Re: Why does a digital mobile phone signal affect digital hearing-aid users ?

Hi George.

That's a really good question. There are a number of mobile telephone standards currently in use throughout the world. I am not absolutely certain which types are available in Australia, but I do know something about the systems used in North America, Europe, and the far East. I suspect that similar systems are used in Sydney.

Many of the mobile phone standards originated in the US. The earliest was an all analog system called "Improved Mobile Telephone Service" or IMTS. IMTS was developed in 1969 and was not suitable for large scale use. I doubt that IMTS was ever used outside the US, it was a very limited and costly system. The successor to IMTS was "Advanced Mobile Phone System" or AMPS. AMPS was developed in 1979 and originated the concept of "cells" or distributed base stations that could hand off calls as handsets moved through a network. AMPS has been used all over the world with reasonable success and is currently the most widely supported system in the US. The analog system you speak of in Sydney is most likely AMPS.

AMPS was a good idea, but it suffered from poor voice quality and poor bandwidth utilization. A digital version of AMPS, known as D-AMPS, was introduced to address some of the shortcomings of AMPS. D-AMPS is also heavily used throughout the US, but I doubt that it is used elsewhere. The most widely used digital standard worldwide is "Global System for Mobile communication" or GSM. GSM is popular because it is an all digital system yielding better bandwidth usage and voice quality when compared to D-AMPS. GSM handsets require less power than other types, resulting in longer battery life. In almost all aspects, GSM is just a better system. The digital system used in Sydney is most likely GSM. In the US, it is has many names: PCS, PCS 1900, as well as GSM.

So why does a digital mobile phone cause a loud squelch in hearing aides? It has to do with the modulation scheme used by GSM. GSM uses a combination of Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA). When you make a call with a GSM phone, you are allocated a frequency and a time slot. The frequency generally changes only when you move from cell to cell, so if you aren't driving, you will use one frequency. Voice and data is transmitted in a series of frames. Each frame is about 4.615 ms long, so it repeats approximately 217 times a second. Each call is allocated one time slot within a frame and lasts about 0.577 ms. That annoying noise you hear in a hearing aid is the burst of each frame as it is transmitted. A standard telephone or even an AM radio near a GSM phone will exhibit the same phenomenon.

AMPS (analog) mobile phones use a simple frequency modulation scheme that transmits continuously. Data is not sent in bursts, so there is no annoying interference. The interference from GSM phones is caused by near field magnetic radiation. Unfortunately, this type of radiation is difficult to shield. It's not impossible, but I am not aware of any GSM phone that is hearing aid friendly. Phone manufacturers could solve the problem, but it would significantly increase cost. Your best bet might be to try one of those hands free sets. This should minimize, if not completely eliminate the problem. The nice thing about near field magnetic radiation is that it drops off as the cube of the distance. This means that if you move the phone even a small distance away from the hearing aid, it shouldn't cause as much interference. Many mobile phone owners have started using hands free sets due to health concerns, so they should be easy to find.

I hope this helps. I have included a couple of links that explain the inner workings of some of the mobile phone systems. If you have any further questions, don't hesitate to send me an email at madhu@madhu.com.

Radio Telephony
Overview of GSM