Compression 101

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 2

Linear • Definition: “A hearing aid circuit that provides the same amount gain for all input levels” Textbook of Hearing Aid Amplification 2 nd Edition. R.Sandlin 3

Compression “Compression has many faces, there is no one simple way to describe it” environment so that all signals of interest can fit within the restricted dynamic range of a hearing-impaired person” sculpture.” Theodore Venema, PhD 4

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.

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Amplification Formula For Hearing Aids

Input

/s/

+ Gain

=

Output

/s/ 6

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 7

Methods of Output Limiting • Peak Clipping • Compression Limiting 8

Peak Clipping vs. Compression Limiting 120 100 80 60 40 0 20 40 60 Input Intensity level, dB 80 100 OSPL90 9

Peak Clipping 10

Compression 11

“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) 12

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.

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Non-Linear Amplification Compression 1. 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) 14

Examples of non-linear processing characteristics…

• I/O functions • Compression Ratio • Output • Bands versus Channels 15

Basics of Compression – Input/Output functions 120 100 80 60 40 OSPL90

TK = 50dB

20dB

CR = 1:1

20dB 50dB 10dB

CR = 5:1

0 20 40 60 Input Intensity level, dB 80 100 16

Input or Output Compression • Input Compression – VC located between the amplifier and the receiver • Output Compression – VC located between the mic and the amplifier 17

Input Compression - AGC

i

120 Amp VC 100 VC = Max 80 VC = Min 60 40 0 20 40 60 Input Intensity level, dB 80 100 18

Input Compression - AGC

i dB Frequency MPO Gain

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Output Compression - AGCo 120 VC Amp 100 VC = Max 80 VC = Min 60 40 0 20 40 60 Input Intensity level, dB 80 100 20

Output Compression - AGC

o MPO dB Gain Frequency

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Output Limiting vs. WDRC 120 100 WDRC CR < 5:1 TK <~ 55dB 80 60 Output Limiting CR > 5:1 TK >~ 55dB 40 0 20 40 60 Input Intensity level, dB 80 100 22

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.

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Advanced Compression functions • Expansion • Dynamic Compression characteristics • Multi-channel hearing aids 24

Expansion 120 100 80 Additional TK 60 40 0 Additional CR 20 40 60 Input Intensity level, dB 80 100 25

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 26

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 27

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.

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Multi-Channel Compression

4000Hz 2000Hz 1000Hz 500Hz 250Hz

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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 = 1.5:1 CR = ?

CR = 8:1 30

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.

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Studies show that… … the choice of the preferred signal processing is influenced by: • Hearing loss (degree and configuration) • Listening situations • Previous experience with amplification • Acclimatization to amplification 36

Troubleshooting with Compression – Part 1 120 Doctor, Traffic noise is too loud, but speech is just right! What can 60 40 0 20 40 60 Input Intensity level, dB 80 100 37

Troubleshooting with Compression – Part 2 120 Doctor, Whenever the fridge runs it sounds too loud. Plus, I don’t conversation loud enough!

60 40 0 20 40 60 Input Intensity level, dB 80 100 38

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.

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That was it!

• Nikolas Klakow, AuD Northwest Customer Trainer Questions to: nikolas.klakow@phonak.com

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