Digital Signal Processing hearing aids

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Digital Signal Processing hearing aids

• GALWAY ToDs Course
• May 24th 2006
• Mary Hostler
• Modernising Childrens Hearing Aid Services
  (MCHAS) team,
• University of Manchester
• www.manchester.ac.uk/mchas/

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What will be covered today..

• Basic introduction to DSP hearing aids
• Outline of the hearing aid fitting process ToDs
  need to help parents understand the new
  procedures..
• selection fitting
• verification
• day to day management
• evaluation
• Introduction to compression how DSP hearing aids
  process sound
• We want children to get the most out of the new
  technology, so we need to know a bit about it!

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Some common misconceptions about digital hearing
aids
Digital is better than analogue
They are smaller. All in the ear, I think.
No more feedback..
Digital hearing aids cut out background noise
dont they?
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What size are DSP aids?
DSP aids look just like analogue aids. They can
come in a range of different forms Behind-the-ear
(BTE) In- the- Ear (ITE) In-the-Canal
(ITC) Completely-in-the-Canal (CIC)
West Sussex Sensory Support Team A.P 2003
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Digital hearing aids egGN Resound Danalogic

• Most children will continue to be fitted with
  BTE hearing aids, and they can look just the same
  as analogue hearing aids!

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Do they cut out background noise?
Many DSP aids are designed to reduce steady kinds
of background noise such as the fan on this
projector. This makes listening more comfortable.
West Sussex Sensory Support Team A.P 2003
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Analogue aids pick up sound and convert it to
small electrical signals. The signals are then
amplified.
What is the difference between analogue DSP?
DSP aids convert the signal from the microphone
into bits of data - numbers that can be
manipulated by the tiny computer in the hearing
aid
West Sussex Sensory Support Team A.P 2003
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What is a digital hearing aid?
A digital hearing aid simply converts the signal
to a numerical form before processing it, then
converts it back to an analogue signal
Software
Filter
A/D
DSP
D/A
Filter
010100 101101
001100 110101
Acoustic Input signal
Acoustic output signal
Electrical Input signal
Processed electrical signal
Digitised signal
Processed digital signal
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Digital hearing aids cont..

• Theres nothing magical about a digital hearing
  aid
• Its the signal processing algorithm that is
  important, i.e. how is the hearing aid set to
  process the sound
• A badly fitted DSP aid would give less benefit to
  a child than a well fitted analogue aid
• Digital technology allows for more sophisticated
  sound processing

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Digital hearing aids

• Advantages current and potential
• Miniaturisation
• Power consumption
• Reproducibility
• Stability
• Programmability
• Complexity
• etc.

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Hearing aid goals

• To put conversational level speech spectrum
  (LTASS) into the middle of the available dynamic
  range
• Avoid discomfort - dont let amplified signals
  become too loud
• The same aims apply to all hearing aids, both
  linear and nonlinear, analogue and DSP aids

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Childrens hearing aids in UK

• Still some good quality analogue linear aids with
  peak clipping or compression limiting (e.g.
  bodyworn)
• Some analogue nonlinear aids (digitally
  programmable)
• Most fittings now should be NHS contract DSP
  aids, fit nonlinearly, though there are still
  some linear fittings.

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How does a digital hearing aid work?

• A digital hearing aid has a computer inside to
  control it

• The computer memory stores settings for its user

• The computer program uses the stored settings to
  tailor the hearing aid sound to suit the user

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What happens?

• The computer program monitors the sound through
  the hearing aid and instantly adjusts the way
  that the hearing aid amplifies the sound

• The objective is, as usual, to adjust the sound
  to be within the users window of hearing or
  dynamic range.

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. what happens .
Different digital hearing aids work in different
ways, but usually...

• The speech spectrum is split into frequency bands
  or slices

• Computer technology allows for precise matching
  of each slice for the specific hearing loss

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. what happens .

• Each slice may be programmed differently e.g.
  more gain where there is more hearing loss (shape
  of amplification)
• Soft, medium loud sounds can be treated
  differently (adjust gain given to incoming
  sounds)
• A maximum output limit can be set (control
  output)

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Multi-channel hearing aids
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. what happens .

• Quieter sounds can be amplified more than loud
  sounds to make them audible Wide Dynamic
  Range Compression
• Louder sounds can be limited so that they are
  kept comfortable
• Different profiles can be stored for different
  situations
• Speech, music, high noise, fm etc.
• Extra algorithms may be programmed in e.g.
  feedback control, noise reduction, speech
  enhancement
• A second microphone may be activated to help hear
  better in noise

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What about user control?

• A choice of programs to suit different situations
• Usually chosen by a switch or button on the aid

• Sometimes there is a remote control device

• Sometimes all programs are set the same for
  younger people

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What about volume control?

• Sometimes there is no volume control wheel on the
  hearing aid(!)
• If there is a volume control it may not have been
  activated
• Some volume controls operate with a limited up a
  bit or down a bit adjustment

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How does a hearing carer do a listening test with
no control?
Always use an attenuator when listening with a
stetoclip for daily testing

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So how is a digital hearing aid programmed?

• Take a computer

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Add some software
The industry standard digital hearing aid
software is called NOAH after all, we are all
in the same boat!

• Plus you will require an add-on module for your
  specific hearing aid/manufacturer

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Add an interface

• Connection to the computer is via a Hi-pro
  universal interface
• Hearing-instrument programmer

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Connecting leads

• Some hearing aids are connected using a lead and
  special shoe

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. Connecting leads .

• Others have a miniature connector hidden under a
  cover plate

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So we have .

• Computer to Hi-pro to hearing aid

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Fitting process1. selection and fitting

• The audiologist needs to
• tell the computer software details of the hearing
  loss
• use software (DSLi/o or NAL-NL1) to generate
  targets (prescriptive method)
• use their expertise
• To select an appropriate hearing aid
• to measure individual ear variations (RECD)
• to tweak the hearing aid to suit the user

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Fitting process2. verification

• The audiologist needs to
• verify that soft, medium, loud v loud sounds
  are both audible comfortable- using probe tube
  microphone measures
• These are called Real Ear Measurements (REMs).
• Real Ear Aided Response (REAR)
• OR
• Real Ear to Coupler Difference (RECD) and an
  ordinary test box measure may be used

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In-situ REAR
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In-situ REAR
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In-situ REAR
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Predicted ULLs
80 dB
65 dB
50 dB
Hearing Threshold dB SPL.
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What happens next?
The audiologist can activate different options
within the hearing aid

• Directional microphones
• Feedback control
• Noise reduction
• Multi-memory to cater for a range of listening
  situations or simply to select fm

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What does the audiologist see when programming ?
Some example computer screen displays follow .
But do not worry too much about specific details
as they vary from hearing aid to hearing aid

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Digital Feedback Suppression
What has been chosen for programme 1?
Will the volume control work?

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How has the volume control been programmed?

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Programme 2 selections . plus . what noise
cancellation is selected?

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Choose FMM for start up program for little
ones. Older children may want to switch to FM
only for lecture style classes.

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Well what does this all mean?

• Mind boggling options
• The ability to closely programme a hearing aid to
  suit an individual user
• No chance of knowing what to expect unless you
  are told how an aid has been programmed
• Access to features/processing not previously
  common on hearing aids

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Fitting process3. Evaluation

• Some speech tests or other aided evaluations may
  take place within the clinic.
• REAL WORLD evaluation is also crucial, and there
  is least concensus about how this is best
  achieved
• A multi-method approach is recommended that
  should include QUESTIONNAIRES to parents, carers,
  ToDs and children themselves where possible.

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Return visits to the clinic for fine tuning may
be required, and the hearing aid fitting must be
reviewed regularly

• We are all individuals and fittings can often be
  improved as a result of real life evaluation by
  users, parents teachers.
• The possibility of human error with using new
  technology
• Babies and childrens ears grow and the fitting
  must be adjusted accordingly

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What can be achieved by switching to DSP aids?

• In some cases, noticeably improved speech
  discrimination.
• In many cases noticeably improved listening
  experiences
• More hearing aids left turned on!
• but it takes time to explain the operation,
  understand the possibilities and get used to
  differently processed sound

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Role of ToDs

• Understand how each childs hearing aids are set.
  (LIAISE WITH AUDIOLOGISTS!!)
• Help child, family and school to get the best
  from the hearing aids
• Help to keep hearing aids functioning (regular
  testing)
• Help in real world evaluation of hearing aids
  monitor development of listening skills and
  speech/language progress using observations,
  checklists, tests, questionnaires.
• feed information back to inform hearing aid
  review.

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Key message from MCHAS experience
The need for good communication is
ESSENTIAL Between
Audiology
User
Education
Carer

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More on compression

• There are many different types of compression and
  different terminology used. Two of the most
  common types are
• Compression limiting high knee-point, high
  compression ratio (e.g. 101) limits Maximum
  Power Output.
• Wide Dynamic Range Compression (WDRC) low
  knee-point, low compression ratio (e.g. 21)
  aims to restore loudness perception.

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West Sussex Sensory Support Team A.P 2003
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Here we go hold on!
West Sussex Sensory Support Team A.P 2003
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Loudness Growth

• Typically, sensorineural hearing loss results in
  recruitment
• Low intensity sounds are inaudible
• Moderate intensity sounds are heard as very quiet
• High intensity sounds are perceived as similar in
  loudness to that normal hearing listener

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Loudness growth functions
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Automatic gain control (AGC)

• An AGC automatically reduces the gain of the
  amplifier as the signal intensity increases
• This change in gain takes a finite time to turn
  on and off
• attack-time The time taken for the AGC to
  respond to an increase in input level
• release time the time taken for the AGC to
  increase the gain again when the input level
  decreases

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Automatic gain control
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Types of compression..cont

• Syllabic compression fast attack time (lt 5ms),
  fast release time (lt 30ms) aim is to boost gain
  for less intense phonemes in speech relative to
  the more intense ones
• AVC automatic volume control slow-acting
  compression reduces the need for a users volume
  control

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AGC - parameters

• As well as attack and release time the AGC has
  other parameters that can often be adjusted in
  hearing aids
• Knee-point
• below a certain signal intensity the amplifier
  behaves linearly, with a 11 compression ratio
• Above this intensity the AGC operates
• Compression ratio
• Above the knee-point the rate of change in output
  with an increase in input is typically less than
  1 dB per dB and is quantified by the compression
  ratio. e.g. if an increase in the input of 10 dB
  results in an increase in the output of 5 dB the
  compression ratio is 21 or the slope of the I/O
  function is 0.5

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Wide dynamic range compression
Intense
Automatic
Non-linear
Non-linear
Moderate
Weak
Normal
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Wide Dynamic Range Compression
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Wide Dynamic Range Compression
Intelligibility benefit
Moore, Johnson, Clark Pluvinage, 1992
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Nearly there, keep holding on.
West Sussex Sensory Support Team A.P 2003
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Linear Aids
Same gain no matter how loud the sound coming in
to the hearing aid
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40
40
West Sussex Sensory Support Team A.P 2003
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Linear Aids
But it might be a bit loud, what do they do?
Just cut off the top..peak clipping or squash
the top (output limiting compression)!
Uncomfortable listening level
West Sussex Sensory Support Team A.P 2003
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Non - Linear Aids
Change in the gain as the input level is increased
Knee point
10
20
30
40
West Sussex Sensory Support Team A.P 2003
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Non - Linear Aids
The sounds above a certain intensity (40 dBSPL)
are amplified by a different factor.
Hmm better, I dont lose my head!
Uncomfortable listening level
10
20
30
40
West Sussex Sensory Support Team A.P 2003
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Hmmm, so what does compression give us?
West Sussex Sensory Support Team A.P 2003
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• High gain for low intensities
• And
• Low gain for high intensities

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Hmm, you mean quiet sounds are made much louder
and loud sounds are made a little bit louder
West Sussex Sensory Support Team A.P 2003
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What can parents/ ToDs expect from modernised
hearing aid services?

• Longer fitting and review appointments
• A role in the evaluation of the hearing aids
  (questionnaire- type input)
• A sometimes tricky initial period of adjustment
  by their child to the new way that sound is
  processed it will sound overall to be quieter
  although speech should be the same, and quiet
  sounds louder!

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What to expectcont

• Information from the audiologist about different
  programs and how to help each child to use them
  (close liaison, joint training)
• Different way (better we think!!) to set up the
  FM radio systems which are STILL NEEDED!!!
• Different ways to test the aids ToDs should
  have new testboxes from the MCHAS project.
• Quicker and more flexible ear mould provision
• Involvement in service development (e.g. family
  friendly) via Childrens Hearing Services Working
  Groups (CHSWG)

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Parents/ToDs role cont

• Getting used to new switches etc may need
  practice, and new models will be a feature of
  ongoing development

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Parents/ToDs role

• Earmoulds will still need checking and cleaning
  daily and changing frequently as the child
  grows.
• Parents or ToDs may receive earmoulds directly
  from the manufacturer once they are confident in
  fitting them, to speed up the process.

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Parents/ ToDs role

• Keeping the hearing aids working day in day out
  is still going to be an uphill task!!
• Keep in mind the basics of good use get in
  close to the microphone, reduce background noise
  as much as possible
• Get to know the hearing aid features and be alert
  to when additional ones may help

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ToDs role

• Keep up to date with new technologies.
• Manufacturers are KEEN TO INVOLVE EDUCATION STAFF
  in training on new products forge links with
  them!! Oticon, GN ReSound, Siemens, Phonak
  (Starkey) all have websites and some great
  resources!!!
• Develop your skills in counselling LISTEN to
  parents and childrens concerns about their
  amplification provision

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ToDs role

• Getting a grip on the new technology, acquiring
  new skills, implementing new guidelines,
  procedures etc is important,
• BUT..
• What goes in to a hearing aid in terms of
  stimulating the child is just as crucial!
• AND ESPECIALLY
• Dont lose sight of the familys and the childs
  needs.

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Maximising the technology requires good
communication

• Good explanations
• Sensitivity
• Inclusive
• Honest and open

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• DSP hearing aids can then give real, not
  just potential, advantages and benefits.
• ANY QUESTIONS?

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Thank you!
Time for lunch?
Thanks to Avril Paylor E. Sussex Sensory
Support Service. Harvey Dillon, NAL, David Evans
(Connevans) Tom Hostler.
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NHS DSP hearing aids

• 2000 (MHAS-P studies)
• Starkey Gemini AV13MM
• Oticon Digifocus II
• AM Selectra
• GN Res. Danalogic 163D
• Widex P37
• Philips Spaceline D71 S40

• APRIL 2003added
• Oticon Spirit
• Phonak Aero 211,211AZ
• AM Select
• baby aid Starkey Gemini 312
• GN Res Danalogic 283D
• Oticon Spirit 700
• Phonak Supero 412
• Widex Senso P38
• AM Triano SP

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NHS DSP hearing aids

• NOVEMBER, 2003
• Starkey Strata
• Oticon Spirit ll Direct
• Siemens UK Prisma 2M
• Phonak Aero 211AZ
• Oticon Spirit ll Power
• Phonak Aero 311AZ
• GN ReSound Danalogic 283D
• Phonak Supero 413AZ
• Siemens UK Prisma 2 DSP

Baby aid
Moderate power
High power (13)
Super power
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Latest additions to NHS contract

• April/August 2005 Oticon Spirit 3, 3VC,3 power
  and Spirit 3 super power
• Dec 2005 Siemens Prisma 2K (baby) Prisma 2 pro
  and Prisma 2D SP
• Imminent Phonak ?? GN ReSound ??
• Developments?
• Better feedback cancellation, adaptive noise
  reduction and directionality

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These DSP aids offer

• nonlinear fitting algorithms and new
  capabilities via new software
• more accurate shaping to fit hearing loss more
  appropriately (multi channel)
• processing flexibility eg
• feedback management or cancellation
• noise suppression
• Speech enhancement
• multiple programmes
• Directional microphones

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MCHAS - General

• Government launched the Modernisation of Hearing
  Aid Services project on 18th January 2000
• Paediatric First Wave started to issue Digital
  Signal Processing (DSP) aids in 2001
  feasibility studies on small numbers
• Training all paediatric sites(England) finished
  2005
• www.manchester.ac.uk/mchas/

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Modernisation targets

• Childrens Hearing Services Working Group, with
  health, education, social services, parents, and
  voluntary sector involved, to consult on service
  development, monitor current service
  arrangements, agree on and advocate for continued
  service developments
• Insert earphone VRA on children with permanent
  hearing loss from 6 months of age.
• Verification of fitting using probe tube
  microphone measures (REMs)
• Earmould protocol implemented

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modernisation cont.

• Parents given copies of all letters and
  assessment results
• Individual written audiological management plan
  agreed between parents, audiology, and education
  services for each child with permanent hearing
  loss, updated at least annually
• Sharing of all necessary assessment information
  between health and education
• Use of patient management systems to monitor
  individual and aggregated data
• Use by education services of standardised
  outcomes measures