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Why do cordless phones give feedback?

Discussion in 'Electronic Basics' started by larrymoencurly, Apr 9, 2004.

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  1. Why has almost every analog cordless phone (not cell) I've tried gives
    some whistling feedback when I talk into it? Don't they either use
    separate frequencies for the transmission and reception or something
    like echo cancellation? The only phone that didn't was a cheap AT&T
    49 MHz model that Consumer Reports said was really good, but I kept
    losing it because it didn't have a paging function.

    I'm using analog because they're about the only 900 MHz phones
    available here that don't sound distorted, and I don't want 2.4 GHz or
    5.2 GHz because they seem to blank out when wireless networking
    equipment or microwave ovens are used.
  2. the Wiz

    the Wiz Guest

    Probably has something to do with the position you're holding the phone in - as
    in it's not against your ear - something that's very common if you're trying to
    use the phone "hands free" by tilting your head towards your shoulder.

    I have one of those cheap AT&T phones that works pretty well, but WILL give a
    feedback squeal if it's placed near a hard surface. The re4ason it doesn't
    squeal in normal use is that the microphone isn't very sensitive - people often
    ask me to "speak up" when I'm on the cordless phone but never when I'm on the
    corded phones..

    More about me:
    VB3/VB6/C/PowerBasic source code:
    Freeware for the Palm with NS Basic source code:
    Drivers for Pablo graphics tablet and JamCam cameras:
    johnecarter [email protected] mindspring com. Fix the obvious to reply by email.
  3. Brent

    Brent Guest

    5.8 (not 5.2) Ghz shouldn't interfere with either of those. 2.4Ghz
    interferes with the microwave because microwave and WLAN because they
    also operate at 2.4Ghz. Some of the newer 2.4Ghz phones say they will
    not interfere with WLAN. I've seen a Vtech that says this.
  4. zack

    zack Guest

    My microwave does not interfere with my 2.4ghz cordless phone unless I
    put the phone about 6 inches away from it.. If you get interference
    when your phone is more than 2 feet away, then you probably have a
    very old microwave and should get it tested for leakage.
  5. That was one of the first things I thought it was, but even pressing
    my ear hard against the phone has never helped.
    The cheap 49 MHz AT&T 4336 that gave me no problems seemed to have a
    fairly sensitive microphone, or at least it amplified my own voice
    plenty back into the earpiece. Consumer Reports said that another
    model with a pager, 4337, was very similar, but they were completely
    wrong -- very different internal circuitry, noticeable distortion, and
    lots of feedback.
  6. Shawn Hearn

    Shawn Hearn Guest

    I gave up on cordless phones years ago after I got tired of the battery
    dying on me at the least convenient moment. I simply have corded phones
    now with long cords. That idea might work for you too. You can buy a
    corded phone with caller-id for $10 or less and a 50 foot telephone
    cable for $1 at many dollar stores.

    When I did use cordless phones, I never heard any feedback though.
  7. I myself at our office have a few 2.4 gHz phones, one on each tele
    line. They are the Panasonic Gigarange series. I have no feedback
    problems or any interferance from the microwave oven in the kitchen
    area. On our fax line we have a cordless 900 mHz Panasonic phone,
    that shares with the fax. This one does not have any problem as well.

    The higher end phones have a good quality RF and frequency control
    design, where they can select the frequency with the least
    interferance, and best reception.

    As for the feedback problems, this can be a problem with your
    telephone company as well as your phone, consering how their system is
    working for the sound duplex. The telephone line is a single twisted
    pair that is by-directional. It is possible to have a fault condition
    where there is a condition that can have some feedback, depending on
    the load on the line and the specifications of the equipment on their
    side, and the customer side together.

    There is also the condition where the microphone on your phone is too
    sensitive, or the return signal back to the telephone companies
    equipment is a bit too high. This can cause some feedback. There is
    also the condition where the ear piece or recieving signal from the
    telephone company is too high.

    When you have the feedback condition, try to lower the ear piece
    volume on the phone, and see if this gets rid of the feedback. If the
    sound is too low, then this can also mean that the microphone is too
    loud, or the signal that is being sent back to the telephone company
    equipment is too high.

    In some of the older cordless phone models, there were internal RX and
    TX level pots. The newer phones are mostly controled by internal
    software, that is not accessable without the proper service setup.

    If you are having the fault condition with many phones, I would guess
    the fault to be with the telephone companies equipment, and not yours.
    The task would be to have them check this all out for you, and
    correct this, if it is the case.

    Jerry G.
  8. JM

    JM Guest


    Sometimes the earpiece can be loud enough that the microphone picks it up.
    That makes feedback.
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