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An Informational An Informational presentation for the presentation for the

audiological professional audiological professional from Connevans Limitedfrom Connevans Limited

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Digital hearing aids can cause Digital hearing aids can cause ‘funny noises’ when used with ‘funny noises’ when used with

personal fm systemspersonal fm systems

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A question for youA question for you

Do you have more reported occurrences of noise interference from fm users using

A – DSP hearing aidsB – Analogue hearing aidsC – Cochlear implants

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Let us predict your answerLet 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 ?

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Electromagnetic interferenceElectromagnetic 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

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Electromagnetic interferenceElectromagnetic 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

FM Receiver

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EMI testing of DSP aidsEMI testing of DSP aidsThe 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

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Diagram of Diagram of Test setupTest setup

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Background noise from test system up to 10MHz, as displayed by the spectrum analyser with the

hearing aid switched off

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Harmonic structure of DSP aid interference up to 10MHz

Note the DSP clock at 991KHz and its many harmonics

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Wideband harmonic structure of DSP aid interference up to 500MHz using 400mm d/i lead

Note the peak at 120MHz near fm radio aid systemsThe peak moves to lower frequencies as the d/i lead length increases

USA UK USAfm systems work here

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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.

Zoomed in structure of interference near 174MHz (UK fm systems) – animated sequence

e.g. channel 51 receiver listens here

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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 …

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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.

Zoomed in structure of interference near 174MHz (UK fm systems) – animated sequence

e.g. channel 51 receiver listens here

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Objectives of this Objectives of this informational presentationinformational presentation To raise awareness of the issue To encourage hearing aid

manufacturers to improve their designs

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SolutionsSolutions 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

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One last questionOne last question

Are FM systems worth the effort?

A positive yes

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Thank you for your time