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Digital Cell Phones

Jeff Joslin jjoslin at nortel.ca
Thu Nov 14 16:57:41 EST 1996


In article
<Pine.A32.3.95.961113215314.108197B-100000 at homer29.u.washington.edu>,
Penny Evers <pevers at u.washington.edu> wrote:

>Could anyone provide any feedback on the use of digital cell phones by
>hearing aid users.  I am specifically interested in the new AT&T PCS
>Digital phones.

I can't give you any information specific to the AT&T phones, but 
the following information depends more on the hearing aid model than
on the phone model.

First, the PCS I'm familiar with is the broadband PCS in the 1.85-1.99 
GHz band, which is, I believe, what is being used for the new digital 
cell phones.  

Many hearing aid users find that they cannot use PCS phones, because
conversation is drowned out by a hum at 217 Hertz (or 434 Hertz).
This occurs as follows:  the phone transmits at 1.9 GHz.  This
1.9 GHz signal switches on and then off again 217 times per second.
In electrical engineering terms, a 217 Hz square wave amplitude-
modulates a 1.9 GHz carrier.  (Good old AM radio is just
amplitude modulation using carriers in the range of 450 kHz to 
1500 kHz.  The signal doing the modulating would be the audio signal
to be transmitted, rather than a square wave.)

The phone is designed (and licensed) to transmit at 1.9 GHz,
so it's working as intended.  Any nearby unshielded electrical
devices will pick up this signal to a greater or lesser extent.  
Unfortunately, the unshielded circuitry inside most hearing aids
tends to demodulate the 1.9 GHz  carrier, leaving behind the 217 Hertz 
artifact (and/or harmonics of 217 Hz).  That audio-frequency artifact 
is amplified and passed along to the hearing aid's output.

So, a perfectly-designed PCS phone can induce unwanted artifacts in 
less-than-perfect hearing aids.  Hearing aid manufacturers are becoming 
increasingly aware of the problem, and I expect that more hearing aids 
will be engineered to be immune to this type of problem.  I suspect,
though, that as of a couple of years ago, almost all hearing aids
were susceptible to the problem.

Hope this helps.  Note that I'm not speaking for Nortel.  Nortel 
happens to be one of the largest manufacturers of PCS phones, but 
that work is done by another division, and so I have no special 
inside knowledge, nor do I have any particular axe to grind, 
except that I've had trouble using PCS phones with my hearing aids.

Jeff

-- 
Jeff Joslin                              All opinions mine.



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