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A cordless phone question

Discussion in 'Electronic Repair' started by Ken, Oct 22, 2011.

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  1. Ken

    Ken Guest

    I have two AT&T model 1475 cordless phones. (I know they are quite
    old, but I like the feel of the phone even though it is a 2.4GHZ phone.)
    I acquired the second one for a few dollars primarily for parts if the
    first one crapped out. Both of the units work just fine in all respects.

    Since they are the same model and the phone from one does NOT work on
    the other base unit, I assume there is a different frequency that each
    is running at. I opened up one unit and I did notice a crystal tacked
    on the back of the PWB that I assume is unique for each pair.

    My question: Just out of curiosity, what do the manufacturers do to
    make their same model cordless phone communicate with only the base it
    belongs? My guess is they change something simple like the crystal on
    both the base and handset so that there is a distinction between units.
    Anyone know?? Thanks.
     
  2. The early ones used different frequencies. Then when the multiple channel units
    came out (in the early 1990's) they added some sort of digital encoding to the
    signal (such as a low level digital subcarrier) to identify one base station
    from another.

    Some of the earlier phones had switches inside to set this, later ones were
    just hard coded in a ROM somewhere or with soldered jumpers on the boards.

    The digital ones just include it in the digital data stream.

    Generally they are designed NOT to allow to change them so that you buy a
    new one when the old one dies.

    The new generation of cordless phones, DECT (there are similar but not
    compatible DCT phones) are IMHO best. DECT phones are wifi friendly they
    operate in a different band (1.8gHz) and listen to make sure a frequency
    is NOT in use by someone else before they transmit.

    They also have the ability to connect multiple handsets to the same
    base station and some have the ability to roam handests between multiple
    base stations.

    Geoff.
     
  3. Rich Webb

    Rich Webb Guest

    It's almost certainly handled in the digital realm rather than a single
    change in the base frequency. The handset and base are paired together
    by something like both sharing the same seed value for a pseudo-random
    number generator. The PRN is used as the basis of digital spectrum
    spreading of the RF signal, frequency hopping or direct sequence, so
    that only units that share the same sequence can inter-operate.

    Some base units and handsets can be "registered" together so that the
    base can operate with more than one handset. Since that capability isn't
    mentioned in the 1475's manual, I'd guess that the pairing is done once,
    at the factory.

    *If* the pairing key in the handset is stored in an identifiable chip,
    say, a serial EEPROM, then it might be possible to get at the key and
    change it.
     
  4. Winston

    Winston Guest

    (...)

    I don't know, but I RTFM and this seems likely:

    See page 36 of:
    http://dl.owneriq.net/5/52d44f17-e87a-46c2-adaf-59da932f7357.pdf

    Place your new-to-you base unit into a carton
    and tape securely closed. Do not touch it
    for about a week.

    Unplug the power from your existing base unit.
    Count to five slowly.

    Take the battery out of the new-to-you handset,
    count to five slowly and then put it back in.

    Place your new-to-you handset onto your base unit
    and plug the base unit power back in.

    This will cause the unit to reinitialize (and
    probably pair your new-to-you handset to your
    base unit).

    Test your old handset to see that it still works.

    If this doesn't work try:
    www.telephones.att.com, or call 1 800 222–3111.


    --Winston
     
  5. Ken

    Ken Guest

    Thanks for your comments. Thanks to everyone I have several ideas now
    that I did not have before.
     
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