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ADSL filters

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ADSL filters

Hi,
I am getting noise on my DECT cordless phone so I thought I would look through the Forums first. A reply to Valvejam caught my eye and got me wondering. I have a filter in the line to my computer because I also have a dial up modem which I still also require. Is a filter necessary with no phone connected to that socket?
newboy
19 REPLIES
Community Veteran
Posts: 14,469
Registered: 30-07-2007

ADSL filters

The filter is only required when you want to connect a normal phone to a socket. Your DECT must be connected via a filter if your ADSL modem is connected anywhere in your house.

See the ADSL: Wiring and Filters tutorial.

Note: if your DECT phone is connected through a filter it may be faulty so try another one.

Also if the noise is still there with your ADSL modem not connected you may have a fault on the line - this would also effect your normal 56k modem's connection speed.

Cheers

Peter Cool
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ADSL filters

You do not need a filter on that socket if the DSL modem alone is connected. However, a normal 56K modem is counted as a phone, so of that is connected, you will....

Your cordless will need to be connected to its socket via a filter.

Is your cordless phone base near to the PC or DSL modem Huh You may be picking up stray interference from the PC / Modem
Community Veteran
Posts: 6,111
Thanks: 1
Registered: 05-04-2007

ADSL filters

You wouldn't happen to have a wireless network would you? That can bring about interference over digital phones (though I have to admit I don't experience that problem). You may also experience noise when using microwaves near the phones...

Thomas
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ADSL filters

Quote

have a filter in the line to my computer because I also have a dial up modem which I still also require. Is a filter necessary with no phone connected to that socket?


Yes you do.

The only device on the line that should remain unfiltered is the ADSL modem.

Analogue modems, phones, answers phones, analogue alarms, SKY, TiVo and anything else you can think of that uses the phone line, should be filtered.
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ADSL filters

Quote
The only device on the line that should remain unfiltered is the ADSL modem.


By this, are you implying that if you connect only a DSL device to a phone point, it doesn't require a filter?
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ADSL filters

A DSL modem does not need a filter period.

Most DSL ports on filters are simply a unfiltered passthrough.

The only real need for the filter in this isntance, is the fact that most DSL modems come with a lead with a RJ11 terminal on each end.
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ADSL filters

There are two lines of thought on this.
A filter supplied by the likes of D Link or the basic
Solwise filter is no more than a low pass filter .
This basically filters the high fequency DSL signal
from your telephone and prevents unwanted noise.
The input for the modem is more or less a direct path to
the telephone socket so actually connecting your modem
directly will make no difference.
However the high frequency DSL signal is also subject to
interference from the low frequency voice signal especially from
analog modems, fax macines, etc. and also is prone to
RF interference from the line.
There may be some advantage in also filtering the dsl
frequencies for some modems, it gives them less work to
do thus improving efficiency and speed and cuts out interference
reducing disconnections.
Filters from the likes of Speedtouch contain high pass
circuitry and they insist that you connect
their modems to the filters supplied rather than direct.
So I'd say depends on modem.
Alec.
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ADSL filters

That's interesting. In dealings with BT in the past, it's been suggested that an unfiltered DSL modem can result in dropped connections as a result of interference from incoming calls. Now my knowledge of electronics is not sufficient to examine this argument critically, but what you say would seem to entirely contradict BT - especially if the DSL connection on a filter is, as you say, actually an unfiltered pass-through. My understanding was that DSL filters worked on both connections, filtering the pstn frequencies on the one hand, and the dsl frequencies on the other. However, I'm quite happy to accept that this might be incorrect since, as I say, my knowledge of electronics is not great, and I've never actually pulled apart a dsl filter!
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ADSL filters

Granted.

It does depend on what filter you actualy purchase though.
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ADSL filters

Quote
That's interesting. In dealings with BT in the past, it's been suggested that an unfiltered DSL modem can result in dropped connections as a result of interference from incoming calls.............

BT have in the past mainly supplied Alcatel speedtouch modems
and routers such as 'The frog' and hence the requirement for
filtered ADSL. It should be a package really.
If the modem is already designed with a proper filter integral,
then there would be little advantage in a high pass filter,
however it's unlikely that many budget modems are,
so in a situation where your line is some distance from the
exchange or your suffering disconnections then it may help to change
filters as long as you research into what your actually buying.
Alec.
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adsl FILTERS

Many thanks for all the comments. However, a lot of them are way over my head. I have solved the problem by waggling the filter which was plugged into a 3 metre extension wire. It would appear that the plug was not a very good fit.
All is now well as I can go on line and my wife can chat away on the phone. Harmony at last!
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ADSL filters

Maybe we do get to carried away on technical issues. The old "give it a kick" answer may work sometimes.
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ADSL filters

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If the modem is already designed with a proper filter integral


Aha! So modern modems have an in-built filter? This was the missing piece of the puzzle. I get it now!
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ADSL filters

Not all DSL modems have some form of input filtering.
The cheaper products may have no input filtering at all, while more expensive units may have "band filters" to allow only the wanted signals to pass.
It's the old story of "you get what you pay for".

You may be getting confused with DSL modems that have inbuilt filters for analog phones, similar to 56K modems. You plug the modem directly into the socket and your phone plugs into the modem !!!