cause a degredation in speed rather than an improvement (ie, less interference getting IN to the cable as well as out) ? Plus the wiring at my place is due for replacement anyway, it's a pretty crappy job at the moment. I've always wondered what the cause of Mum's constant complaining of AM radio not working was! Tongue out

The reason I mentioned a possible degradation in speed was that ADSL uses frequencies up to 1.1MHz as Cyril has pointed out.  At those higher frequencies, it doesn't take much extra capacitance to attenuate the signal and as a result you will lose some of the highest frequency "buckets" used by ADSL.

At the point where the shield starts (assuming it is on the input side of the filter), a "step change" is caused in the impedance of the line and reflections will result.  Those reflections result in extra attenuation which typically affects the higher frequencies more, and thus reduces the maximum possible ADSL throughput.

Adding extra capacitance via a shield can be beneficial sometimes too, if that extra capacitance makes the line termination conform more closely to the characteristic impedance of the line, hence reducing any reflections.  We are getting into transmission line theory here, and it is a complex area, but suffice to say that shielding can be a good or a bad thing for bandwidth depending on various factors, and where the ground connection is.

One rule of thumb is:  NEVER ground the shield at more than one point, or you risk having ground loops set up, which can further degrade the signals carried in the cable.

If your phone wiring needs replacing then I would suggest:

1)  Put a hard-wired filter at the phone jack nearest to the Point-of-Entry
2)  Provide a separate jack for the ADSL modem wired on the input side of the filter
3)  Connect ALL other phone jacks in the house to the output of the filter
4)  Use all 2-wire phone jacks i.e. replace any 3-wire ones
5)  Use CAT5e cable for your phone wiring as it has better twisted-pair characteristics than the standard phone cable and should help to contain any residual DSL signals


Finally, re-educate your mother as to where she can find her favourite stations on the FM dial.  It's surprising how many older people haven't discovered National Radio on FM yet, simply because they've listened to it on AM for all their lives Smile

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  Reply # 101560 23-Dec-2007 15:16
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Finally, re-educate your mother as to where she can find her favourite stations on the FM dial. It's surprising how many older people haven't discovered National Radio on FM yet, simply because they've listened to it on AM for all their lives Smile


Yep sounds like an FM capable radio might make a good Chrisy present for your mum, cheaper than most other options mentioned here. I think most homes have a pretty high level of MF band interference due to DSL, PC's, even processors in the VCR, and one I noticed the other day, by default an AVR with integral loop antenna I was installing had been previously tuned to 567kHz Natradio that is 20km from here, As soon as I turned the new Plasma screen ON, buzzzzzzz, no more Nat Radio, quickly tuned 101.5FM problem fixed.

NewTalkZB's frequencies can be found here.

Cheers
Cyril



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Reply # 101578 23-Dec-2007 19:29
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Wow - what a fantastic response!

Initially I did a search on the net for a solution, but all I seemed to find was AM interferring with ADSL; hence I knew about the cross-over of frequencies.

Grant - I didn't realise there was an FM alternative; I'll give it a try, but I'm pretty sure that's a Kapiti Coast frequency - not Wellington.  The reason I think this is that I've already complained to the Radio Network about their reception in Wellington.  Their response was that there were no plans to broadcast in FM.

If there is no FM, then it looks like I'm pretty much stuck with turning the modem off.  Not a huge deal, but a little annoying none-the-less.

Cheers.

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