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An Informational presentation for the audiological professional from Connevans Limited

An Informational presentation for the audiological professional from Connevans Limited. Digital hearing aids can cause ‘funny noises’ when used with personal fm systems. A question for you. Do you have more reported occurrences of noise interference from fm users using . A – DSP hearing aids

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An Informational presentation for the audiological professional from Connevans Limited

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  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 Do you have more reported occurrences of noise interference from fm users using A – DSP hearing aids B –Analogue hearing aids C – Cochlear implants

  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. FM Receiver Electromagnetic interference • We have identified the cause to be ElectroMagnetic Interference (EMI) generated by DSP hearing aids • As the d/i lead is also the aerial, any radio interference from a hearing aid travels directly into the receiver

  7. EMI testing of DSP aids The tests were carried out in a RF (Radio Frequency) screened chamber • The hearing aid was placed in the test chamber with the direct input shoe and lead connected

  8. Diagram of Test setup

  9. Background noise from test system up to 10MHz, as displayed by the spectrum analyser with the hearing aid switched off

  10. Harmonic structure of DSP aid interference up to 10MHz Note the DSP clock at 991KHz and its many harmonics

  11. Wideband harmonic structure of DSP aid interference up to 500MHz using 400mm d/i lead fm systems work here USA UK USA Note the peak at 120MHz near fm radio aid systems The peak moves to lower frequencies as the d/i lead length increases

  12. Zoomed in structure of interference near 174MHz (UK fm systems) – animated sequence 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. e.g. channel 51 receiver listens here

  13. 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 72nd harmonic (72MHz-USA fm) any frequency drift is 72 times larger • At the 174th 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. Zoomed in structure of interference near 174MHz (UK fm systems) – animated sequence 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. e.g. channel 51 receiver listens here

  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 yes

  18. Thank you for your time

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