122 lines
7.0 KiB
Plaintext
122 lines
7.0 KiB
Plaintext
The following article appears in the October 1989 issue of Computer Shopper
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Page 533
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TELECOMPUTING: BBS OF THE MONTH
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by Curt Phillips
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Line Noise Filter
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Line noise is the bane of on-line life. Trying to read a message on the
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computer screen interspersed with the garbage characters ( *#%^[ and other
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intelligible hieroglyphics) that are the result of line noise in enough to make
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the computer user say, "#&$@&#!!!" (you know what I mean if you read the
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comics).
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Over the past several months there has been a text file transferred about
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on BBSes and on the on-line services going by the name of "Noise-Killer" or
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something similar. If you have seen this file and have wondered if it is
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legitimate, of if you'd like to know how to alleviate the line noise, here it
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is.
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This circuit requires a few electrical components (available at the
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ubiquitous Radio Shack) and the basic soldering skills. Depending on the exact
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source of your line problem, this may or may not be the total solution, but it
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can help almost any situation.
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The original listing of materials where:
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1 (Radio Shack #279-374) modular line cord (if you don't already have one)
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You won't need one if your phone has a modular plug in its base.
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Cost $4.95
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1 (R.S. #271-1720) Potentiometer. This is a 5K (5000 ohm) audio taper
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variable resistor. Cost $1.09
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1 (R.S. #272-1055) Capacitor. Any non-polarized 1.0 to 1.5 ufd (micro-
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farad) (NOT MFD!) capacitor should do, but it should be rated for at
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least 100 volts. (Phone line ring-in voltage is 90VAC - S.F.) Paper,
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mylar or metal film caps should be used, although #272-996 (a
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non-polarized electrolytic capacitor) may work as well. Cost $.79
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1 100 ohm resistor (quarter or half watt) Cost $.19
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1 (R.S. #279-357) Y-type or duplex modular connector. Don't buy this
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until you have read the section on connecting the Noise Killer below.
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(Parts A, B or C) Cost $4.95.
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The original instructions for the assembly of the "Noise-Killer" are as
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follows:
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First of all, open the modular block. You normally just pry them open with
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a screwdriver. Inside you'll find up to six wires. Very carefully cut all the
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wires out EXCEPT the red and green ones. The ones you'll be removing should be
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black, yellow, white and blue.
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These wires won't be needed and might be in the way. So cut them as close
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to where they enter the plug as possible. The other end of the wires have a
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spade lug connector that is screwed into the plastic. Unscrew and remove that
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end of the wires as well. Now, you should have two wires left. Green and red.
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Solder one end of the capacitor to the green wire. Solder the other end of
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the capacitor to the center lug of the potentiometer. (Three lugs total.)
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Solder one end of the resistor to the red wire. You may want to shorten
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the leads to the resistor first. Solder the other end of the resistor to one
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of the side lugs on the potentiometer. It dosen't matter which side. Now wrap
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it up, Make a hole in the lid of the mod block to stick the shaft of the
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potentiometer through. Don't make this hole dead center as the other parts may
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not fit into the body of the mod block if you do. See how things will fit on
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order to find where the hole will go. (Before you put everything together go
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back and check your work. You don't want to fry your modem do you?)
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The instructions for testing and installation are, first, twist the shaft
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on the potentiometer until it stops. You won't know which way to turn it
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until later. It dosen't matter which way now. You also need to determine where
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to plug the Noise Killer into the telephone line. It can be done by one of
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several ways:
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A. If your modem has two modular plugs in the back, connect the Noise
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Killer into the one using the line cord (not to your phone). A line cord
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is a straight cord that connects a phone to the wall jack.(In this case modem
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to wall jack. Usually silver in color.)
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B. If your phone is modular, you can unplug the cord from the back of it
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after you're on-line and plug the cord into the Noise Killer.
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C. You may buy a Y-type modular adapter. Plug the adapter onto the wall
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outlet. Plug the modem into one side and the Noise Killer into the other.
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Call a BBS that has known noise problems. After you've connected and
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garbage begins to appear, plug the Noise Killer into the phone line as
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described earlier. If you have turned the shaft on the potentiometer the wrong
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way you'll find out now. You may get a lot of garbage or even disconnected. If
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this happens turn the shaft the other way until it stops and try again. If you
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don't notice much difference when you plug the Noise Killer in that may be a
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good sign. Type in a few commands and look for garbage characters on the
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screen. If there still is turn the shaft slowly until most of it is gone. If
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nothing seems to happen at all, turn the shaft slowly from one side to the
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other. You should get plenty of garbage or get disconnected at some point.
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If you don't reread this message to make sure you've connected it correctly.
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With all the fear of viruses and such on BBSes, you may have been
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hesitant to try this. Rest assured that this is legitimate circuitry. It's
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technical name is a passive R-C band-pass filter (resistive-capacitive).
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The typical 2400 bps (or less) modem uses tones between 1000hz and 2500 hz
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to communicate. Although the phone lines can transmit over a wider frequency
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range than this, any tones (or noise) are garbage to the modem, and hence to
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your computer.
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Turning the shaft on the potentiometer (variable resistor) modifies the
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"band-pass" frequency of the filter to minimize the noise. Most often the
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total resistance of the potentiometer and the fixed 100 ohm resistor will
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"tune" between 2.5 and 3 Kohms, so if you have a fixed resistor in this range
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try it before you buy a variable resistor. Also, if you have a potentiometer
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over 5K, use it. It dosen't have to be an audio tapper (that simply denotes how
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fast the resistance varies). Again you will probably "tune" to about 3K of
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resistance anyway.
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Capacitors of up to 2 microfarads (2ufd) have been reported to work also.
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As you may have guessed, these components can be varied a reasonable amount
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without interfering with the filtering capability of the circuit.
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This circuit should be most useful to those with inexpensive modems, since
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filtering circuitry if often the first casualty of economizing, but users with
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high-price, big name modems have also reported improvements in performance.
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End of Article.
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Typed by Silver Fox
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This whole thing looks ok but remember putting any thing like this into
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Ma-Bell's circuits could make them annoyed. If you build right you should be
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ok. But if you didn't - watch out! - S.F.
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lectrolytic capacitor) may work as well. Cost $.79
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1 100 ohm resistor (quarter or half watt) Cost $.19
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1 (R.S. #279-357) Y-type or duplex modular connector. Don't buy this
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until you have read the section on connecting the Noise Killer below.
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(Parts A, B or C) Cos |