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What Do Microfilters Do?

A Microfilter separates the voice signal off of the telephone line leaving just the Upstream and Downstream signals. The telephone line is used for 3 things, it sends data to the Internet (the Upstream), it receives data from the Internet (the Downstream) and the voice signal is carried down the line, and these three things happen all at the same time. This is achieved by splitting up the frequencies available in the line, and is explained in how does it work?

Basically a Microfilter splits your line into a voice port and a data port by taking the incoming voice signal off of the line and sending it to the phone and taking the phone signal in and sending down the cable at the correct frequency so it does not interfere with your Broadband data. The Broadband signal is not touched by the Microfilter, it travels straight through so the quality of the Microfilter depends on it's ability to remove the noise caused by the voice signal as efficiently as possible.

They also have the ring capacitor built into them. This means that even if any of your telephone equipment does need the ring wire, which is not very common these days, the Microfilter replaces it if you have removed the ring wire to improve your SNR margin.

The Types Of Microfilters

Active or Passive?

Although Microfilters come in many different shapes and sizes, there are essentially two types of filter coming in three packages. The two types of Microfilters you get are either Active or Passive Microfilters. If being completely honest, I don't know how they filter the signal differently but I understand that active Microfilters use some of the current in the telephone line to help filter the telephone signal out and are generally thought of to be better quality Microfilters. The three packages that Microfilters come in are Filtered Faceplates, Filtered Sockets and Standalone Filters.

Filtered Faceplates

Filtered Faceplate

A Filtered Faceplate is a replacement front module for an NTE5 socket with the filtering components built in and is noticeable for it's telephone socket and data port. These can be used to take care of all filtering for your house, so you don't need to use any other filters. These are generally seen as the best option and if your have problems with your Internal Telephone Wiring you may find a good quality Faceplate will help no end. They can also be used if you want to route filtered signals to all sockets except one socket. This unfiltered line could run to a socket closer to your computer than the NTE5 socket. In this Internal Telephone Wiring configuration it is important that the cabling carrying the unfiltered line is as high a quality as possible.

Filtered Sockets

Filtered Socket

A Filtered Socket looks just like a regular extension socket however, like a Filtered Faceplate, it also has a telephone socket and data port. Filtered Sockets are used to tidy any unfiltered lines. You see every unfiltered socket must be filtered before a telephone is plugged in or the telephone signal will interfere with the Broadband signal. Using one of these instead of a regular extension socket is a way to make sure that any unfiltered lines are getting filtered at the box so you don't need to have any unsightly standalone filter plugged in.

Standalone Filters

Standalone ADSL Filter

Standalone filters, just referred to as Microfilters, plug into an unfiltered telephone socket and come in many different shapes. As already stated if you have any lines in your house that have not previously traveled through a filter, like a regular extension connected to a standard NTE5 socket, a standalone filter must be plugged in before any telephone equipment should be plugged into it. Don't be mistaken, A poor quality Microfilter at the other end of the house from the computer and NTE5 socket could cause no end of problems. Generally you can check for faulty Microfilters by unplugging all Microfilters (and at the same time unplugging the telephone equipment connected to the filters) from the line and replacing them one by one until you find the one causing the problem.

Don't by afraid to try putting two filters in a row, known as double filtering, if you feel a certain attached device may be causing a problem. In the past certain Sky boxes needed to be double filtered as the seemed to place a lot of noise on the line. If when doing this the modem / router also needs to be plugged in here make sure that the modem / router is plugged into the first of the filters. i.e. the filter that the second filter is plugged into.

If you do no have any telephone device plugged into the socket it does not need a Microfilter. However it's always worth putting unused Microfilters on unused, unfiltered sockets just in case someone in the family plugs something into it one day. Although in some cases this has been reported to have a slightly negative effect on your SNR this is not nearly as bad as the effect of someone plugging in and using a telephone on an unfiltered line.

Where Should You Put Your Microfilters?

I must admit, whenever I am now told that all filters have been checked I don't believe it. Just about every problem I've looked at recently the phone line has not had a filter in all the correct places, even if the main problem was not one of filtering. Every device plugged into your telephone sockets must have a Microfilter between it and the telephone line exiting the house. This is maybe not always as obvious to some as it may sound and the correct way depends on how your telephone wiring has evolved since it was first installed.

Below are 6 different types of wiring scenarios. Although yours may be slightly different the message for all of the scenarios is the same. The black telephone cable shown in the pictures is the Drop Cable. This is the cable that comes in to the house and connects, usually, to the Master Socket.

In a scenario 1 the line that comes in to the house, the drop cable, runs into the back of the NTE5. A line then runs out of the standard faceplate to two normal extensions. In this situation the Standalone Filters have been put on all 3 lines before connecting telephones as the lines are not filtered at the source by the standard faceplate and the extension sockets are not Filtered Sockets. All sockets can be used to connect to the Internet. If the NTE5 socket was an old style BT Master socket you would still have to filter the line in this way.

Although the drop cable shown goes directly to the NTE5 sometimes it may go through a small junction box. If you are getting any work done by BT, like changing an old style Master socket to a NTE5, it is worth asking them if they could remove the junction box and run the cable straight through to the NTE5 to remove one last join from the line. This may bear a small charge but not a great amount, just ask how much first.

In scenario 2 the bottom extension socket is a Filtered Socket so this does not need a Standalone Filter. As all the other sockets are unfiltered they need Standalone Filters plugged in even though the top extension is on the other side of a Filtered Socket. This is because usually you connect the cable to the second extension into the same IDC connectors that the incoming cable connects to. This means they connect before the filter not after it. Only if the other Extension socket was a Filtered Socket would it then not need a Standalone Filter. Again, all sockets can be used to connect to the Internet.

In scenario 3 a Filtered Faceplate is used at the NTE5. As it is filtered it does not need a Standalone Filter at the front of the Faceplate. The line coming out of the Faceplate and going to the extension sockets is connected to the filtered connections on the Filtered Faceplate. These will be labeled depending on the manufacturers specifications but are usually numbered 2, 3 and 5 as per usual internal telephone wiring rules. To check this you would need to check either the documentation which came with the Filtered Faceplate or contact the manufacturer or re-seller. As this is the filtered line it will run through the filter in the Faceplate so the extensions do not need a Standalone Filter. These extensions will not be able to be used to connect to the internet.

Scenario 4 is similar to scenario 3, there is a Filtered Faceplate at the NTE5. As it is filtered it does not need a Standalone filter at the front of the Faceplate. The line coming out of the Faceplate and going to the top extension socket is connected to the filtered connections on the Filtered Faceplate and, therefore, does not need a Standalone Filter. The difference here to scenario 3 is that the line going to the bottom extension socket is connected to the unfiltered connections of the Filtered Faceplates. These will be labeled depending on the manufacturers specifications but are usually lettered A and B like the incoming line. To check this you would need to check either the documentation which came with the Filtered Faceplate or contact the manufacturer or re-seller.

As the bottom extension socket is a Filtered socket it does not need a Standalone Filter but if it were a standard extension socket like the top extension socket it would need a Standalone Filter. Also, as it is on an unfiltered line from the Filtered Faceplate you will be able to use the bottom extension socket to connect to the Internet.

Scenario 5 is similar to scenario 3 but both extension sockets are connected to the unfiltered line on the Faceplate. As the top extension socket is a standard extension socket it needs a Standalone Filter but as the bottom extension socket is a Filtered Socket it does not need Standalone Filter but if they were both just standard extensions they would both need Standalone Filters. Either of these extension sockets could be used to connect to the Internet.

The other thing you may see is a star configuration like the one shown in scenario 6. This is tricky to know what is your side and what is BT's side. The NTE5 shown is on the bottom left but, even though it is still the Master socket with the components inside, the drop cable goes into the extension socket in the middle. This is not always a socket, it may be a distribution box. In this situation a Filtered Faceplate is pointless, none of the other extensions would be serviced from the cables coming from it as their line comes from the extension in the center and not the NTE5. In this situation the 2 Filtered Sockets do not need Standalone Filters but the NTE5 and the two standard extension sockets do.

In this situation it is well worth having the center socket swapped, by BT, with an NTE5 and having the extension socket where the NTE5 is currently located. This way you can get the internal wiring all running after the NTE5 as it should be. After the sockets have been swapped the setup should run similar to the Re-wired scenario below. This will allow you to wire all the extensions through the NTE5 faceplate. Either way if your telephones are wired in this way I would definitely remove any ring wires.

The above examples are supposed to demonstrate that it does not matter what configurations your internal telephone wiring uses, the important thing is that before every piece of telecom's equipment there must be a Microfilter between that piece of equipment and the Demarcation point, which is either the Test socket on an NTE5 or the front socket of the old Style Master Socket.

Extension cords

The last things that need to be considered are extension cords which plug into the front of existing sockets, rather than being wired into the socket internally. These should be avoided if possible however, if you plug an extension cord into a socket to provide an additional socket further round the room or in another room you must place the filter before the extension cord and not at the end of the extension cord. This is for a quite simple reason, if you are plugging an extension cord in there is no way to remove the ring wire from the extension lead plug going into the original socket. The idea here is to filter the cable before the ring wire produces interference on the line but, as already stated, if possible it is better to use fixed cabling wired properly into the socket rather than an extension cord because you can never know the quality or type of the cabling used in them.