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Q: OK, so exactly what is a DSL Filter and why is it needed and how do they work?

A:Riss Centaur's 50-cent DSL Filter Primer:

Way back in the dim reaches of time, the good old Bell System (Alexander Graham Bell, The Telephone Company, Ma Bell, Western Electric, Before Divestiture, the Big Breakup, whatever....) came up with the standard for the VOICE BAND or POTS (Plain Old Telephone Service) that all telephones should handle throughout the system. It was decided that telephones would need to carry signals from about 300Hz (cycles) through to about 3,300Hz. This is the range that is needed for intelligible speech. (In truth, the phone line band is 0 to 4,000Hz, just to be safe and carry touch tones.) This is also why music, with its higher frequencies (up to 20,000Hz) sounds lousy over a phone - because you lose the high frequencies.

Ok, so voice is in the band of 300-3,300Hz. Where is DSL? ADSL uses the frequencies from about 26,000 (26K) to 1,100,000 (1,100K) Hz. These much higher frequencies are outside of human hearing, but when shoved down the phone wires can carry data to the modems!

So when it is all put together it looks like this:

We have to keep the Green and the Blue out of the POTS equipment and keep the POTS equipment from messing up that set of high frequency signals.

So now the big moment! The dilemma is this: Most phones or any other type of equipment that connects to the phone line (answering machine, fax, Tivo, cable box, alarm system) were only designed to deal with Voice Band signals! If you suddenly start feeding DSL signals into them you can wind up with all sorts of problems - Noise in the voice band, Shorting out of the DSL signal, Crosstalk of noise into the DSL signal, and god only knows what else!

Can you see the light come on? Bright Idea: Keep the DSL signal out of the equipment that does not know what to do with it. Install "The Filter"!

The Filter - AKA a Microfilter, POTS filter, DSL filter, Splitter, Inline filter, plastic box with wires on it... you get the idea.

Now, a filter works by filtering out the high frequency DSL signals, it does not let them pass through it, it only allows the low frequency voice signals to pass. Thats why in the electronic business this type of circuit is know as a Low Pass filter. It is a fairly simple idea, and a fairly simple circuit to implement with coils and capacitors and resistors. They are passive devices, they usually have no active electronics in them like transistors or IC's, and need no outside power. The filter, in addition to keeping the high frequency DSL signals out of equipment that does not know what to do with it, also keeps that equipment from loading down or shorting out the DSL signal itself!
That is why everything that connects to the phone line EXCEPT THE DSL MODEM needs to be filtered in one way or another.

The two ways of filtering are: Install an inline filter on each phone and other device where it plugs into the wall - or - install a big filter at the main junction (NID) where the phone line comes in and it will handle filtering everything behind it. (And you have to run a home run wire from there to your DSL modem so it has an unfiltered signal.)

Ok, what is the difference between a Filter and a Splitter eh?
Well, they both do the same thing, they filter the DSL signal out from things that should not see the signal. The difference is in the packaging. The first inline filters just had a plug on one side and a jack on the other. You unhooked your phone from the wall jack, plugged it in to the filter jack and plugged the filter into the wall. Thus you inserted the filter between the DSL signal at the wall jack and the phone, and it kept the DSL signal out of the phone and the phone from shorting out the DSL signal.

Now if you have a newer type filter that has 2 jacks, one marked VOICE and one marked DSL, you could think of it as a splitter. In truth all the DSL jack is is an extra connection off the plug fed straight through from the wall, so you have an extra jack to plug your DSL modem into. Also, filters installed at the NID are referred to as splitters, as they filter out the DSL signals from the wires split off of the main phone line.

But Mr. Centaur, how do you install these things?

I'm glad you asked. Lots has already been written on this subject.
Here are various FAQ's relating to filter installation:
»AOL Plus Cable Modem and ADSL Internet »What is a filter? Why do I need them?
»DSL FAQ »Filters?
»DSL FAQ »Filters and line-sharing
»Verizon Online DSL FAQ »What is a microfilter?
»SBC DSL FAQ »Filter vs. Splitter explained..

And NID Splitter installation:
»AT&T Midwest/Ameritech FAQ »NID Splitters and Installation
»Verizon Online DSL FAQ »What is a splitter?
»AT&T West FAQ »How do I install a splitter?
»AT&T Southeast Forum FAQ »Splitter, DSL/POTS ? Where can I get one?
»AT&T Southeast Forum FAQ »Home Security Alarm Wiring with RJ-31X Splitter Homerun Diagram

And as for where to get this little marvels, well, the microfilters usually come in your install kit, or can be had at electronic stores or online. POTS Splitters for installation at the NID can usually be had on Ebay in the $25-$30 range on good days.

Now, arent you glad you asked? ;)

Here are images of the guts of an NID Pots Splitter and a plug in Micro Filter (provided by d_l See Profile)

Corning/Seicor SPS-H50-SR1

Click for full size

Excelsus Z-330TJA Z-Blocker

Click for full size

You will NOT get a reply if you click "Got Feedback" as that is for suggested changes to the FAQ only. If you have additional questions, visit the AT&T DSL Forum.

Feedback received on this FAQ entry:
  • Excellent guide ^^

    2014-10-29 09:08:00

  • Made it easy to understand, great! Wish I knew this earlier.

    2014-05-02 15:10:55

  • Great summary & explanation!

    2014-01-13 18:22:09

  • I've had my DSL for so long that AT&T originally installed a "HI-PASS" filter between the line and my DSL Modem and no filters going to or at the phones. The DSL modem lives on it's own "homerun" IW wire. The filter keeps the POTS voice and ring signal out of the DSL Modem and maybe the -48VDC as well. The phones are tied directly to the cable pair.

    2013-09-04 23:05:53

  • Can the filter be installed in the reverse ?

    2013-06-04 19:28:20

  • very good explanation ....Thank you

    2013-05-23 17:48:15

  • Thank you for al your efforts. Quite clear and concise, as well as an enjoyable read. Now if you could also add a "What to do when you have a 2-Line phone" to the mix (like describing whether line2 is filtered or not, what if I want to move my DSL service from Line1 to Line2 or Vice versa). There doesn't seem to be ANY information for people with 2-Line phones, which are quite common in this day and age.

    2013-05-14 19:38:59

  • Thank you so much for explaining the difference in frequencies!! I've been trying to find an explanation for years (the phone company people were no help). Appreciatively, dave in austin, texas

    2013-03-14 00:10:16

  • grreat information - more people should understand this and you make it simple

    2012-12-13 07:10:50

  • It's almost clear.... the topic isn't really simple to explain, but you did a great job. I've been looking around and I found your explanation the most complete and well written. So, thanks a lot. Guido

    2012-10-05 05:48:01

  • THANK YOU! Now I understand a bit better why the filter is needed. Recently my phone line began to have static and eventually went dead! My AT&T U-verse internet was constantly going down and I passed it off as just plain rotten service. I had an extra single line filter and replaced the old one at the phone....both DSL and phone are working great! I appreciate you explanation here.

    2011-07-30 10:56:01

  • Great explanation. Now....what about the filter that needs to go to the home alarm system? The plug is bigger, did they do that just so we would have to buy new filters just for them?

    2011-07-26 17:14:28

  • Thank you very much! I've been using these for years but have always been curious as to why they're needed and what they do. Thanks!

    2011-06-21 12:30:15

  • Thanks for this, its really great

    2011-04-30 03:31:59

  • Thank you for a very clear explanation! I had been wrestling -- in my head -- for weeks and now I can fairly certain that what I need is an additional filter for me second jack.

    2010-07-24 22:53:58

Expand got feedback?

by Riss_Centaur See Profile edited by lev See Profile
last modified: 2014-05-15 23:17:19