beser@tron.UUCP (Eric Beser) (08/22/89)
I have been fighting with the phone company now for a year trying to get them to clean up the line noise on my modem line. Right now, the static is so bad, that the modem carrier is dropped and disconnected. Most of the noise seems to be incoming. Is there anything I can do to filter the line at the point of the network connector. The modem is the only line on the system, and the cable is connected to the point of entry into the building. There are no powerlines nearby. Can some form of optical coupling with filter caps help? Any suggestings (practical) would be appreciated. The problem is not any one particular modem. There are provisions for 6-conductor wire in the modem, but there is only a two conductor line coming into the building. Thanks for assistance. Eric Beser ebeser@ajpo.sei.cmu.edu eric@sarin.UUCP
ignatz@chinet.chi.il.us (Dave Ihnat) (08/30/89)
The following is the distillation of an item entered here at 'chinet', a
public access Unix system here in Chicago.
Edward Lee wrote:
I got the following file last year from CBBS #1. The original
author was uncredited.
_____________________________________________________________
Modem Noise Killer (alpha version)
With this circuit diagram, some basic tools including a
soldering iron, and four or five components from Radio
Shack, you should be able to cut the noise/garbage that
appears on your computer's screen.
I started this project out of frustration at using a US
Robotics 2400 baud modem and getting a fare amount of junk
when connecting at that speed. Knowing that capacitors make
good noise filters, I threw this together.
This is very easy to build, however conditions may be
different due to modem type, amount of line noise, old or
new switching equipment (Bell's equipment), and on and on.
So it may not work as well for you in every case. If it
does work, or if you've managed to tweek it to your
computer/modem setup I' d like to hear from you.
I'd also appreciate any of you electronic wizzards out
there wanting to offer any improvements. Let's make this
work for everyone!
Please read this entire message and see if you understand
it before you begin.
OK, what you' ll need from Radio Shack:
1 #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. $4.95
1 #279-420 Modular surface mount jack (4 or 6 conductor)
$4.49
1 #271-1720 Potentiometer. This is a 5k audio taper
variable resistor. $1.09
1 #272-1055 Capacitor. Any non-polarized 1.0 to 1.5 uf cap
should do. Paper, Mylar, or metal film caps should be used,
although #272-996 may work as well. (272-996 is a
non-polarized electrolytic cap) $.79
1 100 ohm resistor - quarter or half watt. $.19
1 #279-357 Y-type or duplex modular connector. Don't buy
this until you've read the section on connecting the Noise
Killer below. (A, B,or C) $4.95
First off, open the modular block. You normally just pry
them open with a screwdriver. Inside you'll find up to 6
wires. Very carefully cut out all but the green and red
wires. The ones you'll be removing should be black, yellow,
white, and blue. These wires won't be needed and may be in
the way. So cut them as close to where they enter the plug
as possible. The other end of these 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 (there are
three lugs on this critter). Solder one end of the resistor
to the red wire. You may want to shorten the leads of the
resistor first. Solder the other end of the resistor to
either one of the remaining outside lugs of the
potentiometer. Doesn't matter which. Now to 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 in order to
find where the hole will go. Well, now that you've got it
built you'll need to test it. First twist the shaft on the
potentiometer until it stops. You won't know which way to
turn it until later. It doesn't matter which way now. You
also need to determine where to plug the Noise Killer onto
the telephone line. It can be done by one of several ways:
A. If your modem has two modular plugs in back, connect the
Noise Killer into one of them using a line cord. (a line
cord is a straight cord that connects a phone to the wall
outlet. 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 have to buy a Y-type modular adaptor. Plug the
adaptor into a 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 above. 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 disconnected at some point. If you don't, reread
this message to make sure you've connected it right.
***END OF ORIGNAL FILE***
ADDITION TO ORIGNAL FILE - 2/29/88 - Mike McCauley - CIS
71505,1173
First, a personal recomendation. _THIS WORKS!!!_ I have
been plagued with noise at 2400 for some time. I went round
and round with Ma Bell on it, and after they sent out
several "repair persons" who were, to be kind, of limited
help in the matter, I threw in the towel. I saw this file
on a board up east a few days ago, and thought I'd bite.
Threw the gismo together in about 10 minutes, took another
five to adjust the pot for best results on my worst
conection, and guess what? No more worst connecion! A few
pointers:
1) The pot need not be either 5K or audio taper. I used a
10K 15 turn trim pot. Suggest you use what is handy. 2)
I used 2MFD's of capacitance (two 1MFD's in parallel) Two
R.S. p/n 272-1055 work fine. Remember that about 90
Volts will appear across red & green at ring, so the
caps should be rated at 100VDC+. 3) I ended up with a final
series resistance value (100 ohm + pot) of 2.75K. I
speculate that one could probably use 2MFD and a fixed 2.7K
resistor and do the job 90% of the time. The adjustment
of the pot is not very critical. Changes of +/- 1K made
little difference in the performance of the circuit.
Hope it works as well for you as it did for me.
Mike McCauley
_____________________________________________________________
Second addition:
26 July 1989
The noise killer works extremely well for me. I own a
Product R&D 2400 bps modem and have been irritated by noisy modem
connections to Chinet and CBBS #1 for the past few years. Today, I
went to a Radio Shack store and bought the parts for the noise
killer for less than $4 dollars and built it in less than an hour.
(I am a klutz with a soldering iron.) I used all the parts listed
in the original message except for the modular surface mount jack.
The Radio Shack store that I visited did not have the jack, so I
instead bought a #279-1261 Flexible Telephone Extension Cord for
$1.20 (it was on sale). I already had the modular line cord and
the Y-type modular phone adaptor, but I did not need to use the
latter. Turning the stick of the 5k audio taper potentiometer
counter-clockwise all the way made the connections to Chinet and
CBBS #1 almost noise-free.
Parts list Cost
#271-012 Two 100 ohm resistors (use only 1) 1/2 watt $0.19
#279-1261 Flexible Telephone Extension Cord 30 ft. $4.99 ($1.20)
#271-1720 Audio taper potentiometer 5k ohms $1.09
#272-1055 1.0 MFD @200 volt metalized film capacitor $0.99
-Ed L
Floyd Davidson comments:
Couple of suggestions:
1. Increase the value of the 100 ohm resister to 500 or 1000 ohms.
2. Use smaller cap, like 1 mfd, not 2 mfd.
One warning:
Ringing voltage may be 90 vac, it may be 105 vac too. And that
may have dc peaks much higher. Use at least 200vdc rated caps.
The fact that this works is interesting. All it does is make the
impedance of the line roll off at higher frequencies. It would be
interesting to put a spectrum analyzer on your phone line and see
what is on there. There might be garbage at greater than 3200~
from power lines or cable carrier and what you are doing is filtering
it out. If that is the case a much smaller capactitor (say .1 or .5 mfd)
alone might be even better.
Gregory Higgins added:
At my old office, I happened to mention to the Teleconnect guy that the
phone lines were too dirty to use his service for long distance data.
He got a 300 ohm resistor from RS, popped the top off the outlet and
wrapped the ends of the resistor around two of the posts ( sorry , don't
remember which two ) and the problem went away.
Edward Lee added:
Here's more information:
I elected not to use an unadjustable 2.7K ohm resistor, and I am glad
that I bought the potentiometer. The reason is that the Product R&D modem
will pick up occasional noise if the potentiometer is fixed in any one
position. I suspect that this is because the modem has automatic adaptive
equalization. When the line is relatively quiet, when I am not typing
anything and am waiting for some text to appear, the modem seems to
increase its sensitivity and pick up more noise. So, what I
do is turn the potentiometer until I get a lot of noise and then
turn it back to the relatively quiet setting, effectively deafening
the modem to line noise (I think that is what I am doing). When I
do this, as I did at the beginning of this call, I get absolutely
no line noise.
Here's yet more information. I type the following at the command prompt:
cat>/dev/null
before adjusting the potentiometer. Pressing CTRL Q and CTRL C may be
necessary to get back to the command prompt.elc@well.UUCP (Eric L. Cook) (09/01/89)
On the Chinet posting about a "Modem Noise Killer" I have a few comments about what I saw. First let me say I design modems for a living. What you are doing is not filtering noise but changing the impedance match on the phone line. A nominal phone line should be about 600 ohms, but a lot of phone lines run in the 750 to 1200 ohm range. As you can see there is not a very tight control over this spec. Impedance matching is important to a modem as the modem does a "4 wire to 2 wire conversion". That is 4 wire, (1)TX, (2)ground and (3)RX, (4)ground to 2 wires, Tip and Ring. In a perfect conversion when you transmit on the TX line you will not see your transmit signal on your incoming receive line. ( Its like if you where talking you could not hear yourself talk but you could hear the other person talk. ) Any time there is not a perfect match you can start to hear yourself talk. To a modem this shows up as noise on the receive signal affecting the signal to noise ratio. A 300 BPS modem needs a minimum 8 DB's of signal to noise ratio for a bit error rate of 1 x 10^-5. With a 2400 BPS reception you need a minimum of 17 DB's of S/N ratio for the same error rate! So you can see that noise can play a important part in modem transmission. The reason you are not filtering noise for better reception is that you are filtering the receive signal as well, and in doing this you are not improving the S/N ratio. !!!!!!!!!!!************ WARNING **************!!!!!!!!!!!!! (1) These changes that you are making ARE illegal. You are affecting a "public" network and the FCC has very heavy fines for such violations. (2) I see people quoting 100VDC, or 200VDC caps. The ring signal is 100 volts ***RMS***, not DC. A minimum of 250 volts is needed. Also the cap must be non-polarized. (3) If you live where there is lightning you are exposing yourself to a shock hazard by exposed metal parts. ( Resistor bodies, wires. ) There is the information, you can make the decision for yourself. Eric Cook
feldman@urbana.mcd.mot.com (Mike Feldman) (09/01/89)
In article <9411@chinet.chi.il.us> ignatz@chinet.chi.il.us (Dave Ihnat) writes: >The following is the distillation of an item entered here at 'chinet', a >public access Unix system here in Chicago. > >Edward Lee wrote: >I got the following file last year from CBBS #1. The original >author was uncredited. >_____________________________________________________________ > >Modem Noise Killer (alpha version) ... long (and useful, thanks) message followed. I have had terrible line noise problems at 1200 baud over long distance connections from my father's house (rural), and this filter circuit may help. But my initial reaction was to look at error correcting modems, since I've noticed a few new modems on the market for under $200 that offer MNP class 5 error correction at 2400 baud. From home to work (clean local call) I use an MNP class 3 modem (MultiTech 2400E), and I'm very happy with it, but I haven't been able to find any literature that confirms that MNP class 5 and class 3 are, as I suspect, compatible in error correction, and you just don't get the class 5 compression. Is this true? Also, can anyone recommend any of the low-priced modems, eg. Practical Peripherals? My father's under $100 1200 baud modem-in-a-plastic-box cooked itself in under 10 hours of use, and I don't want to see him or me sink $$$s into that level of quality again. Thanks, Mike Feldman, Motorola Microcomputer Div., (217) 384-8538, FAX (217) 384-8550 Urbana Design Center 1101 East University Avenue, Urbana, IL 61801-2009 UUCP: uunet!uiucuxc!mcdurb!feldman, Internet: feldman@urbana.mcd.mot.com -- Mike Feldman, Motorola Microcomputer Div., (217) 384-8538, FAX (217) 384-8550 Urbana Design Center 1101 East University Avenue, Urbana, IL 61801-2009 UUCP: uunet!uiucuxc!mcdurb!feldman, Internet: feldman@urbana.mcd.mot.com
epsilon@wet.UUCP (Eric P. Scott) (09/03/89)
In article <984@urbana.mcd.mot.com> feldman@urbana.mcd.mot.com (Mike Feldman) writes: >Also, can anyone recommend any of the low-priced modems, eg. Practical >Peripherals? My father's under $100 1200 baud modem-in-a-plastic-box >cooked itself in under 10 hours of use, and I don't want to see him or >me sink $$$s into that level of quality again. We have/had 2 Practical Peripherals 2400SA modems from younger -and-more-(penny-wise-and-pound-) foolish days. They've never quite worked right, and multiple firmware replacements have helped, but not cured them. I say "had" because one of them recently failed completely, and has been sent back to the manufacturer. As soon as we get the bucks, these turkeys are history. -=EPS=-