Electromagnetic interference

1
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

2
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

3
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

4
Diagram of Test setup
5
Background noise from test system up to 10MHz, as
displayed by the spectrum analyser with the
hearing aid switched off
6
Harmonic structure of DSP aid interference up to
10MHz
Note the DSP clock at 991KHz and its many
harmonics
7
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 approx 120MHz near fm
systems The peak moves to lower frequencies as
the d/i lead length increases
8
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.

9

• 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

10
Objectives

• To raise awareness of the issue
• To encourage hearing aid manufacturers to improve
  their designs

11
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