From: John Woodgate
Subject: Re: passive circuit excitation
Date: Wed, 8 Jan 2003 11:36:20 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Wed, 8 Jan 2003 13:14:31 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Joe Legris
wrote (in <3E1AEAF7.email@example.com>) 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
- 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.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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