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

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

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.

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.

Amplification Formula For Hearing Aids Input + Gain = Output /s/

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

Methods of Output Limiting Peak Clipping Compression Limiting

Peak Clipping vs. Compression Limiting 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB OSPL90 Basic graph showing the different input/output function of a linear and output compression circuit. Quick explanation of what happens to amplified sound when it comes up on the ceiling. Lead into the next slide

Peak Clipping Graphic demonstration of a sound wave reaching saturation level and the distortion it creates. Santa is a different demonstration of the same concept.

Compression Graphic demonstration of the same sound as the previous slide as it gets compressed to avoid distortion. Same for Santa example.

“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) Figure from study showing the detrimental effect of peak clipping on a sound. This supports the previous three slides.

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. Summary of previous information

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)

Examples of non-linear processing characteristics… I/O functions Compression Ratio Output Bands versus Channels

Basics of Compression – Input/Output functions 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB OSPL90 TK = 50dB 10dB 50dB 20dB Points to be made: TK explanation CR explanation 20dB CR = 5:1 CR = 1:1

Input or Output Compression Input Compression VC located between the amplifier and the receiver Output Compression VC located between the mic and the amplifier Ask group if they can differentiate between the two types of compression before showing the difference.

Input Compression - AGCi 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB Amp VC Rec Mic VC = Max VC = Min Graphical demonstration of i/o curves for input compression circuit and where the VC is placed in the amplification chain. Kneepoint stays at the same place for this type of compression since it has been set in the compressor/amplifier before the volume control can make changes. Important: gain above the kneepoint changes with the VC adjustments. MPO also changes.

Input Compression - AGCi MPO dB Continuation of previous slide. Since the VC is before the amplifier, the lowered voltage entering into the system results in a change in gain and MPO. This type of compression works well for mild to moderate hearing losses since the gain requirements are not that high, risking setting the MPO too loud. Gain Frequency

Output Compression - AGCo 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB Mic Amp VC Rec VC = Max VC = Min Output compression depiction with VC before the amplifier, resulting in changes in kneepoint location. VC Min has TK at 75dB resulting in 95dB output; therefore 20dB gain. VCMax has TK at 40dB and output at 90dB; therefore 50dB gain. Used for more severe hearing losses where a more linear approach provides greater gain for all inputs and MPO is the same.

Output Compression - AGCo MPO dB Gain Frequency

Output Limiting vs. WDRC 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB WDRC CR < 5:1 TK <~ 55dB Output Limiting CR > 5:1 TK >~ 55dB Differences in TK and CR that define WDRC and Output Limiting compression.

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. Basic definitions of the differences between channels and Bands. There will be a visual explanation of multi-channel compression later in the ppt.

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

Expansion 120 100 Output Intensity level, dB 80 Additional TK 60 40 Input Intensity level, dB Output Intensity level, dB Additional TK Graphic demonstration of the changes that occur to the gain of a hearing aid when the CR and TK are changed at the soft end of the amplification spectrum. As the TK is raised the gain is reduced for sounds below that TK. This will attenuate gain for soft inputs without affecting gain for moderate and loud levels. TK adjustments the same concept as output compression, just at a very low level. Additional CR

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

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

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.

Multi-Channel Compression 4000Hz 2000Hz 1000Hz 500Hz 250Hz Graphic demonstration of the various compression characteristics that multiple channels can have in order to provide more precise amplification for the varying hearing loss thresholds. This means more appropriate amplification for all frequencies compared to a hearing aid that has fewer or only a single channel. Some channels may appear more WDRC and others more like output limiting. Significant benefit of multi-channel digital amplification to have this flexibility.

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 = 8:1 CR = 1.5:1 CR = ? The risk of having significantly different compression characteristics in neighboring channels is that since they overlap, certain inputs need to be averaged rather than getting two significantly different commands, which would result in worse sound quality.

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! Due to flexibility of multi-channel amplification, care must be taken to provide appropriate compression. “: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.

Rationales for Compression Avoid Distortion and Discomfort Decrease inter-syllabic level differences Increasing sound comfort Normalizing Loudness

Why So Many Different Types? Isn’t “Digital” all we need today? Digital hearing aid have _______________ type of signal processing A conventional hearing aid (yes the ones with trim pots) are always linear? (True/False) In terms of fitting adults or pediatrics, signal processing is not very important. (True/False)

Things to consider with adult fittings… Amplification history Length of time using previous technology Goals for new hearing aids Counseling, realistic expectations, acclimatization Choice of hearing aids Severity of loss; does it matter?

Things to consider with pediatric fittings… Amplification history? WDRC IS the gold standard Yes digital is ideal Maximizing the fit to ensure Audibility above all (DSL: Does this ring a bell?) Older pediatric patients with a history of linear amplification. What to do? Severity of loss does it matter?

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 We know that the choice of the best signal processing strategy goes beyond the client’s hearing loss. It should also take into account personal sound preferences, the sound environments the wearer is in and their previous amplification history. A ‘one size fits all’ signal processing concept simply does not address the varying needs and experiences of hearing impaired clients, let alone their various hearing loss configurations. The use of different processing and performance enhancer settings results in greater user benefit and speech understanding in different listening environments. For more detail on this, see the Valeo SoundSelect Manager background story.

Troubleshooting with Compression – Part 1 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB Doctor, Traffic noise is too loud, but speech is just right! What can you do for that? Increase CR . Result: Loud sounds more comfortable, without affecting speech! Δ TK to lower level. Result: Loud sounds more comfortable, but speech is softer!

Troubleshooting with Compression – Part 2 40 60 80 100 20 120 Input Intensity level, dB Output Intensity level, dB Doctor, Whenever the fridge runs it sounds too loud. Plus, I don’t seem to hear conversation loud enough! In this example the important point is the priority of the complaints. Since hearing speech is of greater importance it should be handled first; then the second complaint can be tackled with expansion controls. If the clinician focuses on the noise complaint first, the focus is away from the primary reason of the instrument. Stress that making changes with the amplification should occur only after the full picture of the situation is known.

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.

That was it! Nikolas Klakow, AuD Northwest Customer Trainer Questions to: nikolas.klakow@phonak.com