Cable/POTS Crosstalk
June 16, 2006 8:09 PM   Subscribe

My dialup is crosstalking with my wifes cable internet connection. She works from home and has a dedicated cable internet connection with VOIP. Only her fax is connected to the phone line. She can hear my system dial up and sometimes can hear my phone call conversations over her VIOP. My office phone connection passes through my modem. The phone is not a wireless. Both services share an underground conduit coming into the house. The cable company sent a Tech out who could not find any problems on my wifes system. What can I check out on my POTS before I blow $100 on a visit from a phone Co. Tech?
posted by Raybun to Computers & Internet (9 answers total)
A couple of things, the outlets that the electronics are plugged in to are possibly not properly grounded. It's also possible the cable isn't properly grounded as well. With VOIP there's almost no way you should be able to get cross talk with POTS.
posted by iamabot at 8:20 PM on June 16, 2006

If she can hear your phone conversations over her VOIP, the interference must be arising after the signal is decoded-- as iamabot says, I can't see how it could happen any other way. What kind of VOIP phone does your wife use?
posted by justkevin at 8:34 PM on June 16, 2006

Yeah, it's impossible. But thier might be some crosstalk if she is using a regular phone with VOIP. But not the actual cable line.
posted by bigmusic at 9:58 PM on June 16, 2006

That's simply not possible.
posted by delmoi at 10:46 PM on June 16, 2006

Well, the only way it could happen is on the way from the sound card too the speaker that she uses. Does she use a headset or something?
posted by delmoi at 10:48 PM on June 16, 2006

The reason it's not possible is that VOIP is just ones and zeros, and not only that but it's packetized and compressed. The ones and zeros don't "Look" anything like a normal audio signal untill you run it through special software on your computer.

In order for your phone signals to get into her VOIP, your voice would have to be converted into ones and zeros, then compressed into the proper format, and then inserted into the TCP packets, without altering things like the checksum and TCP signal encoding sequence number. You would need to buy very expensive networking equipment, and write special software to do it.

In fact, you'd actually have to decode the call, mix in your voice, and then re-encode the call.

So you can see why this can't happen.
posted by delmoi at 10:55 PM on June 16, 2006

TCP signal encoding sequence number

posted by delmoi at 10:56 PM on June 16, 2006

So if you were to plug your voip "box", for lack of a better word, in to your homes twisted wire cabling you *could* get cross talk...but it really wouldn't be cross talk, it would be direct interference, and there is no way around this.

Low pass and high pass filters won't take care of something like this, you'd need to segregate the wiring and make sure you're using seperate pairs (most homes have 2-4 pair cabling). If the pairs aren't terminated properly at all of the jacks you can have this happen, sometimes folks bridge pairs so they don't have to be specific about what the jacks are terminated on.
posted by iamabot at 10:59 PM on June 16, 2006

Like other phones, VOIP phones have feedback from the microphone to the earphone, so that you can hear yourself in the ear you're listening to the other party with as you talk. This feedback signal must be analog rather than digital. As a wild guess, the fax machine amplifier, which is probably always on, is picking up the signal from your POTS, amplifying it and inducing it into the feedback circuit which goes to your wifes earphone.

Test this by unplugging the fax from the power outlet and the phone line. If that's it, just move stuff around the room till it (the problem) goes away.
posted by jamjam at 11:03 PM on June 16, 2006

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