Interference Car Speakers – Stories Behind The Buzz

Overview Digital Mobile Network 

THE issue of interference on the GSM Digital Mobile Network is a contentious one that needs to be put into perspective. The service vendors would have us believe that digital mobile phones cause little or no interference. Others imagine that the entire world will blow up if you activate your digital mobile. In reality, the interference caused by a digital mobile is on a par with that generated by, say, your electric power drill.
car speaker

As a user of the digital mobile network for the past couple of months, I have noticed minor interference in certain circumstances. In most circumstances it is simply not an issue. In other specific cases it may be of some concern. As a qualified communications and electronics engineer who has worked on digitally controlled radio systems as long ago as 1979, before analogue and digital mobile existed, as well as on satellite systems, I know a bit about how radio frequency electromagnetic radiation propagates.

Here are some of the effects I have noticed as a user: A buzzing in the ear a couple of seconds before the digital mobile rings when you’re on the POTS (Plain Old Telephone System) phone. It’s so obvious, you either have to stop talking on the phone or switch the mobile off. I never had that with the AMPS phone. My wife tells me to go away when I walk into the kitchen with my mobile on air, and she’s talking on the phone (the kitchen phone is a Telecom 200, wall- mounted).

A faint buzz from the car radio speaker, just before a « briiinngg » from the digital mobile. I have learnt that I must turn the volume down on the radio when I hear the buzzing noise (a manual mute) as the mobile is about to ring. The computer monitor screen shimmers and I get pretty colors everywhere if I place the digital mobile within a few inches of it. Sitting it directly on top of my NetComm Trailblazer V32 results in the familiar buzzing noise from my modem, too.

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The line drops immediately a call comes in on the mobile. These are artificial circumstances. I wouldn’t normally leave my mobile sitting in these places as it’s usually on the charger. I did it only to see what would happen. Other tests for interference to our domestic life support systems such as the VCR, electronic washing machine, dishwasher and (most importantly) the TV, indicated they were all immune.

The OPTUS cardboard box containing my new GSM digital phone was sealed with stickers warning of interference that could be caused to externally worn medical equipment. It is this issue of interference to medical equipment that is of most concern, and has been recognised as such by organisations such as the Monash Medical Centre, Telecom, and the Therapeutic Goods Administration (TGA – Australia’s equivalent of the US Food and Drug Administration). In January this year Telecom Research Laboratories and the Monash Medical Centre published a report that established that, when used within a distance of two metres, both AMPS and GSM digital mobile phones interfere with certain types of electronic medical devices, such as drug infusion systems, infant incubators and ventilators.

It found that « the new digital GSM phones are of greater concern than the older AMPS models ». As a result of the report, the TGA has advised hospital administrators to restrict the use of mobile phones in hospitals. Fair enough, considering the potential risks to patient safety (there is one reported incident of a death in Japan). So, what is that buzzing noise my car speaker and other equipment detects so well? It’s the transmitter being turned on and off as packets of data are sent to the digital mobile network.

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Essentially, part of your electronic equipment has the side effect of behaving like an antenna. This means it receives some of the radio frequency power transmitted by the mobile. Some of this power is converted into electrical currents, which in turn behave like AM radio. Once the spurious signal is demodulated (decoded as a buzz) it is then amplified by whatever amplifiers are in the equipment and out the speaker it goes. Monitors have amplifiers in them which control the horizontal synch, hence the shimmer.

Hearing aids have amplifiers, too. A simple fix for hearing aid users is to get the Ericsson digital mobile with the headset option. Put the headset on and place the phone on your desk. Just a few centimetres usually makes all the difference. In some cases, where logic circuits are involved (computers and micro- processor-controlled equipment) zeros can become ones due to the interference and in so doing can cause a failure. The consequences of the failure depend on your circumstances, hence the concern in the medical arena. Using my trusty interference detector (cheap car radio), it is possible to detect what the GSM mobile does under different conditions.

Conclusion

For example, if the Power Save feature is disabled, the phone will transmit more or less continuously, searching for a base station if it can’t hear one. This essentially means the battery will go flat faster when the phone is out of range. With the Power Save enabled, the phone seems to only search periodically, hence using less power. Also, when a conversation is under way, the AM radio only buzzes when speech is going into the microphone. This implies that the phone is doing silence compression, also saving power.

I’ve also noticed that switching the mobile off blanks the display and makes the phone look off. But a few little buzzes are still emitted by the 977D – and detected by my car radio – just after it has been switched off. This probably means the digital mobile is telling the network it’s gone and don’t call me anymore. So don’t pull your batteries straight out like you did with the AMPS phone.

Switch it off, give it a couple of seconds to communicate its status to the network, and then pull the batteries out. The conclusion is that GSM digital mobiles do cause a degree of interference, but this is a minor concern for the vast majority of users, on a par with the potential chemical hazard of the battery. I can’t live without mine. Phillip Bertolus is the principal of a Melbourne-based software company.