Compression in Hearing Aids
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1 Compression in Hearing Aids [Photo of Presenter] Presenter: Ted Venema PhD
2 From the desk of our Attorney The views expressed in presentations made at International Hearing Society (IHS) educational events are those of the speaker and not necessarily of IHS. Presentations at IHS events, or the presence of a speaker at an IHS event, does not constitute an endorsement of the speaker's views.
3 Agenda Here s what we ll learn about: Setting the Scene for Compression Evolution of Compression Dynamic Aspects of Compression Compression in Digital Hearing Aids Is Linear Processing So Bad? Q&A Total Time (if you re good) 10 min 30 min 10 min 20 min 10 min 10 min 90 min
4 Learning Objectives: Learning Objective 1 Describe the concepts of various compression types as they historically appeared in hearing aids Learning Objective 2 Compare the applications of compression types for mildmoderate versus severe-profound SNHL Learning Objective 3 Explain the clinical benefits of directional microphones and digital noise reduction
5 I. Setting the Scene for Compression Speech in Noise + Hearing Aids Why? = Problems in Noise
6 Hearing Aids must do two things: 1. Provide gain for the HL 2. Increase signal-to-noise ratio (SNR) There are three things to note here: 1. The Cochlea is a WDRC circuit 2. Compression is a Gain issue 3. Dmics & DNR address SNR This won t be covered here
7 The Cochlea is a WDRC Circuit
8 Fitting the Eye vs Fitting the Ear Intact Retina Damaged Hair Cells = Hair Cells of the Cochlea are the Retina of the Ear
9 Cross Section of Cochlea Spiral Ligament Helicotrema Temporal Bone Scala Vestibuli Scala Media Scala Tympani Basilar Membrane Spiral Ganglia Fig 1-2, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
10 Note: Basilar membrane Wide at apex Narrow at base From Yost, WA Fundamentals of Hearing (1994)
11 Inner vs Outer Hair Cells Tectorial Membrane IHC OHC OHC OHC Afferent fibers Efferent fibers Basilar Membrane Fig 1-3, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
12 It s all about hair cells Inner vs Outer Hair Cells Watercolor by Arnold Starr
13 Inner hair cells Outer hair cells Hair Cells: A Closer Look Picture from Australian Hearing Hear & Now, issue 4, 1998
14 From Yost, WA Fundamentals of Hearing (1994) Normal Outer Hair Cells
15 When Stereocilia Are Gone, Cells Often Gone From Yost, WA Fundamentals of Hearing (1994)
16 Relative sizes: Stereocilia Hair Cell Human Hair From M. Killion ppt
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20 Inner Hair Cells (IHCs) From Yost, WA Fundamentals of Hearing (1994)
21 Outer Hair Cells (OHCs) Picture from Australian Hearing Hear & Now, issue 4, 1998
22 Normal Inner & Outer Hair Cells Fig 1-7 Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
23 Damaged Hair Cells (mostly outer) Fig 1-8 Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
24 Picture from Canadian Hearing Society
25 Picture from Cdn Hearing Society
26 Here s a Passive Traveling Wave A wave without outer hair cells Apex Base Basilar Membrane Fig 1-4, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
27 Time: 400 msec i n f o r m a t i o n Model representation of basilar membrane movement in response to word information. Audiology 19: Length of BM: 36 mm
28 Outer Hair Cells Sharpen the Peak! They are the muscles of the cochlea They usually get damaged first Apex Base Lows Basilar Membrane Highs
29 Outer Hair Cells: The Active Cochlear Mechanism Note how embedded OHCs actually pull tectorial membrane down
30 Fig 1-6, Venema, T. Compression for Clinicians 2 nd edition Cengage 2006 It s the sharpening thing that s the main challenge This is why hearing aids for ears aren t like glasses for eyes Natural shape of fluid wave: 2 peaks from 2 tones close in Hz Hair cell damage results in: smaller rounded peaks Hearing aids make wave bigger: but cannot sharpen it
31 The Frequency Resolution is Compromised: Person cannot separate frequencies close together Trouble separating speech from background noise Solution is increase signal-to-noise ratio (SNR) Done by Dmics & Digital Noise Reduction
32 Recall, Hearing Aids must do two things: 1. Provide gain for the HL 2. Increase signal-to-noise ratio (SNR) Recall also: 2. Compression is a Gain issue 3. Dmics & DNR address SNR This won t be covered here
33 Compression: A Gain Issue What it can do to otherwise good people
34 II. Evolution of Compression
35 Good Old Linear Amplification 1. What is the gain here? 2. Just by looking at the graph, how do you know the gain is linear? 3. What will raising/lowering the MPO trimmer do on this graph? 4. What will raising/lowering the volume control do on this graph? Output Input/Output Function for 2000 Hz Input
36 Compression Amplification Output = input + gain Output I/O Function for 2000 Hz Compression circuits: different gain for different inputs Knee point: at 60 db inputs 80 Line at rising angle: compression outputs increase < inputs Input For inputs > 60 db SPL compression limits MPO Fig 5-1, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
37 Mic Rec Mic Rec Input versus Output Compression: Volume Control Effects Output (AGCo) Input (AGCi) Output Output VC = Max VC = Max VC = Min VC = Min Input Input Compression Compression VC Amp Amp VC
38 Input versus Output Compression: Volume Control Effects on Frequency Response Output (AGCo) Input (AGCi) db MPO db Gain Gain Gain Gain VC = Max VC = Min Gain Gain Gain Gain VC = Max VC = Min Frequency Frequency
39 Output Limiting vs Wide Dynamic Range Compression (shown as I/O Functions) Limiting WDRC Output Output : : Input Input Fig 5-7, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
40 Gain Gain Output Limiting vs WDRC: Displayed as Frequency Responses 40 db 60 db 40 db 60 db 80 db 80 db Frequency Frequency
41 Gain Differences with Different Compression Ratios 1:1 Input Output Gain * Linear Gain Output Limiting Compression 10:1 Input Output Gain * Output Limiting Compression 20:1 Input Output Gain * Wide Dynamic Range Compression 2:1 Input Output Gain * *Asterisks show knee-point Underlined values show input, output & gain past knee-point (compression)
42 Loudness Growth & Types of Compression Output Limiting WDRC Too Loud Too Loud Loud Loud Comfortable Comfortable Soft Soft Very Soft Very Soft Fig 5-8, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
43 Different Ways of Adjusting AGC (shown as I/O Functions) MPO Adjustment TK Adjustment Output Output control=max control=min control=max control=min Input Input Output limiting control: Affects Kneepoint & Output TK control: Affects Kneepoint & Gain Fig 5-5, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
44 Mic Rec Mic Rec Remember This? Output (AGCo) Input (AGCi) Output Output VC = Max VC = Max VC = Min VC = Min Input Input Compression Compression VC Amp Amp VC
45 Mic Rec Mic Rec Output WDRC was always found on Input Compression or AGCi Here s Why: Input (AGCi) Output Output (AGCo) VC = Max VC = Max VC = Min VC = Min Input Input Compression Compression Amp VC TK Amp VC VC placed here gives above effect Allows placement of TK Control here to give above effect
46 Output That s Why WDRC had to be on Input Compression (AGCi) AGC o Effects of VC Output AGC i Output Output Limiting Input Adjusting Compression Output WDRC Input Input Input
47 Output Limiting Compression and MPO Adjustment Output 120 OLC 10:1 Output MPO Adjustment Input Input
48 Wide Dynamic Range Compression and TK (gain for soft inputs) Adjustment Output 100 WDRC 2:1 Output TK Adjustment Input Input
49 MPO vs TK Adjustment Listening to the Differences MPO adjustment adjust while speaking loudly into hearing aid notice volume go up and down because it affects MPO TK adjustment adjust while speaking softly into hearing aid notice volume go up and down because it affects gain for soft inputs
50 WDRC: A Word of Caution People who were accustomed to linear hearing aids did not initially appreciate WDRC reported WDRC sounds too soft for speech Reason: WDRC has low knee point & low ratio provides maximum gain for soft inputs less gain for moderate inputs
51 Gain Gain Remember BILL & TILL? Two 1 st Types of WDRC BILL TILL Frequency Fig 5-10, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006 Frequency
52 BILL: Reduce upward spread of masking Improve speech understanding in background noise low-hz noise: bulls in china shop high-hz consonants: china tea cups compression focused on lows Intense Low-Hz traveling wave moves entire Basilar Membrane Intense High-Hz traveling wave moves Basilar Membrane only at base Basilar Membrane Displacement Basilar Membrane Envelopes
53 Upward Spread of Masking Intense Low-Hz traveling wave (TW) moves entire Basilar Membrane Note that the Low-Hz envelope covers a soft High-Hz TW envelope Basilar Membrane Envelopes Basilar Membrane Displacement
54 It doesn t work the other way around Soft Low-Hz TW moves mainly the apical Basilar Membrane Intense High-Hz traveling wave still moves Basilar Membrane only at base Basilar Membrane Envelopes Basilar Membrane Displacement
55 Gain TILL: Found in the K Amp TM Rationale: Most people have high-hz SNHL So, make the action of WDRC active for treble, not bass compression thus focused on highs TILL Frequency Also, for high intensity inputs, gain is most like REUR Makes hearing aid for loud inputs acoustically transparent
56 Gain TILL: Most Clients Have High-Hz SNHL So, make compression concentrate on high Hz s Make gain for loud inputs look like natural ear canal resonance Makes HA acoustically transparent TILL 2700 Hz Frequency
57 Multichannel Amplification with BILL & TILL Became a fitting flexibility issue Philosophies disappeared Gain Frequency Cross-over Low-Frequency Channel BILL High-Frequency Channel TILL Inputs 80 Frequency
58 Categorizing Input Compression, WDRC, BILL & TILL INPUT COMPRESSION WDRC BILL TILL
59 Summary: Applying Compression in Analog Hearing Aids Hz } } Input Compression WDRC TK Adjustment Output Compression OLC MPO Adjustment
60 Summary A Clinical Spectrum of Hearing Aids Linear Limiting WDRC Always associated with: MPO adjustment Associated with: MPO adjustment Associated with TK adjustment Fig 5-13, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
61 III. Dynamic Aspects of Compression Sudden changes to input intensity over time Attack Time Hearing aid responses to the changes Release Time Fig 5-14, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
62 1 st Types of Dynamic Compression Peak Detection fixed attack/release times Adaptive Compression fixed attack, variable release times Average Detection variable attack/release times
63 Advantage of Adjustable Attack/Release Times Threshold Level Ambient Sound level Gain Average Detector Peak Detector
64 Newer Types of Dynamic Compression Syllabic Compression short attack/release times shorter than the average syllable often used in combination with BILL Automatic volume Control long attack/release times like the sports telecasts meant to imitate time to change VC
65 Amplitude Twin Average Compression Detectors Slow detector Fast detector m s Input sound level Time
66 Interaction Between Static & Dynamic Compression ANSI specs: compression ratio obtained w/pure tone stops and starts with speech are different fast attack/release times reduce compression ratio In general, most distortion occurs with: fast attack/release times & high compression ratios Less distortion occurs with: fast attack/release times & low compression ratios
67 Today s Dynamic Compression Defaults Syllabic Compression used most commonly with WDRC most gain can be applied to soft consonants in words optimizes audibility/speech intelligibility not always most subjectively pleasing though longer release times reduces pumping perception Defaults today Syllabic compression for low frequency channels Average detection for high frequency channels
68 IV. Compression in Digital Hearing Aids INPUT Microphone Analog Signal Processing Amplifier Receiver OUTPUT Acoustic Electrical Electrical Acoustic Microphone Digital Signal Processing Receiver A/D DSP D/A Binary Fig 7-1, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
69 Level Amplitude Sampling & Quantization Sampling rate Sample Index Time Fig 7-2, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
70 Digital Hearing Aids & Compression You may recall this I/O Function (a conventional way of viewing compression) 1:1 Output 2:1 4:1 Input As compression ratio increases, gain decreases
71 Digital Hearing Aids & Compression Output You may also recall this I/O Function (compression to restore normal loudness growth) 4:1 2:1 1:1 Too Loud Loud Comfortable Soft Very Soft Input As compression ratio increases, gain increases
72 Loudness Growth & Compression Ratios Output TK control setting Left-most or lower kneepoint Right-most or higher kneepoint Compression ratios hinge from here Too Loud 4:1 2:1 Loud Comfortable 1:1 1:1 1:1 Soft Very Soft Input
73 Remember the Traveling Wave & OHC Action?
74 Cochlea and OHCs Are a WDRC Amplifier!
75 Different Ways of Adjusting Compression Left-most kneepoint Right-most kneepoint 1:1 Output 2:1 4:1 Output 4:1 2:1 1:1 Input As compression ratio increases, gain decreases Input As compression ratio increases, gain increases Fig 7-6, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
76 Two Kneepoints, with Linear, WDRC, Output Limiting TK 2 Output TK 1 Expansion Input Fig 7-7, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
77 Adjustable Kneepoints Vertically & Horizontally Output Input
78 Output Raise Left TK Vertically... MPO 10:1 5:1 2:1 TK1 Input TK2 Increases compression ratio Increases gain for soft/mid-level inputs Fig 7-8, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
79 Move Left TK to the Left... Output MPO 10:1 5:1 2:1 TK1 Input TK2 Does nothing to compression ratio Increases gain especially for soft inputs
80 Raise Right TK Vertically... Output 2:1 MPO 10:1 10:1 10:1 10:1 5:1 TK1 Input TK2 Compression ratio decreases Gain increases for intense input levels Fig 7-9, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
81 Raise Right TK Vertically... Output 2:1 MPO 10:1 10:1 10:1 10:1 5:1 TK1 Input TK2 Compression ratio decreases Gain increases for intense input levels Fig 7-9, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
82 A Multi-Kneepoint Input/Output Function Output Output limiting compression WDRC Linear gain Expansion Input
83 You ve Seen Compression...
84 Try Expansion!!
85 Expansion Output 60 input = 90 output (gain = 30) 50 input = 85 output (gain = 35) 40 input = 80 output (gain = 40) 30 input = 60 output (gain = 30) 20 input = 40 output (gain = 20) 10 input = 20 output (gain = 10) 0 input = 0 output (gain = 0) With WDRC, expansion means greatest gain at (and only at) the kneepoint Input A greater-than 1:1 compression ratio Fig 7-11, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
86 Expansion Output Input/Output Function Gain Input/Gain Function WDRC Input Input Gain is reduced for inputs below 40 db Fig 7-12, Venema, T. Compression for Clinicians 2 nd edition, Cengage 2006
87 Benefits of Expansion Reduces microphone noise in quiet Addresses noise complaint of K Amp, WDRC, etc. no extra drain from battery
88 V. Is Linear Processing SO Bad? No! as long as it isn t saturating or peak clipping actually, linear processing sounds quite clean and crisp Compression requires: knee-points, ratios, attack/release times, etc. all these together in separate channels cause distortion too How about using linear processing as much as possible? ie; not just below some knee-point or b/w 2 knee-points but always, unless output is inaudible or too loud
89 ADRO Adaptive Dynamic Range Optimization TM A digital algorithm: started w/cochlear implants; now in some hearing aids Typical compression: gain adjusted with fixed knee-points & ratios ADRO: statistical distribution of aided output samples; fixed rules are applied Audibility criterion: up to 30% aided outputs can be < some predetermined level Comfort criterion: up to 90% aided outputs must be < some predetermined level
90 ADRO Gain is essentially linear until criteria are violated Output Typical WDRC Linear gain, knee-point, 2:1 compression ratio Once adjusted, they are fixed W/input changes, gain only follows function 70 Output Inpu t Inpu t Comfort boundary Audibility boundary ADRO Linear gain unless criteria violated! eg, 40dB gain until 90% output exceeds comfort 25dB gain until 30% output becomes inaudible
91 Unaided Input Speech WDRC Aided Output Speech
92 Unaided Input Speech ADRO Aided Output Speech
93 ADRO Application of Comfort & Audibility Rules Distribution of Output constantly changes w/listening environment But criteria remain constant 100% Audibility boundary 30% rule Comfort boundary 90% rule Probability of having some output 0% Specified Range of Output db SPL Output Level in one Hz Channel
94 Output ADRO: I/O Function Comfort rule applies 100 Audibility rule applies More Linear Gain Less Linear Gain 70 Background noise rule applies Input
95 Normal Input Dynamic Range
96 Reduced Input Dynamic Range with SNHL
97 WDRC Does This
98 ADRO Does This
99 ADRO Processing Might Reduce Need for Programs Programs: The baggage that comes along Straight (but changing) amounts of linear gain Produces a rather bright and clear kind of sound With less processing Might just reduce need for alternating among programs
100 Recall: Two things we must do for hearing loss: 1. Improve audibility A gain issue achieved with Compression 2. Improve signal-to-noise ratio Achieved with Dmics Attempted (but not achieved) with DNR!
101 Hair cell damage & Speech in Noise Loss of outer hair cells dulls the traveling wave peak soft sounds no longer naturally amplified Loss of inner hair cells mixed speech & noise sent on to brain hearing aids make mixed up sound louder
102 Hearing Aids Make Soft Compromised Sound
103 Into Louder Compromised Sound
104 Solutions for Speech in Noise Directional mics objectively improve speech/noise performance Digital noise reduction subjectively enhances comfort in noise Together they make a good team But we ain t covering these here Or we ll never get outa here!
105 Conclusion The cochlea is a fascinating place And there s lots more to learn
106 Ted Venema Hearing Instrument Sciences Program Ozarks Technical Community College Springfield MO 2 nd Edition 2006 Cengage Publishing ISBN rd Edition 2017 Plural Publishing To be released at AAA 2017
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