Tetech ADSL Splitter/Filter

What does an ADSL Line Filter do?

ADSL Line Filters are the ubiquitous little white box you’ll see on many phone sockets in Australia. What does an ADSL Line Filter do and do you need one?

ADSL Frequency Plan
ADSL frequency plan – Licensed under Creative Commons Attribution 2.5 via Wikimedia Commons

An ADSL Line Filter or ADSL Low Pass Filter is a device which separates signals intended for ADSL modems from voice signals. Most of the human voice is at sound frequencies below 3000hz. ADSL works by sending signals along a phone line which are at higher frequencies than those used for human voice. A simple ADSL filter will block signals above 3000hz from coming out the phone side of the filter. It will also stop voice frequencies above 3000hz from being carried on the phone line, where they would interfere with the ADSL.

Do you need an ADSL Filter?

If you have any sort of telephone(fixed handset or wireless) or fax machine connected to your phone line, the answer is yes. Any phone, fax machine or any device other than an ADSL Modem must have an ADSL Line Filter attached between the telephone wall socket and the device. However ADSL modems do not need to have an ADSL filter. Some ADSL filters are also line splitters, they have a socket marked phone and a socket marked ADSL. This is for convenience only, so you can connect a phone and a modem at the same socket. If you’re only connecting an ADSL modem to the socket, no filter is needed. Likewise if you’re using a naked DSL type product, again no filter is required.

Types of ADSL Line Filters

ADSL Inline Filter

A basic ADSL Inline Filter is a simple white box with a phone socket or short cable attached at either end. One side of filter will usually be marked “Line” as is connected to the incoming phone socket. The other end is usually marked “Phone” and is connected to the telephone or other device.

ADSL Inline Splitter/Filter

Tetech ADSL Splitter/Filter
Tetech ADSL Splitter/Filter

An ADSL Inline Splitter/Filter is similiar to an ADSL Inline Filter, except it will have three connections. One to go into the wall socket and on the other end one marked “ADSL” and one marked “Phone”. The purpose of this is to make it easy and convenient if you need to connect both a phone and a modem at the one socket. Plug the ADSL Modem into the side marked “ADSL” and the phone into the side marked “Phone”. As with the ADSL Inline Filter, the connection marked “Line” goes into the wall socket.

ADSL Central Filter

An ADSL Central Filter
An ADSL Central Filter

An ADSL Central Filter is usually installed as a fixture of the house or building. The purpose is to eliminate the need to have an ADSL Inline Filter at every phone or other device on the line. The ADSL Central Filter splits the phone line into two, an ADSL line and a phone line. The phone output is connected to the existing wiring that feeds the phone sockets throughout the house. A separate phone line just for the ADSL is connected to just one socket in the building, where the ADSL Modem will be installed. This doesn’t necessarily require a whole new cable to be run, as in many cases an existing unused cable pair can be used.

A benefit of an ADSL Central Filter is that it can result in better line sync speed. This is because less of the building’s wiring is carrying the ADSL signal which reduces the potential for minor line faults and other issues to reduce signal quality. If you use ADSL I strongly recommend central filtering.