POTS Line interference and Humm 2

[FlyingDevilFish]

I have a recording device bridged to a POTS line and 2500 set. The recorder starts on an incoming call when the 2500 set goes off hook. The conversation is crystal clear. WHen the 2500 set goes back on hook the recorder is picking up a hum on the line and thus recording the hum. (The unit has no on-hook voltage drop intelligence, just stops recording when it there is silence). I tested the line and there is maybe .01ma leaking on the line in the on-hook state, which is in telco spec.. Any ideas what can cause this or if there are any filters available?

Being the conversation is clear I can't see it being an external RFI/EMI source, but I may be wrong.

 

[mattKnight]

I have had problems in the past with similar issues.

What sort of recorder and card are you using?

Take Care

Matt
If at first you don't succeed, skydiving is not for you.

 

[FlyingDevilFish]

Matt, It is a exacom unit. It is the customers unit so I am no familiar with it. There tech support keeps pointing to the lines. Its a digital recorder so the card is acts as a modem and records the conversation to disk. There are 4 lines connected to the unit, 2 lines per connection X 2 RJ14 interfaces. This is the second exacom line card and the problem is still there on all 4 lines. What were your solutions?

 

[FlyingDevilFish]

More info...I have tried swapping out the 2500 sets with my butt set, and eliminated the bridged wiring to the 2500 set. Toned out the wiring to make sure there weren't any other open drops. No change. The line card is 2 feet from the telco demarc where the bridge is on the 110 block.

John

 

[ISDNman]

Does the recorder have a low cut filter? If not this is a design flaw. There's no particular advantage to have frequency response below 300 Hz. Remember, the 2500 (and most butt sets) *don't* have much response at 60 Hz.

Does the recorder device have a level adjust? If you back that down a tad that might be a suitable workaround.

To troubleshoot this properly you need to beg/borrow/steal a transmission test set (a good source for a used unit is ebay) and get the number for a silent line. Then measure C message metallic noise (e.g. noise tip to ring). While you are at it measure noise to ground (tip to ground and ring to ground). With these number you can calculate circuit balance, which is tarrifed as well as metallic noise.

The C message noise has a weighting curve and it is weighted to be *less* sensitive to low frequency noise. This is why the recorder should have a low cut filter. In any case, measuring it would tell you for sure.

For more on measuring loss and noise see:

http://www.sandman.com/longitud.html

If you firmly but politely demand it you may be able to get the Telco to come and measure the noise. Makes sure you are there to watch them do it. Otherwise the odds are pretty good they'll take a listen with a butt set and then close the ticket with "no problem found".

Best of luck.

 

[bikechuck]

move your recording device to another line. If the hum is still there fix your equipment.