Microfilters - what they are and where to put them
When your phone line is enabled for ADSL, an additional signal is placed on it, alongside all the normal telephone activities of voice, ringing, dial tones, caller ID signalling etc.
This new ADSL signal is used by your ADSL modem to talk to the rest of the ADSL system and thereby the Internet. Unfortunately, if we allow the ADSL signal to enter telephones, we will hear noise and distortion, so it is important to exclude the ADSL part of the combined signals from standard telephone devices.
Note the use of the word devices here; all conventional telephone equipment is designed to do its job using the range of frequencies provided by the phone system for its primary purpose - transmission of human speech. So faxes, Sky Digiboxes, Caller ID displays and your old 56k analogue computer modem all count as telephone devices.
Since ADSL is added on to the conventional phone system, it has to be able to tolerate all the normal things that happen when phone calls occur. This is where the microfilter plays an important role.
When ADSL was first rolled out in the UK, it was installed by a phone company engineer. The usual practice was to fit a special double socket, with the filter incorporated into it. One of the two sockets provides telephone services, and the other the ADSL service. The disadvantages of this approach were:
In contrast, microfilters are fitted by the user and any phone socket on an ADSL-enabled installation can be used for broadband services.
The filter splits apart the telephone voice signals from the ADSL signal, so that the performance of any telephone device attached to an ADSL-enabled phone line will not be degraded.
The long and short of this is:
Every device attached to an ADSL-enabled phone line must connect through the appropriate port of a filter
There are some minor exceptions to this rule - the ADSL port on a microfilter is actually a straight-through circuit. So your ADSL modem does not need a filter by itself, but you must fit a filter if you wish to connect a phone to the same phone socket as the ADSL modem. If you don't need a filter, then you must find an RJ11 to BT Plug 431A lead to be able to connect your ADSL modem directly to the phone socket.
What kind of filter should you buy?
There are several varieties of filter in the marketplace. Remember that you get what you pay for. The better units are about the size of 2 matchboxes and cost in the region of £10 each (though you may find them for less). BT supply the "Excelsus" brand. You may encounter some which are shoehorned into a modem adapter style connector - these generally do not work so well and may cause you problems. Bear in mind that you may be paying £20 a month or more for a Broadband connection - why compromise it for the sake of a few pounds of initial investment? ADSLNation have a useful comparison of some of the more commonly encountered microfilters.
What happens if you don't have all the filters in place?
Connectors
In practice, you will meet, in the UK at least, three distinct styles of connector.
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