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Re: Getting matching transformer from telephone

Discussion in 'Electrical Engineering' started by Tomi Holger Engdahl, Dec 29, 2008.

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  1. Ross Herbert

    Ross Herbert Guest

    On Mon, 05 Jan 2009 18:25:18 +0000, Eeyore

    :Ross Herbert wrote:
    :> :Ross Herbert wrote:
    :> :
    :> :> Since your PC is mains powered and it may not have the required isolation
    :> :> between the mains side and the sound card input you can do your own thing
    :> :> using an approved 600:600 transformer with 3kV isolation rating to
    :> the
    :> :> telephone line to the sound card input.
    :> :
    :> :He DOES NOT need a 600 ohm transformer since the input impedance of the
    :> :card is not 600 ohms <sigh> !
    :> Since the application is merely detecting signal "voltage" it hardly matters
    :> that the secondary impedance of the transformer is 600 ohms and the input
    :> impedance of the sound card is more like 10Kohms. The only reason one tries
    :> match impedances is where one needs to maximise "power transfer" and that
    :> doesn't apply in this case.
    :Untrue. A non-optimally loaded audio transformer will not have a flat frequency
    :response. Nor is it about power transfer.

    With regard to a POTS line the VF bandwidth is some 300 - 3400Hz - hardly hi-fi
    - so optimal flat frequency response is not an issue.

    The fact that the secondary impedance of the 600 ohm transformer does not match
    the input impedance of the sound card is totally unimportant in this
  2. krw

    krw Guest

    How well do V.92 modems work or is the bandwidth really 4K (8K line
    cards, and such)?
  3. Also in the UK. Floyd seems to be referring to the US Bell standards.
    The CCITT standards (which apply in most other parts of the world) are

  4. Are we discussing the same thing, I wonder ? The "Local Loop" is the
    exchange - subscriber path, and is not bandwidth limited other than by
    the basic line parameters. Hence the reason a local loop can (typically)
    support 8Mbit ADSL.
    Which standard do you quote from ?
    Could you cut and paste some text from this standard in support of this
    statement ?
    This may well be the case as the designers will work to the worst
    (international) case.

  5. Eeyore

    Eeyore Guest

    By WHOM ?

  6. Eeyore

    Eeyore Guest

    Or up to 24 Mbps ADSL2+ !

  7. Eeyore

    Eeyore Guest

  8. Eeyore

    Eeyore Guest

    So your attitude is "it's so bad it doesn't matter messing it up even more".

    A proper 600:600 transformer will be more expensive than a 10k:10k one too.

  9. Palindrome

    Palindrome Guest

    PCPaul wrote:
    Did you ask them what they felt like about fighting alongside French,
    German or Italian forces? I'd bet a pineapple upside down cake that they
    would have been even more nervous... However, let's face it, the odds
    are that if anyone is supplying someone to fight alongside Brits - it is
    going to be the US and so the possibility becomes a realistic and not
    just a theoretical one.

    Of course a Brit is at greater danger from a Yank than another Brit -
    the odds are that the Yank actually has a gun/missile/helicopter/torpedo
    that /actually works/. Or, more to the point, actually *has* one,
    without having to go and scrounge it from another Yank.

    There is greater risk, of course. The Yank may not be aware of the
    British Army's traditions. Such as sending out soldiers with battlefield
    communications that don't work. In aircraft where spilt fuel has to be
    mopped off the deck after each IFR. With body armour that they have to
    take in turns wearing...

    I don't know many UK service personnel. Those I do have said that, when
    they have been in a fix, they have prayed for US air support to turn up.
    Not because it will be any better than Brit air support - but because
    they are praying for something that could actually happen and not just
    hoping for a miracle.

    US casualties in Iraq = 4147 US population =304 million
    Rate = 13 per million

    UK casulaties in Iraq = 178 UK population = 60 million
    Rate = 3 per million

    Yep, that sounds a good reason to feel nervous about fighting alongside
    the US military - they go into harm's way.
  10. tony sayer

    tony sayer Guest

    When you live -in- an exchange that is;!...
  11. Palindrome

    Palindrome Guest

    Michael A. Terrell wrote:
    "North of London", in terms of British accents, covers just about all of
    them. I don't think anyone ever loses an accent - at most it is
    temporarily mislaid. They tend to return to the surface when
    circumstances dictate, with or without any deliberate intention.

    I'd suggest that accents, even regional accents, really become
    interesting when used in a different language. Which, from what I
    understand of Florida, could easily have applied in the case you
    mention.. :)
  12. Ross Herbert

    Ross Herbert Guest

    On Tue, 06 Jan 2009 04:53:17 -0900, (Floyd L. Davidson) wrote:

    :>On Sun, 04 Jan 2009 20:36:09 -0900, (Floyd L. Davidson)
    :>:> voice frequency circuits were all 300 - 3400Hz in my day.
    :>:The PSTN is specified from 400 to 2800 Hz, with 24 dB SNR.
    :>:Individual channels on various carrier systems, and some
    :>:private line voice circuits are specified with more
    :>In Australia PSTN is specified for 300 - 3400 Hz bandwidth.
    :I doubt it.

    Well let's give some examples...

    When I was involved in junction commissioning (unloaded copper inter-exchange VF
    junctions) with Telstra, transmission measurements were carried out over the
    300-3400Hz range. This hasn't altered.

    When looking at the transmission characteristics of an international telephone
    exchange (ITU-T Recommendations)the only frequency range mentioned is

    And the following document recommends all channel terminal equipment be lined up
    using 300-3400Hz.!!PDF-E&type=items

    Note that ITU-T G235 (3KHz spacing) is supesrseded and is no longer recommended
    for international connections.

    As for the specification relating to customer equipment connected to the PSTN
    the frequency range used for testing is 100Hz - 4KHz.

    In other documents from this website the definition of VF telephony or Voiceband
    is 300-3400Hz.

    There probably a number of other publications if I had the time to research them
    but suffice to say that in Australia the VF telephony channel bandwidth is
    specified as per ITU recommendations ie. 300-3400Hz.
  13. krw

    krw Guest

    My SIL is a Brit. In the 35+ years she's been here she hasn't lost
    her accent, though has lost her ignorance.
  14. Good, accurate stuff, Ross.

    I really don't know where Floyd L. Davidson is coming from. He is grimly
    sticking to an assertion the the UK and Australian PSTN is specified as
    "400 to 2800 KHz", but has been unable to quote his source in terms of
    unambiguous specifications.

    I can only assume - from his location - that he has some experience as a
    technician in military comms systems, and has some view of the US Bell
    system. Hence the belief in "400-2800". As has been fully demonstrated
    by several others, this is a profound misunderstanding of the
    international PSTN.

    This thread has wandered far and wide from the OP's question, but it's
    getting a bit too far off the subject for me. It's also degenerated into
    a classic battle of the Trolls.

    If Floyd wishes to make further wild and inaccurate statements, perhaps
    he would like to open a new thread ?

  15. Ross Herbert

    Ross Herbert Guest

    On Wed, 07 Jan 2009 06:52:48 -0900, (Floyd L. Davidson) wrote:

    :>Good, accurate stuff, Ross.
    :All of it referenced the specifications for individual
    :channels on various facilities. None of it had to do
    :with the overall minimum allowed specification for an
    :end to end connection via the PSTN.

    Hold it, that's a dumb statement.

    The ITU recommendation for international VF channel bandwidth is 300-3400Hz, so
    why would it be necessary to "allow a minimum" bandwidth of 400-2800Hz? If the
    300-3400Hz applies to ALL channels used in an end-to-end international link,
    then it follows that the overall bandwidth is 300-3400Hz.

    Obviously the ITU spec is greater than the so-called "minimum" of 400-2800Hz, so
    it is not necessary to "allow a minimum" bandwidth of less than this.

    The fact that some administrations may not have adopted the ITU recommendation
    and continue to use 400-2800Hz simply means that they are not prepared to
    upgrade their equipment and are therefore behind the times. Any communication
    carried over such links will mean the overall bandwidth is degraded even if some
    sections do conform to the ITU recommendation of 300-3400Hz.

    :>I really don't know where Floyd L. Davidson is coming from.
    :More than three decades in the long distance
    :telecommuncations industry (but not with the Bell

    And I have 37 years of Telco experience from 56 - 93 in both metro and long-line
    installations. In all those years we used 300-3400Hz.
  16. Eric Tappert

    Eric Tappert Guest

    The smaller bandwidth was used on undersea cables to increase capacity
    (16 channels in the same bandwidth as 12 standard channels). Such
    usage has gone away with the introduction of fiber optic cables. TASI
    (Time Assignment Speech Interpolation) is still used to about double
    the capacity of modern undersea cables. The operating principle of
    TASI is to disconnect a channel with no voice detected and give the
    circuit to an active user. This game is played continuously and
    allows the operator to about double the capacity of a link. The
    downside is the speech detector tends to clip the first syllable a

  17. Ross Herbert

    Ross Herbert Guest

    On Tue, 06 Jan 2009 19:47:17 +0000, Eeyore

    :Ross Herbert wrote:

    :> With regard to a POTS line the VF bandwidth is some 300 - 3400Hz - hardly
    :> - so optimal flat frequency response is not an issue.
    :> The fact that the secondary impedance of the 600 ohm transformer does not
    :> the input impedance of the sound card is totally unimportant in this
    :> application.
    :So your attitude is "it's so bad it doesn't matter messing it up even more".

    Not at all. Any practical measurement of the degradation of a 300-3400Hz signal
    using the 600 ohm transformer would be insignificant. If your assumption that
    audio degradation when using a 600:600 transformer was valid then why is there
    no available audio transformer, with a 3.5Kv isolation rating, to match a 600
    ohm telephone line to a high impedance (eg. 10K ohm) input? The answer has to
    be, "it's not necessary".

    :A proper 600:600 transformer will be more expensive than a 10k:10k one too.

    That may be true but how many readily available 10K:10K audio transformers have
    a 3.5Kv isolation rating?

    In order to conform to the required isolation rating of 3.5Kv between the mains
    powered PC and the telephone line, the only transformer which is readily
    available, and complies, is an approved 600:600 transformer.
  18. tony sayer

    tony sayer Guest

    Ummmm... those figures are not -quite- that meaningful unless we have
    some sort of reference level ..

    i.e. 300 to 3400 +/- ? dB ...
  19. Ross Herbert

    Ross Herbert Guest

    :What do you suppose happens to the bandwidth of a signal as it
    :is passed through successive audio devices that are band
    :limited to 300-3400 Hz?
    :Would you expect to get 300-3400Hz +- ndB response in an end
    :to end loopback test when you use white noise as the test
    :signal? (n =ITU spec)
    :Going from a handset to a PBX to MUX to a CO into the network
    :and back if each device has a 300-3400Hz response the looped
    :back signal should be more band limited than the first device
    :in the signal chain with 300-3400Hz response originating test
    :signal by quite some bit.

    I suggest you do some research on the various ITU documents, eg.

    or pick any relevant section from the main page
  20. Eeyore

    Eeyore Guest

    Bell don't count as they're not International.

    Since everyone else says ITU-T say 300-3400Hz I suspect we're seeing
    another case of you being stuck in a time warp.

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