Fm advantage

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Fm advantage

• new procedures for setting up
• fm systems (MCHAS Guideline)

2
An audio demonstration

• Lets remember basics first
• listen to a hearing aid alone recorded through a
  coupler .... and then with an radio aid to
  improve the quality of sound
• Play demo
• Which sound would you prefer?

3
New procedure for setting up fm systems..

• Whats different?
• We are moving from an Equal output approach to
  anfm Advantage approach

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BALANCING traditional method of equal output

• Summary
• balance or match outputs using 65/75 dB
  inputs 65dB SPL into hearing aid matches 75dBSPL
  into Tx.
• Some maximised FM output keeping distortion
  less than 10.
• Some advised older children to adjust volume
  control according to circumstances but ensured
  that this was not causing high distortion.

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BALANCING - background

• Need to COMPROMISE- the FM output can be greater
  than target output if it is LESS than target
  maximum output the system is not in saturation
  (distortion less 10).
• In practice many children tweaked the radio
  receiver up a few dB with equal output approach
• ASHA (2000) recommend a 10dB FM advantage,
• Children able to manage their own amplification
  can adjust their own radio receiver volume as the
  situation demands.

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fmAdvantage why?

• We want children to have a better fm advantage in
  classrooms
• We can maximise the benefits of fm use by
  utilising the way the DSP hearing aids work
• We can then obtain a beneficial s/n ratio

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Design of DSP hearing aids
FM signal enters before any DSP, in parallel with
the microphone input
The loudest input will drive the compressor
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WDRC more gain for quiet sounds, less for loud
sounds

• 80dB into the Tx means low gain is applied
• Teacher is well audible, noise into the
  microphones is less audible.

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Gain Curves on FP35
Linear or Non-linear?
Linear or Non-linear?
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
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DSP WDRC aid listening
Relative levels
Repeat demo
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General assumptions

• The hearing aid(s) have been separately assessed
  as working normally
• The hearing aid(s) are recognised as being
  correctly fitted for the user
• The fm Advantage setting up procedures are
  carried out with the hearing aid left at the
  normal user settings

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Before you start

• Understand the different programmes and ask which
  hearing aid programme is intended for fm use?
• Listen to the whole system (FM hearing aid)
  before you carry out the fm advantage procedures
• Set the test box is to display output
• Use a lapel microphone for the procedure

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Stetoclip with variable attenuator
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fm Advantage new procedures

• Digital or analogue ?
• Whether the hearing aid is digital or analogue is
  not important, the relevant factor is whether the
  hearing aid is operating in linear or non linear
  mode

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The fmAdvantage approach for WDRC (nonlinear) aids

• 65dB input level for the hearing aid
• 65dB also for the fm system
• 80dB SPL the reality part of the test, will we
  get an fm advantage?
• By balancing with a 10dB lower level into the
  transmitter than traditionally the receiver
  output is set 10dB higher

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Linear test levels

• 65dB SPL for the hearing aid
• 70dB SPL for the fm radio system
• 80dB SPL the reality part of the test
• Here, we are increasing the receiver volume
  by 5dB more than in the old procedure.70dB is
  chosen to minimise the risk of distortion in a
  linear aid

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Hearing aid in test box
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Curve 1 hearing aid alone
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Hearing aid radio system
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At this point in the procedure..

• Always adjust the radio receiver volume upwards,
  from a low position.
• Find the lowest volume setting where the match is
  best.

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Curves 1 2
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Curves 1, 2 3
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Ear level receivers

• It is quite often the case that transparency
  with the HA performance will not be achieved when
  the MLx is coupled.
• The FM advantage with MLx will vary between
  hearing aids (as it will with conventional fm
  systems), and each aid to be used with MLx should
  therefore be individually assessed before
  fitting.

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Ear level receivers

• The programmable MLx-S wireless radio receiver
  can be adjusted via a toaster and LEXIS, which
  is not multi frequency, is adjustable via a screw
  pot.
• Some DSP aids will only work in FMM mode,
  regardless of how the switch is set on the MLx or
  Mlx-S There is quite often a better performance (
  approx 5dB) on the FMM setting.

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fm issues

• Comparison with ASHA 2000 guidelines
• (were more in line but not identical)
• Distortion
• Listen to the system measuring it just produces
  artefacts
• Noise reduction feature
• Not a problem with a DSP stimulus, but you can
  still do the procedure with a pure tone sweep or
  composite signal wait for the FRC to settle
• Interference from DSP hearing aids see MCHAS
  website

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Interference

• DSP Hearing aids, personal FM systems and
  interference is there a problem?
• J Bamford, M. Hostler, G. Pont
• Ear Hearing (2004) Vol 26 (3) pp341-349
• Summary As with some CI processors, DSP hearing
  aids can cause interference to personal fm
  systems. On the present NHS contract, some aids
  are clean others have quite high levels of
  interference. We are in dialogue with NHS
  supplies manufacturers on this issue.

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Fm issues work in progress

• Quality standards (candidacy, fitting, evaluation
  etc)
• FM CI
• Evaluation of design integrated fm receivers
• Further development of procedures
• NHS contract requirements advising about
  programming, DAI shoes, etc