his 240 - common digital hi algorithms

COMMON DIGITAL HI ALGORITHMS

Software algorithms for digital hearing instruments are written to create various combinations of compression, frequency response, gain and output for the purpose of processing/modifying acoustic sound input energy received by that digital hearing instrument.


Particular digital hearing instrument performance characteristics (which have been created by the algorithms) may then be chosen, applied and modified for appropriate stimulation of the patient/client’s residual hearing ability.


There are six fundamental algorithms written into the software for digital hearing instruments. They are:1. Automatic feedback reduction2. Combinations of compression types3. Expansion4. Dynamic noise reduction5. Multiple channels and bands6. In-situ audiometric testing (using the HI)


Automatic Feedback ReductionThere are two basic types of feedback reduction. They are: 1. Notch filtering2. Phase cancellation.


NOTCH FILTERS—have limitations regarding the number of peaks in the frequency response they can affect, and the decibel amount of reduction they can create within a certain few milliseconds.


PHASE CANCELLATION—involves the creation of an out-of-phase frequency signal by the digital HI processor. It’s limitations occur when it is unable to “recognize” the input sound as feedback or, more often, inappropriately recognizes the input sound and generates the phase cancellation signal when not required.


COMPRESSION TYPESDigital HI algorithms may be written for multiple configurations of compression. For example, multiple kneepoints may be integrated into one channel of processing.

Let’s view Venema page #163


COMPRESSION TYPESNote: on the input/output graphs for these multiple kneepoints, all compression ratios below the 1:1 compression line represent output compression, and all compression ratios above the 1:1 line represent input compression.


COMPRESSION TYPESMost digital HI compression strategies fundamentally use SYLLABIC compression for low frequency channels and AVERAGE DETECTION compression for high frequency channels.


COMPRESSION TYPESNOTE: An interesting type of compression is Adaptive Dynamic Range compression it uses audibility and comfort criterion to automatically adjust the gain and output characteristics by input frequency. It is an attempt to electronically simulate the hair cell functions of the human cochlea.


EXPANSIONExpansion is the opposite of compression. • It actually reduces gain below 40dB SPL.• This gain reduction for soft inputs was designed to eliminate the microphone and amplifier noise of digital hearing instruments.


EXPANSION• It acts like an internal noise “squelch filter”—in fact some manufacturers call it a “soft squelch” feature. • It is only “offered”/present below the left-most compression kneepoint of a digital hearing instrument.


DYNAMIC NOISE REDUCTIONThere are four primary types of noise reduction algorithms used within today’s digital hearing instruments. They are:1. Spectral Subtraction2. Phase Cancellation3. Spectral Enhancement4. Speech Synthesis


SPECTRAL SUBTRACTION• This type of algorithm basically

attempts to differentiate the noise components of the frequency spectrum from the speech components. • It then attempts to subtract the spectral

noise components from the received frequency input thereby, allowing the spectral speech components to remain and become enhanced/amplified.


PHASE CANCELLATION• This algorithm design identifies spectral

noise and then creates an opposite spectral phase signal to cancel the noise. • It is the fundamental process used in the

BOSE noise cancelling headphones. • NOTE: The input to a hearing instrument

already has the speech and noise mixed so, it cannot operate like a BOSE noise cancellation headset.


SPECTRAL ENHANCEMENT• Once again, this type of algorithm

basically attempts to separate the noise components of the frequency spectrum from the speech components. • However, its premise is to increase the

reception of the spectral components of speech. • This enhanced signal should provide a

greater opportunity to differentiate speech from noise.


SPEECH SYNTHESIS• This type of algorithm has a similar

performance to the spectral enhancement algorithm. • However, it takes the identified

spectral speech input components, and adds additional (and similar) spectral information to the input of the digital amplifier.


SPEECH ENHANCEMENT & DNRLet’s view Venema page #175

• These two spectral intensity graphs represent an acoustic input signal into a digital algorithm.• You can see that when spectral intensities do

not fluctuate over a defined time period, the algorithm determines that input to be noise and that spectral input is reduced. • If the spectral intensities do fluctuate over

time (such as speech) those inputs are not reduced.


SPEECH ENHANCEMENT & DNR• There is a lot of variation in the DNR algorithm effectiveness among the various manufacturers. • It is the generally accepted wisdom today, that there is no objective benefit for improved speech recognition within background noise.


• Perhaps there is not quantifiable objective benefit because the patient/client’s ability to perform that hearing-in-noise activity has not been measured? • You will learn more about patient/client speech-in-noise residual ability performance assessments in HIS 230.


MULTIPLE CHANNELS & BANDS• All digital algorithms involve multiple channels

and bands to create the desired performance characteristics of that specific algorithm. • The more compression channels that are

available, the greater the opportunity to create custom algorithms for patient/client psycho-social demands. • Remember, you can have more bands than

channels to create algorithms, so multiple algorithms may be created off of few channels.


IN-SITU AUDIOMETRY• Many of today’s digital hearing instruments

can generate their own electroacoustic signal without any outside acoustic input. • This provides the dispensing professional

the opportunity to measure the patient/client’s residual hearing ability as modified by the insertion of a digital hearing instrument. • Pure tone threshold and supra-threshold

measurements may thus be created “in-situ”.


• It is interesting to observe the departure from a required knowledge of compression types and other technological features on the part of the dispensing professional. • The current trends appear to be more

toward the psycho-social issues of the patient/client.• Minor compression and other electro-

acoustic settings appear to depend more on creating specific answers to patient/client psycho-social situations and their various listening conditions.

his 240 - common digital hi algorithms

Technology

spectral input

input frequency

compression typesnote

compression line

spectral components

spectral intensitiesdo

speech synthesis

spectral enhancementalgorithm