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How to prevent iPhone audio interference in speakers
Authored by: westont on Jan 07, '08 12:45:37PM

This hint highlights an aspect of GSM-based cell-phones that is experienced by thousand of users each day. While the steps taken here may have alleviated the problem, they may also (as a previous post indicated) have reduced the effectiveness of the receiver in the phone.

There is almost certainly nothing wrong per-se with the phone or the car audio system. It is just an unfortunate fact that the type of bursty modulation used in GSM phones (Time Division Multiplexed GMSK/8PSK) results in an amplitude modulated component in the audible range (around 200Hz) which then has a propensity to be demodulated and amplified by susceptible audio equipment.

You may also find that experimenting with the location of the phone relative to the car audio system may help considerably, Naturally though, your mileage may vary as it may not be possible to eliminate the problem entirely by doing so.

Unless specific steps have been taken with the circuit design, most audio equipment will exhibit the problem to a greater or lesser degree - the quality of the shielding and the design just dictates how physically close the phone has to be before the problem is heard.

Interestingly, this is an inherent design problem for the audio circuitry of the GSM phone that needs careful attention to ensure adequate interference rejection with minimal impairment to voice quality!

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