Transcript Slide 1
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.
5
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.
13
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
19
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
21
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.
23
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.
28
Multi-Channel Compression
4000Hz 2000Hz 1000Hz 500Hz 250Hz
29
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.
31
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.
39
That was it!
• Nikolas Klakow, AuD Northwest Customer Trainer Questions to: [email protected]
40