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What is a DSL Filter?

R. Kayne
R. Kayne

Digital Subscriber Line (DSL) filters, also known as micro-filters, are small in-line devices sometimes required for DSL installations. They filter line interference to standard telephone equipment when phones share the same lines as DSL service. Not all setups require the use of a DSL filter, though they are often needed for each phone connection.

Basic Design

DSL filter.
DSL filter.

A DSL filter is a small, rectangular device that has phone connections on both ends. The person setting up a DSL system simply unplugs a telephone line from a wall jack, inserts the filter into the wall connection, and then plugs the telephone line into the filter. Fax machine lines and answering machines tend to also require DSL filters. Filters separate the voice and data signals sent through phone lines, ensuring that neither signal interferes with the other.

Split DSL Setups

Fax machines require a DSL filter.
Fax machines require a DSL filter.

There are two methods for installing DSL service: split, or splitterless. A DSL filter is not required if the split method is used. In this case a technician visits the premises and installs a splitter in the main telephone cable. Two lines emerge from the splitter; one line feeds into standard phone jacks, while the other line is used exclusively for a DSL modem. A DSL filter is not required in this case because the splitter has separated the voice and data signals.

Splitterless Setups that Require Filters

The second method, which has gained popularity because it does not require a technician, can be user-installed. In this case, the end-user can setup a DSL modem along with several micro-filters. DSL service is sent through the main service line so the end-user only has to connect the modem to any existing phone wall jack. However, the signal reaches all connected phone jacks, creating possible bleed-over noise on telephone, answering machine, and fax machine lines. A DSL filter connected to each of these lines blocks interference that might be created by the service.

Advantages of DSL Setups

DSL has three great advantages over dial-up service: it is always on, it is up to 50 times faster, and it does not tie up a phone line, even though it uses the same connection. Standard telephone equipment uses an analog signal, while DSL communicates on a higher digital frequency. This lets a user navigate the Internet using DSL service, while talking on the phone at the same time.

Other Filters and Troubleshooting

A DSL filter is typically a small device, though some other equipment can have filters built into them. Some surge protectors or power strips, for example, include a filter to help with DSL connections and to protect equipment in case of a power surge through a phone line. Issues that arise with signal clarity may be related to faulty or improper filters, though flaws in telephone cables or damaged hardware can also result in poor performance.

Discussion Comments


This is so silly. My phone is not working and I can't get any one to answer me and tell me what is going on.


My phone has stopped working and Google is no help.


I started having trouble with my call ID working sometimes and sometimes not. I had recently installed a Wi-Fi modem and moved the phone over to the same jack the DSL uses.

So I used this new filter for the phone that came with the new modem.

Well I was going to try the old filter, then the next step was to move the phone back to the old jack, but I then noticed the filter had a splitter made into it, but was marked DSL/Phone.

I had had them crossed. The phone tech was here too, and he had plugged his test equipment into the splitter/filter, and had not noticed they were crossed.

So far, (keeping fingers crossed) Caller ID is working well again.


I have ATT U-verse DSL. Everything worked fine for a few months, then the wireless phone began having static. I have filters installed and the net works fine. I tried a regular cord phone into the jack and filter and it produced static as well.

I think because it was initially clear, then developed static later, it indicates something, but I'm not sure what.


I have an Ooma system (voip). I was able to receive and call out without any problem from another country until six months ago. Now I can only receive calls and cannot dial out. I have an app in my iphone via internet and this works. I assumed that this app uses the same ip address as my landline. So I assumed that is not the problem. Help.


if you are getting static on a wireless house phone, check the battery. When they get old, they don't work like they should.


I had the same problem. As a Comcast user (this applies to many services), I was told to get a line filter. I went to Lowe's and bought one. On the way home, it occurred to me that there was one already in my power strips and UPSs. Rather than open the package, I ran a line from the wall to the power strip and from there to the fax. This solved the problem.

Be certain to plug in the lines correctly, they are directional.


My phone line suddenly starting hissing and crackling. And my DSL internet connection became intermittent; sometimes it worked, sometimes it didn't. I called AT&T. A technician came out, hear the phone line hissing, and said, "That's your modem."

We went outside and he examined my old phone box. The wires were corroded. He said that corrosion is a semiconductor, and that the signal from the DSL wire was traveling through the corrosion to the land line wire. So he replaced the phone box.

The hissing and static on the phone line are gone, and now my internet works. If you can't figure out why your internet connection is lousy and your phone line hisses, old corroded phone wires outside your house might be the problem. Hope this info helps.


I have had static on one of my two lines - sometimes I can't hear anything but static, other times it's clear. Verizon has been called seven times in 10 days, they split my phone and DSL lines, and I have filters on all phones.

The guy is perched up a pole somewhere now, but they can't seem to find the noise. When tested, it tests clean, but the techs have all heard the noise! Any suggestions? Thanks so much.


When I installed DSL, I was told to filter all phones connected to the line. Was also told that the modem should be connected directly to the line. I had a problem with download speed and was told by the ATT tech to filter the modem line. My speed increased from 1.9 to 2.6 on a 3.0 mb service.


The solution to static problems and phone / DSL interference (phone doesn't work when modem is connected) is a "DSL Splitter". This physically separates the wires carrying the DSL signal from the wires carrying the phone connection. A technician can install them, but you can also find out how to do it online.


Great article. Still, I don't know if the filters are tuned to a carrier frequency or not.

A few days ago, my DSL developed a peculiarity.

Suddenly the line filters became useless. The sound of the DSL comes right through.

Yes, I tried two other modems and several other filters, and no filters at all.

There is no difference whatsoever.

My telephone is useless when I'm on the internet.

I'm still waiting to hear from my ISP.

Does anyone have any ideas?


I was told that a DSL filter will prevent any electrical surges in the phone line. Is that true.


Just had dsl installed with verizon. i had lots of problems at first and no phone. now there are low, quiet voices on all phones both ways. Can I put two filters on the jack that holds the modem and a phone. My single jacks branch out to an answering machine and fax and speakphone. do I need *only* one filter on that jack that holds three items or 3 filters on each item.


where are the answers to these questions ?


My answering machine greets phone calls with a very low volume since installing DSL filters. How can I improve this?


I have two line service for my phone. When I went to set up my high speed internet, I noticed that the filter only has 2 pins. I need four. Is there such a thing? What other options are there?


I have an aDSL line and there is a tremendous amount of static on the phone line. When the modem is disconnected, there is no static. I have changed out modems, DSL filters, cords and even plugged the DSL filer directly to the main interchange with no change in the static. The AT&T service man said that when he tried the main interchange connection that was not connected to the modem, there is no static and that the problem is with the modem. Since a new modem did not help, what else is there that could be causing this static?!


does non use of ASDL filters reduce download speed?


We have had DSL for about a year. We have filters on all outlets. Recently we started to get mild static on all phones. We have a cordless phone and answering system on one of the outlets. When we unplug the cordless phone the static is eliminated. I read on your website that the cordless radio frequency could create static and that a different filter is required on the cordless phone jack. Please advise.


As stated by anon6839, the filter also prevents the analog devices from interfering with the DSL connection. I had problems with my DSL Modem not getting a strong signal (and me not being able to connect to the internet). The culprit turned out to be my satellite TV receiver box which was connected to a phone jack without a filter.

I installed a filter and the problem was resolved immediately.


A very good explanation of what the filter does comes from the article posted on wisegeek: "what-is-the-difference-between-dsl-and-adsl.htm". That article states: "DSL accomplishes this by incorporating a filter on the phone jacks in a location that will have telephones connected to them. The signals on the wire under 4Khz are considered voice signals and anything above 4Khz is considered a data signal, the filter helps to ensure that these signals never cross."

Makes sense!


My understanding is that it merely filters out the digital "noise" so that it can't be heard on the phone. Others have told me it prevents the phone from interfering with the DSL signal.


I have a question regarding the use of DSL filter. I have an equipment at home and it's only compatible with an analog line but I don't have exiting analog line. All phones are all DSL. If I will use DSL filter will it give me a smooth connection or or still hit or miss connection.

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    • DSL filter.
      DSL filter.
    • Fax machines require a DSL filter.
      By: Rob Byron
      Fax machines require a DSL filter.