Compression 101. 2 Goals Achieve a basic understanding of various compression characteristics Why is...
36
Compression 101
-
Upload
walter-skinner -
Category
Documents
-
view
213 -
download
0
Transcript of Compression 101. 2 Goals Achieve a basic understanding of various compression characteristics Why is...
Compression 101
2
Goals
• Achieve a basic understanding of various compression characteristics
• Why is it important– Do I really need to commit this to memory?
• Clinical impact• Adult versus pediatric considerations• Troubleshooting with compression
3
Linear
• Definition: “A hearing aid circuit that provides the same amount gain for all input levels”
Textbook of Hearing Aid Amplification 2nd Edition. R.Sandlin
4
Compression
“Compression has many faces, there is no one simple way to describe it”
“To appreciate compression function, we can liken it to a piece of sculpture. We need to walk around and view it from several different angles to appreciate the entire piece of sculpture.”
Theodore Venema, PhD
“The major role of compression is to decrease the dynamic range of the signals in the environment so that all signals of interest can fit within the restricted dynamic range of a hearing-impaired person”
Harvey Dillon, Ph. D.
5
Compression Quiz
• Input– The signal in dB SPL coming into the microphone of the hearing
aid.• Gain
– Gain is the amount of amplification added to the input signal. Output minus input equals gain.
• Output– The signal in dB SPL which is delivered into the patient’s ear.
• Threshold Kneepoint– Point at which input-output function changes. (gain line takes a
bend)• Compression Ratio
– Function determining how much gain is provided by the hearing aid for the incoming sound.
6
Amplification Formula For Hearing Aids
Input + Gain = Output
/s/ /s/
7
Where is the limit?
• Maximum Output (SSPL): The level beyond which the output can not exceed.– Measured when hearing aid is in saturation– OSPL 90
• Importance of output limitation– Keep output of hearing aid from exceeding
tolerance level of hearing aid user– Preference for limiting output without
distortion
8
Methods of Output Limiting
• Peak Clipping
• Compression Limiting
9
Peak Clipping vs. Compression Limiting
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
OSPL90
10
Peak Clipping
11
Compression
12
“Comparison of Sound Quality and Clarity with Asymmetrical Peak Clipping and Output Limiting Compression” Hawkins, D. Naidoo, S.: J Am Acad Audiol 4: 221-228 (1993)
13
Two Basic Categories Of Hearing Aid Signal Processing
• Linear– Unity gain until SSPL has been
reached.– Results in peak clipping
• Non-Linear – Gain changes as a function of
input levels.– Amplification characteristics
dependant on type of compression.
14
Non-Linear Amplification
Compression1. Input/Output Compression(all WDRC is input compression)
• Compression Controls: 2. Regular vs Threshold Knee Points (TK’s)3. Output Limiting vs Wide Dynamic Range
Compression (WDRC) (BILL/TILL: types of WDRC)
15
Examples of non-linear processing characteristics…
• I/O functions• Compression Ratio• Output• Bands versus Channels
16
Basics of Compression – Input/Output functions
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
OSPL90
TK = 50dB
CR = 5:120dB
20dB
CR = 1:1
50dB
10dB
17
Input or Output Compression
• Input Compression– VC located between the amplifier and the receiver
• Output Compression– VC located between the mic and the amplifier
18
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
VC = Min
VC = Max
Input Compression - AGCi
Amp VC
Rec
Mic
19
Input Compression - AGCi
Frequency
dB
Gain
MPO
20
Mic AmpVC
Rec
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
VC = Min
VC = Max
Output Compression - AGCo
21
Output Compression - AGCo
Frequency
dB
Gain
MPO
22
Output Limiting vs. WDRC
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
WDRC
CR < 5:1
TK <~ 55dB
Output Limiting
CR > 5:1
TK >~ 55dB
23
Bands and Channels
• Bands are distinct frequency regions that can be shaped to change the dynamics, however compression characteristics cannot be altered.
• Channels allow for adjustments to be made to the dynamics and also the compression characteristics.
24
Advanced Compression functions
• Expansion• Dynamic Compression characteristics• Multi-channel hearing aids
25
Expansion
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
Additional TK
Additional CR
26
Dynamic Aspects of Compression
• Unlike static features like TK and CR, dynamic aspects of compression deal with the constantly changing intensities of our environment.
They are:• Attack and Release times
27
Attack/Release times
• Attack times are usually very short (<50ms) in order to quickly react to the changed SPL.
• Release times can vary from fast (50ms) to very slow (several seconds).
• “Syllabic compression” seems to be a popular model since the attack and release times fall within the time of average syllable length – 200-300ms
28
Attack/Release times – cont’d
Possible pitfalls:• Release time too fast: “pumping” sound is heard.• Attack time too slow (150ms) the device will not
react quickly enough to dynamic changes in environment.
• Release time too slow may not allow amplification to return to higher gain levels once louder sound has ceased, thereby under-amplifying softer consonant following louder vowels.
29
Multi-Channel Compression
250Hz
500Hz
1000Hz
2000Hz
4000Hz
30
Channels and Compression
• Compression can vary in multi-channel hearing aids.
• In order to maintain a smooth response (no distortion) averaging of compression characteristics may occur.
• Averaged levels are less precise than actual levels.
CR = 8:1CR = 1.5:1
CR = ?
31
Multi-Channel Compression Concerns
• Based only on the output graph, it is unclear what the compression ratios are for the varying channels.
• This is a less than ideal compression situation!
“:any multi-channel compression device risks distorting the normal loudness relationships internal to phonemic speech elements, relationships which provide cues for phoneme identification.”
Mead C. Killion, Ph.D.
36
Studies show that…
• Hearing loss (degree and configuration)• Listening situations• Previous experience with amplification• Acclimatization to amplification
…the choice of the preferred signal processing is influenced by:
37
Troubleshooting with Compression – Part 1
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
Doctor,Traffic noise is too loud, but speech is just right! What can
you do for that?
Δ TK to lower level.Result:Loud sounds more comfortable, but speech is softer!
Increase CR .Result:Loud sounds more comfortable, without affecting speech!
38
40
60
80
100
20 40 60 80 100
120
0
Input Intensity level, dB
Out
put
Inte
nsity
leve
l, dB
Troubleshooting with Compression – Part 2
Doctor,Whenever the fridge runs it sounds too loud. Plus, I don’t
seem to hear conversation loud
enough!
39
Take Home…
• Keep asking questions when the patient makes statements about problem situations. It may uncover different, more appropriate, solutions.
• Know your programming software. Know the rational for the changes you are making.
• Understand the concept, and you will be able to apply it to the fittings and adjustments.
