From: Joe Legris
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Subject: Re: passive circuit excitation
References: <3E171373.email@example.com> <3E1AEAF7.firstname.lastname@example.org> <5Sv7CsA00AH+Ewemail@example.com>
Date: Wed, 08 Jan 2003 11:47:43 -0500
NNTP-Posting-Date: Wed, 08 Jan 2003 11:46:39 EST
Organization: Bell Sympatico
John Woodgate wrote:
> I read in sci.electronics.design that Joe Legris
> wrote (in <3E1AEAF7.firstname.lastname@example.org>) about 'passive circuit
> excitation', on Tue, 7 Jan 2003:
>>>All phones sold in the United States are required to broadcast signals
>>>so hearing aids can pick them up. The hearing aid runs off the EM
>>>signal in addition to the actual sound. This might be worth looking
>>That sounds improbable for 2 reasons:
>>1) I've never heard of such a thing and have never seen evidence of
>>transmitters in telephones.
>>2) If the hearing aid works, why not just amplify the sound of the
>>phone? It's not the hearing that cannot hear, it's the wearer.
> You have been mislead by the word 'broadcast'. I don't know whether it's
> true that all US phones have to have one, but the device is a magnetic
> antenna - a coil fed with the earpiece current. Some hearing aids are
> fitted with a magnetic receiving antenna ('telecoil') which can be
> switched in circuit instead of, or together with, the microphone. This
> picks up the magnetic field from the coil in the phone and relays the
> signal direct to the hearing aid earpiece.
> I was told some years ago that few US hearing aids have such a receiving
> antenna, but recently I was told the opposite.
> It is possible to get amplifiers that fit on to a phone, but there are
> two issues:
> - with the diversity of shapes of handset, it's now difficult to get an
> amplifier that fits all phones.
> - if the amplifier has a microphone and an earphone, acoustic feedback
> limits the gain, especially if the amplifier doesn't fit the shape of
> the handset closely.
> An amplifier having a telecoil would not suffer from acoustic feedback,
> but I don't know that any such product exists.
Thanks for the clarification. I wasn't aware of the magnetic antenna,
which makes a lot of sense. I thought that P!erCer above was referring
to a some sort of ring-signal transmitter.