TUCoPS :: Hardware Hacks :: mdmfilte.txt

How to make your own line filter.

Hang on, extracting file...

The following article appears in the October 1989 issue of Computer Shopper
Page 533

                       TELECOMPUTING: BBS OF THE MONTH

                               by Curt Phillips

                              Line Noise Filter

    Line noise is the bane of on-line life. Trying to read a message on the
computer screen interspersed with the garbage characters ( *#%^[ and other
intelligible hieroglyphics) that are the result of line noise in enough to makethe computer user say, "#&$@&#!!!" (you know what I mean if you read the
    Over the past several months there has been a text file transferred about
on BBSes and on the on-line services going by the name of "Noise-Killer" or
something similar. If you have seen this file and have wondered if it is
legitimate, of if you'd like to know how to alleviate the line noise, here it
    This circuit requires a few electrical components (available at the
ubiquitous Radio Shack) and the basic soldering skills. Depending on the exact

source of your line problem, this may or may not be the total solution, but it
can help almost any situation.
    The original listing of materials where:
    1 (Radio Shack #279-374) modular line cord (if you don't already have one)
      You won't need one if your phone has a modular plug in its base.
      Cost $4.95
    1 (R.S. #271-1720) Potentiometer. This is a 5K (5000 ohm) audio taper
      variable resistor. Cost $1.09
    1 (R.S. #272-1055) Capacitor. Any non-polarized 1.0 to 1.5 ufd (micro-
      farad) (NOT MFD!) capacitor should do, but it should be rated for at
      least 100 volts. (Phone line ring-in voltage is 90VAC - S.F.) Paper,
      mylar or metal film caps should be used, although #272-996 (a
      non-polarized electrolytic capacitor) may work as well. Cost $.79
    1 100 ohm resistor (quarter or half watt) Cost $.19
    1 (R.S. #279-357) Y-type or duplex modular connector. Don't buy this
      until you have read the section on connecting the Noise Killer below.
      (Parts A, B or C) Cost $4.95.

    The original instructions for the assembly of the "Noise-Killer" are as

    First of all, open the modular block. You normally just pry them open with
a screwdriver. Inside you'll find up to six wires. Very carefully cut all the
wires out EXCEPT the red and green ones. The ones you'll be removing should be
black, yellow, white and blue.
    These wires won't be needed and might be in the way. So cut them as close
to where they enter the plug as possible. The other end of the wires have a
spade lug connector that is screwed into the plastic. Unscrew and remove that
end of the wires as well. Now, you should have two wires left. Green and red.
Solder one end of the capacitor to the green wire. Solder the other end of
the capacitor to the center lug of the potentiometer. (Three lugs total.)
    Solder one end of the resistor to the red wire. You may want to shorten
the leads to the resistor first. Solder the other end of the resistor to one
of the side lugs on the potentiometer. It dosen't matter which side. Now wrap
it up, Make a hole in the lid of the mod block to stick the shaft of the
potentiometer through. Don't make this hole dead center as the other parts may
not fit into the body of the mod block if you do. See how things will fit on
order to find where the hole will go. (Before you put everything together go
back and check your work. You don't want to fry your modem do you?)
    The instructions for testing and installation are, first, twist the shaft
on the potentiometer until it stops. You won't know which way to turn it
until later. It dosen't matter which way now. You also need to determine where

to plug the Noise Killer into the telephone line. It can be done by one of
several ways:

    A. If your modem has two modular plugs in the back, connect the Noise
Killer into the one using the line cord (not to your phone). A line cord
is a straight cord that connects a phone to the wall jack.(In this case modem
to wall jack. Usually silver in color.)

    B. If your phone is modular, you can unplug the cord from the back of it
after you're on-line and plug the cord into the Noise Killer.

    C. You may buy a Y-type modular adapter. Plug the adapter onto the wall
outlet. Plug the modem into one side and the Noise Killer into the other.

    Call a BBS that has known noise problems. After you've connected and
garbage begins to appear, plug the Noise Killer into the phone line as
described earlier. If you have turned the shaft on the potentiometer the wrong
way you'll find out now. You may get a lot of garbage or even disconnected. If
this happens turn the shaft the other way until it stops and try again. If you
don't notice much difference when you plug the Noise Killer in that may be a
good sign. Type in a few commands and look for garbage characters on the

screen. If there still is turn the shaft slowly until most of it is gone. If
nothing seems to happen at all, turn the shaft slowly from one side to the
other. You should get plenty of garbage or get disconnected at some point.
If you don't reread this message to make sure you've connected it correctly.
    With all the fear of viruses and such on BBSes, you may have been
hesitant to try this. Rest assured that this is legitimate circuitry. It's
technical name is a passive R-C band-pass filter (resistive-capacitive).
The typical 2400 bps (or less) modem uses tones between 1000hz and 2500 hz
to communicate. Although the phone lines can transmit over a wider frequency
range than this, any tones (or noise) are garbage to the modem, and hence to
your computer.
    Turning the shaft on the potentiometer (variable resistor) modifies the
"band-pass" frequency of the filter to minimize the noise. Most often the
total resistance of the potentiometer and the fixed 100 ohm resistor will
"tune" between 2.5 and 3 Kohms, so if you have a fixed resistor in this range
try it before you buy a variable resistor. Also, if you have a potentiometer
over 5K, use it. It dosen't have to be an audio tapper (that simply denotes howfast the resistance varies). Again you will probably "tune" to about 3K of
resistance anyway.
    Capacitors of up to 2 microfarads (2ufd) have been reported to work also.
As you may have guessed, these components can be varied a reasonable amount

without interfering with the filtering capability of the circuit.
    This circuit should be most useful to those with inexpensive modems, since
filtering circuitry if often the first casualty of economizing, but users with
high-price, big name modems have also reported improvements in performance.

End of Article.

Typed by Silver Fox

This whole thing looks ok but remember putting any thing like this into
Ma-Bell's circuits could make them annoyed. If you build right you should be
ok. But if you didn't - watch out! - S.F.
lectrolytic capacitor) may work as well. Cost $.79
    1 100 ohm resistor (quarter or half watt) Cost $.19
    1 (R.S. #279-357) Y-type or duplex modular connector. Don't buy this
      until you have read the section on connecting the Noise Killer below.
      (Parts A, B or C) Cos

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