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Digital Signal Processing hearing aids

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Title: Digital Signal Processing hearing aids


1
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/

2
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!

3
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?
4
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
5
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!

6
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
7
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
8
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
9
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

10
Digital hearing aids
  • Advantages current and potential
  • Miniaturisation
  • Power consumption
  • Reproducibility
  • Stability
  • Programmability
  • Complexity
  • etc.

11
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

12
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.

13
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

14
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.

15
. 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

16
. 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)

17
Multi-channel hearing aids
18
. 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


19
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


20
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


21
How does a hearing carer do a listening test with
no control?
Always use an attenuator when listening with a
stetoclip for daily testing

22
So how is a digital hearing aid programmed?
  • Take a computer


23
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


24
Add an interface
  • Connection to the computer is via a Hi-pro
    universal interface
  • Hearing-instrument programmer


25
Connecting leads
  • Some hearing aids are connected using a lead and
    special shoe


26
. Connecting leads .
  • Others have a miniature connector hidden under a
    cover plate


27
So we have .
  • Computer to Hi-pro to hearing aid


28
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


29
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

30
In-situ REAR
31
In-situ REAR
32
In-situ REAR
33
Predicted ULLs
80 dB
65 dB
50 dB
Hearing Threshold dB SPL.
34
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


35
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

36
Digital Feedback Suppression
What has been chosen for programme 1?
Will the volume control work?

37
How has the volume control been programmed?

38
Programme 2 selections . plus . what noise
cancellation is selected?

39
Choose FMM for start up program for little
ones. Older children may want to switch to FM
only for lecture style classes.

40
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


41
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.

42
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


43
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


44
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.

45
Key message from MCHAS experience
The need for good communication is
ESSENTIAL Between
Audiology
User
Education
Carer

46
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.

47
West Sussex Sensory Support Team A.P 2003
48
Here we go hold on!
West Sussex Sensory Support Team A.P 2003
49
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

50
Loudness growth functions
51
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

52
Automatic gain control
53
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

54
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

55
Wide dynamic range compression
Intense
Automatic
Non-linear
Non-linear
Moderate
Weak
Normal
56
Wide Dynamic Range Compression
57
Wide Dynamic Range Compression
Intelligibility benefit
Moore, Johnson, Clark Pluvinage, 1992
58
Nearly there, keep holding on.
West Sussex Sensory Support Team A.P 2003
59
Linear Aids
Same gain no matter how loud the sound coming in
to the hearing aid
40
40
40
West Sussex Sensory Support Team A.P 2003
60
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
61
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
62
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
63
Hmmm, so what does compression give us?
West Sussex Sensory Support Team A.P 2003
64
  • High gain for low intensities
  • And
  • Low gain for high intensities

65
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
66
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!

67
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)

68
Parents/ToDs role cont
  • Getting used to new switches etc may need
    practice, and new models will be a feature of
    ongoing development

69
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.

70
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

71
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

72
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.

73
Maximising the technology requires good
communication
  • Good explanations
  • Sensitivity
  • Inclusive
  • Honest and open

74
  • DSP hearing aids can then give real, not
    just potential, advantages and benefits.
  • ANY QUESTIONS?

75
Thank you!
Time for lunch?
Thanks to Avril Paylor E. Sussex Sensory
Support Service. Harvey Dillon, NAL, David Evans
(Connevans) Tom Hostler.
76
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

77
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
78
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

79
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

80
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/

81
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

82
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
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