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Noise issue

Discussion in 'General Electronics Discussion' started by Prince Charles, Jul 17, 2015.

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  1. Prince Charles

    Prince Charles

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    0
    Jul 17, 2015
    Hi Everyone

    I wonder if you can help me with any suggestions. I have 2 telephone lines which I have set up to record my calls. The one that has a joint output through my BT broadband socket records a strong humming noise. This noise isn't detectable when making or receiving calls. But it is detectable when listening to the playback. I assume that the phone is filtering it out, which is why I can't hear it. The hum is ever present, even when the phone is off. I know this because the phone recording software shows a little volume bar by each telephone input line (see Pic). The socket has a filter built in and I've even tried adding a second filter outside the socket but obviously it's not filtering the right thing.

    So my question is, how can I detect what this noise is, Is there some kind of multimeter I could get?

    or alternatively how can I filter it out?

    Thanks

    Mark

    NCH.JPG
     
  2. Colin Mitchell

    Colin Mitchell

    1,418
    314
    Aug 31, 2014
    The hum is coming from your recording equipment because it is powered from a plug pack connected to the mains.
     
  3. Prince Charles

    Prince Charles

    3
    0
    Jul 17, 2015
    Here's a couple of samples:
    1) Is with the phone not being used
    2) With the talking clock
    Hi,

    No, it can't be as it only happens on one line, the other is ok. Both lines feed directly into my PC, neither needs a power supply. I've swapped them over and it still happens on the same line. Also, the hum only gets picked up on recording not during the call.
     
  4. Martaine2005

    Martaine2005

    2,673
    722
    May 12, 2015
    Hi Prince, Camilla here,
    Had to edit this because you answered my question about filters.
    Have you got an answerphone that you can use just to check if that picks up the noise?

    Martin
     
    Last edited: Jul 17, 2015
  5. Prince Charles

    Prince Charles

    3
    0
    Jul 17, 2015
    Hi Martin

    Like most phones these days, the answerphone is an "all-in 1" with the phone. - It doesn't capture the hum either.

    So somehow the phone is filtering it out.
     
  6. Martaine2005

    Martaine2005

    2,673
    722
    May 12, 2015
    I don't know what socket you have. But some have a test socket inside the cover.
    You can try and remove the two screws on the front and connect your phone inside the socket.
    Not sure this will help though as it's still the 'master' socket.
    If you have an IDC tool, you could check the wiring inside the socket.
    We used to use colour codes but I think they don't bother so much now.
    Anyhow, you should have white/blue and blue/white connected to 2 and 5 in the socket. They may have used orange/white instead.
    If I remember correctly, another connection to 3 or 4 in the socket used to solve many issues.

    When you say the socket has a built in filter, are you talking about the yellow cap?

    Internet wasn't around when I was a phone engineer let alone BB down the line.
    But if you have another single EXT socket, you can also try connecting it to the master socket via 2,5 and 4 using the same colours from both obviously. If no change, try connecting 3 as well.

    Other than that, you might have to call BT for a line check.

    Martin
     
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