[cisco-voip] FXO/Analog line hum - 60Hz - Noise

[Wes Sisk]

Credit to Tom Bamert for this one:

Buy enough phone cable to run from server room, down the hall, and out 
to building dmarc.  In an outage replace the house wiring with the hall 
wiring. If hum is gone then cabling is your problem.

My approach:
Hairping a call out to the PSTN and back: 
ipphone1--(analog)--pstn--(analog)---ipphone2
mute both phones.  perform a packet capture of the audio stream. 
alternatively use pcmcapture on the voice gateway.  Get the audio from 
dump or packet capture.  Use CoolEdit or similar to look at frequency 
plot of audio.  If you see a spike at 60Hz then it has to be 
interference introduced in CO or in the analog cable runs.

For this client we presented all points:
* No noise using hall cabling
* No echo using hall cabling (noise distorted outgoing/incoming signal 
so ecans were ineffective)
* frequency plot showing 60Hz spike

It made a strong enough argument for the client to pay for pulling new 
insulated cables that bypassed sources of possible interference.

/Wes



Carter, Bill wrote:
> In a post on "Echo Cancellation and 7911 Phone Loads", Wes's answer
> included: "Examples of noise include any slips or errors on T1's and
> electrical cables (60Hz, fluorescent lights especially) located too
> close to analog (FXS/FXO) connections."
>
> I have a customer with Analog lines connected to FXO ports with constant
> hum. If I have a call from an IP Phone out the FXO ports, put both
> phones on mute, both phones still hear hum.
>
> The analog lines run from outside the building, through a dropped
> ceiling, into the wiring closet. The wiring closet is actually a room
> with the AC blower/furnace and hot water heater.
>
> I have suspected electrical interference for awhile. They sometimes have
> a weird event where users will suddenly hear a screeching sound on the
> phone. It will last for 1-2 minutes. My theory is this happens when the
> blower on the AC/furnace kicks on. I have been able to have a user tell
> me if the screech happens at the same time the blower kicks on.
>
> How can I check for 60Hz interference? Is there an electrician tool I
> can connect to the analog line to see this? Someone suggested adding an
> analog line DSL filter to the lines.
>
>
>
>
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