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Measuring SNR of telephone line

Discussion in 'Electronic Basics' started by Jack// ani, Dec 24, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi there,

    How to measure it??

    TIA
     
  2. BobG

    BobG Guest

    record a received tone of 1KHz. Analyze it. Anything that's not 1KHz is
    distortion. Take the ratio of signal amplitude to noise amplitude.
     
  3. Jack// ani

    Jack// ani Guest

    Few confusion....as usual

    Why only 1Khz...not something else, like voice signal? How to seperate
    voice signal and the noise signal?
     
  4. BobG

    BobG Guest

    You already know the phone line has a bandwidth of about 300-3K. If you
    also know that the max signal is 0dbm or .775V coming in, just measure
    the noise level of a quiet line. sn is 20log(noise/.775)
     
  5. Don Bowey

    Don Bowey Guest

    What test equipment do you have to measure with? Once we know that we will
    be able to help you.
     
  6. Pooh Bear

    Pooh Bear Guest

    Because accurate calibrated measurements are made with respect to a
    'reference level'. It's simplest to provide that with a single tone. 1kHz
    is also in the centre of the telephone frequency band, so a good choice.

    Graham
     
  7. Jack// ani

    Jack// ani Guest

    Nothing more than a DMM and PCScope!! How should I proceed next.....

    Thanks
     
  8. Pop over to greed-bay, and see if you can find an auction for the
    appropriate test gear. If it's just a simple noise or line-quality
    measurement you're looking for, you can probably get away with something
    like a Wilcom T236 (analog meter) or T336 (digital meter) subscriber
    test set.

    If you're really serious about this, as in wanting to do detailed
    readings of noise, phase jitter, and other in-depth variables, search
    ebay for 'Hekimian' or 'HLI,' and you will likely find any number of
    their high-end transmission test sets that, while old (they date back to
    the mid-1980's), are reasonably cheap and will still do the job.

    If Hekimian hardware doesn't tickle your fancy (and fair warning
    -- Hekimian Labs has been out of business for years, so there's really
    no manufacturer support for their stuff), you could try for something
    like an HP 4935A or 3551A transmission test set.

    Keep in mind that, no matter what kind of test gear you have or
    get, you still need the originating central office to send out a 1kHz
    test tone on the line in question. For that, you would need the
    cooperation of your local phone company (good luck!) in the form of
    telling you what the local test number is to get said tone sent back.

    If what I've said above only increases your confusion, then you
    need more education than I'm in a position to give regarding phone lines
    and TIM (Transmission Impairment Measuring).

    Happy hunting.


    --
    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
     
  9. Jack// ani

    Jack// ani Guest

    I was wondering how to send 1Khz signal on my own line.......your post
    left no more confusion now!!!

    Thanks a lot
     
  10. BobG

    BobG Guest

    Use your computer as a signal generator. Have it put the tone in the
    speakers. Put your cell phone in front of the computer speakers. Call
    your home phone. Record the received waveform at the earpiece (gator
    clips?). Analyze the recorded wave.
     
  11. Ahhh... I beg to differ. That's going to introduce all kinds of
    distortion that a reference test source, hardwired to the CO frame, will
    not. The level will also be uncertain as blazes, whereas a reference
    generator puts out a tightly-controlled 0dB.

    Even if this idea works, the results will not be accurate. Better
    just to hit up one of the local telco's field people for the test tone
    number. That's how I used to do it.

    Keep the peace(es).


    --
    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
     
  12. You're welcome. To clarify further, the test tone is typically
    produced by a device at the CO called a 'Test Line Responder.' It is
    designed to, upon various commands (usually by TouchTone signaling),
    produce test tones of varying frequency and intensity. It is usually set
    for a 1kHz reference tone by default.

    The most common one for your purposes would be 1kHz (1004Hz, to be
    exact), injected at a level of 0dBm from the central office. This will
    allow you to take very accurate readings (assuming the correct test
    gear) of your line's loss and noise levels.

    The best way I've found to obtain test numbers is to (politely!)
    ask a telco field person. Technically, they're not supposed to give
    those out, but I've run into many that do if they detect a kindred
    techie-spirit in you.

    Happy hunting.


    --
    Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
    (Known to some as Bruce Lane, ARS KC7GR,
    kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
    "If Salvador Dali had owned a computer, would it have been equipped
    with surreal ports?"
     
  13. Jasen Betts

    Jasen Betts Guest

    You'll get distortion from the amplifier in the speakers piickig up signals
    from the cell phone, distortion from the speakers, the microphone in the cell
    phone, distortion from the encoding used to translate the audio into a
    96kbps digital signal. distortion in the recording device (or if dirrectly
    connected to the computer distortion from unbalabcing the phone line)

    one rough and ready way to measure phone line distortion is to dial out to
    an ISP using a 56k modem and see what connect rate you get.

    using a terminal program it's possible to get information from the modem
    about the line quality etc...

    since the signals are digital from the CO to the ISP the only distortion is
    on your local loop.

    at&v1

    TERMINATION REASON.......... LOCAL REQUEST
    LAST TX rate................ 26400 BPS
    HIGHEST TX rate............. 26400 BPS
    LAST RX rate................ 49333 BPS
    HIGHEST RX rate............. 49333 BPS
    PROTOCOL.................... N/A
    COMPRESSION................. N/A
    Line QUALITY................ 025
    Rx LEVEL.................... 016
    Highest Rx State............ 67
    Highest TX State............ 67
    EQM Sum..................... 00A2
    RBS Pattern................. 00
    Rate Drop................... 00
    Digital Loss................ 2000
    Local Rtrn Count............ 00
    Remote Rtrn Count........... 00
    V90 9481834246E0
     
  14. Don Bowey

    Don Bowey Guest

    Milliwatt test numbers are usually the office lowest nxx plus 1000. For
    example your office may have 581, 582, 583, numbers; try 581-1000. If that
    doesn't work try the rest.

    Or........

    Your telephone line attenuation is going to be in the range of about 2dB to
    8.5 dB. You could ASSUME it's about 4dB and use that in your calculations
    instead of measuring a test tone from the CO.

    Don
     
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