annoying noise issue?

Discussion in 'General Electronics Chat' started by Mathematics!, Dec 17, 2009.

  1. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
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    I am wondering how to fix the zzzzzzzzz sound in my line in port when plugging my device in.

    do I need to make a high pass or low pass or what to get rid of it?
    I can hear this zzzzzzz sound even in the regular phone receiver forgetting for the moment when I rigged up the phone recorder circuit into the line in port....

    What I think is happening is the line in port is amplifiying the tiny zzzzzz sound that the telephone mic is making probably.

    The problem is I don't know what frequency to filter for or what can I do to get rid of it .

    Attached is the sound clip of what I am hereing.


    Please don't just dismiss my post because it is illegal to talk about telephone recording.... THIS POST IS NOT THAT IT IS ABOUT LEARNING HOW TO FILTER NOISE.... I HAVE THE SAME PROBLEM WITH MY INTERCOM CIRCUIT (ALOT OF SQUEEKING cause by the mic )

    hopeful somebody can solve this problem for me and then I can apply what I have learned to fix the intercom circuit...
    Thanks for any help
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
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    Sounds like you have earthed one side side of the phone line which is a definite no-no and will probably show up at the exchange as a fault.
    The obvious solution would be to disconnect any external device or amplifier you have connected to the phone and learn something about electronics before you start messing with things you don't understand.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    If you've downloaded Linear Technology's LTSpice as I suggested the other day, you can download the attached file into your SwitcherCad directory.

    Then start LTSpice, and load the simulation.
    Edit the line:
    V1 Wavnoisy 0 wavefile=.\phone1.wav
    by right-clicking on it, and change the path to where your phone1.wav file is; or copy the phone1.wav file into your SwitcherCad directory.
    Run the simulation, and click on the top of R1. You'll see the waveform displayed as a plot.

    Right-click on the plot, go to the bottom of the list to View, select FFT, and click OK (V(wavnoisy) is selected already)

    You can then zero in on the frequency that is causing you the problem.

    Then you can go to Texas Instrument's website, and download FilterPro, another free utility.
    http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

    Design a single-pole notch filter to remove the undesired frequency.
     
    Last edited: Dec 17, 2009
  4. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    Not very good advice Sgt.
    If the OP has earthed one side of his phone line then in all probability he is also effecting other people who are unfortunate enough to be on the same cable. Inserting a filter may fix his problem but won't help his poor neighbours.
    The only sensible fix is to check for an earth on the line and remove it.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    It is not just a 120Hz sine-wave hum tone from mains pickup because the input is not isolated. It is a 120Hz buzz that has many harmonics. A notch filter will not reduce the level of the harmonics.
     
  6. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
    4
    Ok , I did what you said attached are the voice graph and the view FFT graph.

    My problem is how from these graphs can you determine the cause of the noise???

    And was their any particular reason why you used a resistor value of
    R1 = 1Kohm because this just would reduce the amplitude of the current.
    But any resistor value would do (just a higher wave amplitude)
    Just wondering if their was some reason behind that.

    What will clear this up / filter this out with out killing the audio as well???

    Also when you said earthed do you mean that some part of the circuit/ wires are some how touching the earth/ground.
    I will check this again but I doubt this.

    Thanks for any help

    Could it be that the phone mic is giving out noise sound harmonics that are being amplified?
    Could it be because close wire is giving off stray capacitance/inductance ?
    I also open the wave file in audacity and I can zoom into an piece easier if that helps.
     
    Last edited: Dec 17, 2009
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Gee, you're a mathematician, right? You understand that the FFT is a graphical display of a fast Fourier transformation, right?

    What you might not have realized is that you can "zoom in" on just a section of the FFT by clicking/dragging to form a rectangle. LTSpice will re-scale the FFT plot on the area of interest.

    [/QUOTE]And was their any particular reason why you used a resistor value of
    R1 = 1Kohm because this just would reduce the amplitude of the current.
    But any resistor value would do (just a higher wave amplitude)
    Just wondering if their was some reason behind that.[/QUOTE]
    It was just to create a complete circuit. In the simulation, the actual value of the resistor didn't matter. It could've been 1 Ohm, it could've been 100 million Ohms.

    No. But, the FFT shows you where your noise problem is.

    You have an error in your circuit somewhere.

    Disconnect it from the phone line until you can figure it out.

    Otherwise, the phone company will likely turn off your service until it is corrected.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    A phone line is supposed to be perfectly balanced. Each wire has the signal at exactly the same level but out-of-phase. Then mains hum is picked up by both wires and is cancelled.

    You have something connected to the phone line or phone that disturbs the perfect balance because the buzz is at 60Hz (the mains frequency) and has many harmonics.

    Maybe you used unshielded cable to connect the phone to your recorder.

    Frequently an audio transformer is used to isolate the balanced phone line from your circuit or recorder.

    I just tried Audacity for the very first time. The buzz is actually 60Hz plus harmonics.
     
  9. lmartinez

    Active Member

    Mar 8, 2009
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  10. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
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    Well that could be ... I was building this
    http://www.solorb.com/elect/phone/tap/

    I built it with every component the same except I didn't have a .22uf so I was using random capacitor values in it's place like 22uF , or 0.1uf.. (thought the capacitors main purpose was to block out dc so it won't matter what value I used but I guess I am wrong)

    Maybe that could be the problem? I did check the circuit for grounding but their is no question that this is not the problem.... I still think it is amplifying some noise from the mic or some element of my phones....?


    Either way their is some stuff I don't understand about the POTS..

    Attached is a picture and circled is the components I don't understand...

    In the pic where does these audio 2uf caps connect to (from the left)? Are they connected to the switch which is connected to the other persons phone...?

    Also the relay coils I don't know exactly what their purpose is the page I am reading say's they are for current limiting but I don't understand why you would ever need to limit the current since having more telephones in parrell and longer wire runs would limit this enough???

    Also are these actual relays like the ones used to control high voltage from low voltage or are these totally different components then typical relays...? (because if the relays where switch off then won't the battery + and - be totally open circuit...

    I just don't understand the purpose of the red square in my diagram...?

    Also is their any particular reason why they use 2uf capacitors just before the audio. If this is just to block out dc and pass the audio signal then the values could have been anything . Yes/no ? Like they could have used 2X20uf caps ,...on and on

    Thanks for any explaining on this
    I am still looking thru the FFT to see how I can limit this noise got to tell you I am not really good with making filters yet????
     
    Last edited: Dec 17, 2009
  11. SgtWookie

    Expert

    Jul 17, 2007
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    The FFT in LTSpice provides that capability from a .wav sample.

    It's nice when you can get the equivalent of an expensive piece of equipment for a free download, right?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    He didn't want to hear that option.

    He just wanted to know why he had a buzzing.

    I simply told him where he could find the tools to determine what frequency was causing the buzzing, and more tools to help eliminate it.

    You and I know that removing it is the only sensible option until he fixes the problem. But he would not be receptive to that option.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, it's a fundamental 60Hz with harmonics.

    I tried to pose it as a learning experience for our OP.

    Perhaps I failed to some extent; but at least I know that they downloaded LTSpice and experimented with it. That in itself is a good thing.

    I can only hope that our OP will start exploring the SPICE simulator, and get a great deal of their questions answered.
     
  14. lmartinez

    Active Member

    Mar 8, 2009
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    Sorry, I should of read the responses GIVEN ABOVE. I provided an answer based on the analysis of the frequency components of the wave file provided above. I simply run it through my spectrum analyzer!! Thanks for making a note of it........:rolleyes:
     
  15. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
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    OK , I disconnected the device and am going to work thru the filtering problem.

    But in the mean time to get the whole picture of the POTS can somebody answer my previous posted questions about the relay and audio connection.

    Thanks

    Also I should have mentioned that this is not really the pstn network I am working with I have VOIP from comcast that my phone plugs into.... But I measured the components voltage/current on the line and it seems to be identical with the POTS 50vdc , 90vac 20hz ring , 35mA current ,...etc of the PSTN.

    But maybe the humming 60hz harmonic distortion is from the actual modem or modems power supply sense it is using the 120vac mains power supply at 60hz somewhere down the line..... either way if this is the issue then I should beable to create a high pass filter that cuts off frequencies below 100 hz or so to get rid of the 60hz harmonic distortion.

    LTSpice is cool I think I will practice doing some ossilator circuits with it to see if it gives the correct carrier wave based on the 1/2*pi*sqrt(LC) resonanting frequency formula....
     
    Last edited: Dec 17, 2009
  16. Audioguru

    New Member

    Dec 20, 2007
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    The circuit called "Isolated Telephone Interface" uses a transformer for isolating the balanced telephone line from your recorder.
    Didn't you use the audio transformer?

    Disconnect the circuit from the red and green phone line wires and listen to see if the hum goes away. If it is still there then you must use shielded audio cable from the circuit's audio jack to your recorder.
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    If a value is given, it's given for a reason.
    If you don't use the value given, don't expect the same results.

    If you bought an item from me that I said was priced at $1, and you paid with a $100 bill, and I gave you $4 back in change, would you complain?

    If you bout an item from me that I said was priced at $1, and you paid with a $100 bill, and I gave you back $4,999 in change, would you complain?

    Electronic circuits don't quite work the same way; that was just an analogy. In both scenarios, something was very wrong.

    No.

    You circled in red an inductor that is 5 HENRIES in value. That is a very large inductor. Send a big truck to pick it up. The capacitors are 2uF, or two microFarads.

    You need to understand that the equipment you circled is the telephone company's equipment. You do not have access to it.

    They are at the telephone company office.

    Because they have to ensure that their other customers have telco service even if you try to bugger it up.

    Yep.

    Limit transients (5H inductor), limit maximum current (200 Ohm resistor).

    Caps block DC, and pass the effects of AC. Using a large value of capacitance would not be productive, as people won't be able to hear the low frequencies passed in the headsets of their phones. Using smaller caps would make everyone sound like chipmunks.

    It takes quite a while to become really proficient with filters.

    But you're a math wizard, right? Filters are mostly math.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    I played the noisy wav file in Audacity program. It sounds exactly like a 60Hz square-wave but it barely looks like a square-wave.
     
  19. Mathematics!

    Thread Starter Senior Member

    Jul 21, 2008
    1,022
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    ok , so the relay coil is to limit current to a single home ,...etc so that the person doesn't draw to much current from the telec companies battery thus killing the phone network completely ... are these relay coils disconnect the use if he shorts his green/red wires or something...

    Like when the current starts to get to high does the relay switch off....

    And are these relays put on everybodies line since if it was one big relay that everybody was connected to...won't this just disconnect or open the battery circuit for everybody (thus disconnecting the everybodies ability to use the phone) I would think you would have to have a relay on each persons line at the telecom side in their local office ... etc to prevent total disconnect of everybodies line all at once?

    As for the 2uf caps on the audio I know this is the telcom side I was just wondering where the audio goes after the 2uf caps? does it go to some type of DSLAM or computer that digitalizes the 400hz to 3.4khz audio by sampleing it at 2 the frequency 8khz or is their more circuitary before it reaches the machine that digitalizes the audio and makes the connection with the person on the other end??? Curious


    Also does the relay prevent somebody from damaging their battery...
    Not saying I would do this but what happens if somebody connected a battery or 120vac 60 house outlet to the green/red wires :eek:

    Other then causeing a fire and melting the wires insulation I would think they should have the relay to work both ways so if it gets a current that would damage their battery from the customers house they should have the ability to open the ciruit??? Just a security issue they probably already thought about it.

    Still a little shaky on what this 5H is is it just a coil of wire or is it an actually relay that switches on and off?
    Because if it is a coil of wire then this doesn't have the advantage of completely disconnecting the persons line based on the high current like a normal circuit breaker or relay switch could be used to do....

    Yes I got the same thing in audacity that is a great program for free it allows you to convert wav to mp3!
    Anyway this could be interference from the thing I posted about VOIP in previous posts and is being amplified by the soundcard .... Maybe a ferric band on the green/red lines could clear it up?
     
    Last edited: Dec 17, 2009
  20. lmartinez

    Active Member

    Mar 8, 2009
    224
    6
    So what you are saying is that once you take the Laplace transform of an impulse you get 1 divided by what!!!!
     
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