1	An Informational presentation for the audiological professional from Connevans Limited
2	Digital hearing aids can cause ‘funny noises’ when used with personal fm systems
3	A question for you
4	Let us predict your answer We guess that reported occurrences of fm noise interference from analogue hearing aids is rare We guess that with digital hearing aids and cochlear implant processors reported fm noise interference is more often … yes ?
5	Electromagnetic interference A number of digital hearing aid users have reported occasional loud noises when using their fm system Occurrences were unpredictable Mostly noticed when the transmitter was turned off The sound was a ‘whooshing’ noise
6	Electromagnetic interference We have identified the cause to be ElectroMagnetic Interference (EMI) generated by DSP hearing aids
7	EMI testing of DSP aids
8	Diagram of Test setup
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12	"The peaks are EMI noise..." The peaks are EMI noise from the hearing aid – they move with time and will eventually cross the fm receiver frequency, which is why noise is only sometimes heard.
13	"The fundamental processor clock frequency..." The fundamental processor clock frequency in the DSP aid running at around 1MHz is not stable The frequency drifts with time, battery level, temperature & processor activity At the 72^nd harmonic (72MHz-USA fm) any frequency drift is 72 times larger At the 174^th harmonic (174MHz-UK fm) any frequency drift is 174 times larger This means that the EMI drifts across all fm receiver frequencies at some time Let’s look again at the animation …
14	"The peaks are EMI noise..." The peaks are EMI noise from the hearing aid – they move with time and will eventually cross the fm receiver frequency, which is why noise is only sometimes heard.
15	Objectives of this informational presentation To raise awareness of the issue To encourage hearing aid manufacturers to improve their designs
16	Solutions Some DSP hearing aids produce much less interference than others so these will become preferred for use with fm systems Keep the fm transmitter on when the receiver is in use Use a transmitter microphone mute facility when sound is temporarily not needed Switch off receiver when not needed
17	One last question Are FM systems worth the effort? A positive
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