Thursday, September 20, 2012

ADSL Filters - An Easy Mistake (Links Updated)

High-speed Internet (Broadband) is now routinely provided in the UK over telephone wires to most areas. As a result, adjoining wires in the cable and on telegraph poles pick up the signal by induction or capacitative coupling and carry it to all telephones in the neighborhood. Consequently, all domestic phone sockets in the UK should now be fitted with an Asynchronous Digital Subscriber Line (ADSL) Filter to remove these extraneous signals, whether or not broadband service is provided to the property in question.

ADSL filters are not much bigger than a matchbox and are simply plugged in between the telephone and the phone socket but not all ADSL filters are suitable for all phones and the two basic types provide differing levels of protection.Though they look almost identical, the two type are easily confused.  

ADSL Filter Types:

Two-Wire: These provide the best filtering protection but some phones will not ring, even though they will otherwise operate normally.

Three-Wire: These provide less protection because the third (ring) wire usually passes straight through, unfiltered. This type will however ring all types of phones provided all 3 wires have been properly connected in the wall outlet.

How To Tell:

It is not always easy to establish the type by looking. Catalogues and product descriptions usually do not say. An easy way to tell is if only 2 of the little gold contacts are visible in the BT style plug. Some filters are fitted with plugs which have 4 contacts present, though by sight it is not easy to tell whether 2 or 3 of them are connected. 

Another way to tell is if a phone will not ring with an ADSL filter plugged in, then the chances are that the filter is the more effective 2-wire type. Ask your supplier which type they have but many customer service staff are ignorant of this somewhat fine distinction.

More expensive filters may also have a ferrite choke fitted around the wire to the socket (to eliminate higher frequencies) but in many situations, additional benefits may be inconsequential.


To further reduce electrosmog reaching the head, telephones should be of a type which do not have electronics in the handset (the bit normally held to the head). 'Gondola' type phones, with push buttons or displays in their handsets are therefore to be avoided. Likewise, hearing-aid compatible handsets which use induction to connect to the hearing aid. Use of a speakerphone facility eliminates the need to hold the handset to the head.


Most speakerphones require a mains transformer which adds to electro-pollution in the vicinity. Some speakerphones are ‘line driven’. That is, they use power provided by the telephone exchange and are thus not favoured by phone companies. Phone catalogues often do not indicate whether models are line-driven, battery powered or mains (transformer) powered, making line-driven speakerphones difficult to obtain.  

End Note: 

Whilst price is not always an adequate reflector of quality, cheap (<£10) or Free ADSL filters may not be very effective at attenuating the EMR accompanying the ADSL signal that can reach the handset. The signals may well be inaudible. 

For the EHS, and all who wish to minimise their EMR exposure, handsfree phones with ADSL filters are helpful.

Click the following link for a very helpful and informative page on Broadband Filters and Speeding Up Your ADSL