Common Digital HI Algorithms

1. 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.

2. 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.

3. There are six fundamental algorithms
written into the software for digital
hearing instruments.
They are:
1. Automatic feedback reduction
2. Combinations of compression types
3. Expansion
4. Dynamic noise reduction
5. Multiple channels and bands
6. In-situ audiometric testing (using the HI)

4. Automatic Feedback Reduction
There are two basic types of feedback
reduction.
They are:
1. Notch filtering
2. Phase cancellation.

5. 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.

6. 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.

7. COMPRESSION TYPES
Digital 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

8. COMPRESSION TYPES
Note: 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.

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

10. COMPRESSION TYPES
NOTE: 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.

11. EXPANSION
Expansion 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.

12. 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.

13. DYNAMIC NOISE REDUCTION
There are four primary types of noise
reduction algorithms used within today’s
digital hearing instruments. They are:
1. Spectral Subtraction
2. Phase Cancellation
3. Spectral Enhancement
4. Speech Synthesis

14. 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.

15. 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.

16. 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

17. 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.

18. SPEECH ENHANCEMENT & DNR
Let’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

19. 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.

20. • 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.

21. 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

22. 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

23. • 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