Do I really need another ADSL filter?

  Wilham 22:33 04 Oct 03
Locked

My BB kit came with the ADSL modem and two DSL filters. Tiscali will sell additional filters at £10 ea plus delivery. We have just bought a digital cordless 'phone from Argos and I wonder if just putting an extra filter inline is the best solution?
I know there's a lot of tech know-how/experience out there, so can you help me on any of the following...
(i) Fiddling with an ohmmeter, I guess inside a filter are four 10 ohm resistors as the uprights of letter H with a capacitor across the middle, ie a simple balanced low-pass filter flowing
from top down. Could I be right?
(ii) The two filters are positioned outside the telephones. Doesn't this weaken the signal to the BB modem? Wouldn't it be better if the filters were wired the handpiece sides of the cradle switche and thus in circuit only when voice is used?
(iii) My son suggests it would be better if I connected the voice 'phones together to share the same filter. Is there a reason against this if the wiring is possible?
Many thanks.

  LastChip 22:50 04 Oct 03

The only reason you need a filter, is because when data is traveling along your telephone line, it causes noise in the handset. Indeed, if NO data is flowing, you can hold a perfectly normal telephone conversation on a ADSL enabled line.

So, if you have a cordless telephone, the telephone analog signal is arriving at the base station, which should be filtered in the normal manner. Once it is re-transmitted to the handset, it requires no further (re)-filtering, as ADSL data is no longer an issue.

Hope that makes sense!

  LastChip 22:53 04 Oct 03

Should read;

Indeed, if NO data is flowing, you can hold a perfectly normal telephone conversation on an ADSL enabled line without any filters at all.

  Wilham 23:00 04 Oct 03

I thought same as you. So I switched off BB, removed filter, and mush was still there. I guess the noise comes from other BB users, I believe up to 20 at once.

  Wilham 23:28 04 Oct 03

Before, I switched off BB and left the desktop off-line. I have checked again and this time switched off the power as well,- and the mush cleared. So the modem alone drawing power from the usb is enough to cause the noise.
Freedom to use the computer anytime makes some form of filter essential. I find your comments interesting, and learnt something. Thanks.

  DieSse 23:38 04 Oct 03

All you ned to know about ADSL filter and microfilter circuits. click here - courtesy of a simple google search.

If you don't use a microfilter, and your phone rings while you are using ADSL, it'll usually cause a break in your connection.

  Wilham 00:00 05 Oct 03

This is useful, but not quite all at first survey. I need the loading figures. They may be there, I need to find the drain on the BB signal imposed by the filter(s).
It's great to even see inside the boxes,- thanks.

  LastChip 00:33 05 Oct 03

When I first got broadband, it was at the beginning of the "boom" and I found myself in the unenviable position of having an ADSL enabled line, but no modem or filters to setup the system.

As I rely heavily on my telephone (don't we all!), I telephoned the suppliers of the equipment to explain my predicament. They told me, exactly what I have passed on to you, and sure enough, although I had no equipment installed, the telephone worked perfectly normally. This really bares out what you have discovered by turning off your modem (in my case, it didn't exist). So, could it be you have a fault with your new modem?

The other thing that springs to mind, is there was an issue with some USB modems, (Alcatel in particular) where the available power via the USB port, was barely enough to run the modem. This did lead to some strange symptoms. The fix was to buy a USB powered hub, and run the modem from there. Since then, I believe there has been a driver upgrade which helps the problem. I'm not saying this is the answer, but it may be worth consideration. USB ports are normally restricted to 500ma output, but if your computer power supply is being strained, you may not even be getting that.

I don't see how other broadband users could affect your line, as to a great extent, it is specific to you. The contention ratio talked about, is simply the amount of potential users compared to the available resources.

  Wilham 11:16 05 Oct 03

You have made me think again about the modem. But my first concern is the loading of the BB signal by the filters. Reading reviews at click here made me see how little info was given with the filters. DieSse above suggests Google is simple. I'd just spent half hour on it looking for the BT standard specs for DSL filters,- without success.
I'm going to try filter sharing to see how it goes.
Your comments are appreciated, thanks.

  DieSse 11:22 05 Oct 03

During all this, you haven't actually said you have a problem - do you?

Why not just get another filter, this would be the normal course to take.

  Wilham 12:06 05 Oct 03

Now and then my BB connection is awkward. It may happen to all, but in S lincolnshire I want to make sure I have done my best at my end in handling the signal. Designers using filter circuits usually are cocerned with a one track flow. Filtering implies a rejection of something. This rejection can be dissipated as heat or bounce back to source. This source includes the BB signal.
There is a compromise between preserving the BB and providing cheap, practical filtering of mush from the voice carrier.
We already have the REN limitation. Separately I've heard assurance that up to five filters can be OK. Truth is that all such loading affects the BB signal to some extent, ...I'd like to know I'm using the best arrangement.
OK, you're right, easy answer is to buy a third filter. But if nobody points out an error I'm going to join two circuits together through one filter. If it goes well I'm happy. and happier still if I can organise things even more efficiently. Thanks.

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