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Digital Cell Phones

dybala at utdallas.edu dybala at utdallas.edu
Sat Nov 16 22:21:28 EST 1996

A recent article came out in the Hearing Journal by
Ross Roeser on this topic.  I helped with the
data collection.

Paul Dybala
Jeff Joslin (jjoslin at nortel.ca) wrote:
> 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.
Thank you for your support,
Paul Dybala
dybala at utdallas.edu

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