sundance-communications.com/forum Forums TDM Business Telephone Systems Toshiba Telephones & Systems "Humming" on DID Toshiba CIX

Hello, I have 2 DID trunks going to a RDDU. I get a humming or buzzing on calls coming through both trunks. I have this problem at 2 locations in the same 4mile radius. The provider says that the hum only appears when connected to my switch. They mentioned impedance mismatch because they are far from the co and that I need to match the impedance of their cable pair. I tried changing the db level, I tried every position of the HML jumper. I tried wink, immediate and timing start, the provider is set to wink but I made them change it to immediate start and still getting the noise. Do any of you have any idea what to do with this!?

Thank you

this is common on RDDU cards. The cable pairs from the Telco have to be perfectly balanced. Hard to get them perfectly quiet.

When they say impedance, don't they mean ohmage?

I could be wrong, but if I am right, I would put an ohm meter between the line and where it connects to the system and see what it is on a regular line. (on each wire of the pair)

Then do the same on the DID line.

Get two resistors and put them in series on each wire to match the DID line resistance to the regular copper line.

Of course I could be full of gas but then I could be right too.

You would actually have to use an impedance bridge to get an accurate measurement. An ohm meter will only measure a DC resistance, without considering factors such as reactance and frequency.

Also, the 48vdc voltage coming off the DID card will prevent you from getting a proper impedance measurement on the equipment side. Regardless, the card is designed to look at a 600 ohm impedance. It is usually the line that is out of spec due to the distances involved. It is the telco's responsibility to balance the circuit to this spec.

Ohm's and Impedance is a measurement of resistance, commonly to ground, Tip to Ring, and Ring to Tip.

I believe the test you are referring to is a metallic balance, which puts sound on the TIP side and measures it on the Ring side, then puts sound on the Ring side and measures it on the tip side. The noise should effect each other equally thus being metallicly balanced.

This is commonly what causes the country hum, however what it sounds like is the tester from your provider ran a line test from the CO and is seeing an unbalanced metallic trouble when your equipment is connected. I would call your install and have him troubleshoot the issue in person.

Glad you guys know more than I do, I was guessing trying to help.

This is almost a day to day issue I deal with/work on/am aware of.

Trying to install VDSL circuits at .5 - 1 mile can be a bit of a challenge if you don't pay attention. But, Definitely having a trained technician there in person trained on your specific system will do much better then any suggestions here.