Digital Signal Processing Flashcards
What is digital signal processing?
It converts an analog electric signal into digital data (numbers)
It can be manipulated and isolated
How is sound converted into digital?
Analog to digital converter
What do analog digital converters require?
Sound sampling and quantization
What is a sampling rate?
Measures equally spaced moments of time along the analog electric signal
Are high sampling rates better?
Yes, they better represent the original signal
What is nyquists frequency?
It quantifies the highest frequency that can be reconstructed into a signal for a particular sampling rate
A sampling rate is 2x higher than the hearing aids bandwidth (a 20 kHz sampling rate becomes a 10 kHz bandwidth)
What is bit depth?
It is the vertical measurement representing the amplitude of the signal
What is quantization?
Combines bit depth and sampling rate
Creates a digitalized version of the signal
What is the bit value of most hearing aids today?
16 bit
What is quantization error?
The difference between the original and the digitized signal
What does quantization error create?
Noise
Soft random noise referred to as the noise floor
What is an algorithm?
The analytical calculations applied to the digital signal
These are applied to acoustic scenes
Can algorithms analyze and classify acoustic scenes to determine which algorithmic signal processing scheme to apply?
Yes
They can identify steady state, transient, or reverberant signal and apply calculations to attenuate them
Can the accuracy of algorithms affect how the user hears?
Yes
What happens to the signal after digital signal processing?
It is converted back to an analog electric signal
What are the two ways to return a digital signal to an analog electric one?
Digital signal is converted to an analog signal and then goes to the receiver
Digital signal is converted and then amplified by an output amplifier (AGC-o) before entering the receiver
What is the processing speed?
The time it takes an acoustic signal to be picked up by the mic, amplified, digitized, processed, back to electric signal, converted to acoustic signal and delivered to the ear canal
What is the average processing speed for modern hearing aids?
About 2 to 10 msec
When are slow processing speeds an issue?
When direct signals are mixed with amplified signals
Sound quality issues are greatest for open domes
Comb filtering effect occurs
What is the solution for direct signals and amplified signals mixing?
Faster processing speeds
0.5 ms preferred
Do normal hearing adults understand speech in poor SNR settings?
Yes because speech is highly redundant
Do auditory filters need to be intact to efficiently process complex signals in the cochlea?
Yes
The cochlea analyzes signals using a bank of overlapping band-pass filters
These filters allow regions of the cochlea to paw attention to a specific freq region while ignoring freq outside the band
How big are the critical bands in the low freq?
Narrow
How big are the critical bands in the high freq?
Wide
Does noise mask signals in adjacent critical bands?
Yes
Filters overlap
Does poor freq resolution alter the shape of the critical bands?
Yes
It broadens the filter mainly on the low freq side of the filter
This increases the susceptibility to low freq masking
How much can an intense 250 Hz noise mask?
Not only masks the 250 Hz region, but masking spills over to overlapping higher frequency bands
Impacts audibility up to 1.5 kHz
What are the three different types of noise that significantly impact speech intelligibility?
Steady state signals
Random noise with an intensity-frequency spectrum like speech
Room reverberation
What three domains are analyzed in hearing aids before applying an algorithm?
Spatial domain (location) - identifies a signals location to control the signal
Temporal domain (time) - identifies a signals timing to control the signal
Spectral domain (frequency) - identifies the signals frequency to control the signal
How is noise in the spatial domain managed?
With directional microphone technology
What is automatic microphone switching algorithms?
A device can automatically change from an omnidirectional to a directional mic in the presence of background noise
When a device switches algorithms based on background noise, what are the two options it can switch to?
Fixed directional (one single polar plot) or adaptive directional (multiple polar plots)
Should mics revert to omnidirectional in quiet?
Yes
How does null steering work?
Algorithm digitally manipulates the internal time delay to shift the azimuth of a polar plot null based on the sounds location
Can adaptive directional mics have different polar plots for different frequency ranges?
Yes
When is adaptive directionality better?
When there are only a few noise sources present
When is fixed directionality better?
In the presence of multiple noise sources
What is beamforming?
Bandwidth of directionality narrows as the environmental intensity increases
What are modulation rates?
The number of times a signal fluctuates per second
What is modulation depth?
The amplitude variations between the loudest and quietest portions of the signal
What is the modulation rate for speech?
Speech modulates at a slow rate
What is the modulation rate for noise?
Fast
What is the modulation depth for speech?
Highly variable
What is the modulation depth for noise?
Steady over time
What is digital noise reduction?
Applies an algorithm to any signal that remains steady over a long period of time (steady state noise)
Only applies to signals with fast modulation ratees and low modulation depths
The amount of noise reduction varies based on the intensity of the signal or program settings
The more broadband the noise, will the DNR have a greater effect on speech?
Yes
DNR is not perfect
If noise is in the high freq, you will attenuate some of the speech too
What can digital noise reduction not do?
Improve speech intelligibility
What can digital noise reduction do?
Improve listening comfort
Reduce listening effort
Reduces cognitive load
What is the theory behind sound cleaning in the spectral domain?
If noise if predominantly below 1.5 kHz, reducing output in the low freq will improve intelligibility in noise
What is the issue with the theory behind sound cleaning in the spectral domain?
Noise exists at higher frequencies too
Another speech signal could be the undesired signal, and this low freq output reduction diminishes audibility of the desired speech signal too
What is adaptive wiener filter-based noise reduction?
A spectral subtraction approach
Whenever there is a pause in the speech signal, a quick measure of noise is done (modulation depth and rate)
It is then subtracted from the entire signal
Does the adapter wiener filter-based noise reduction improve intelligibility?
No
But it improves acceptance of background noise
What is a feedback loop?
An amplified signal leaks out of the ear canal and is reamplified by the mic
When do feedback loops occur?
When the device is too loose
When the vent is too large
When the mic and receiver are too close
What are three ways to reduce external feedback?
Increase the snugness of the mold or decrease the vent size
Digital notch filtering
Digital feedback cancellation
What is digital notch filtering?
Removes a narrow band of frequencies around the feedback
manually reduces gain between 2-4 kHz until feedback stops
What are some potential issues with digital notch filtering?
Could reduce speech intelligibility bc 35% of intelligibility come from 2000 Hz
What is digital feedback cancellation?
When feedback is detected, the phase cancellation algorithm mimics the feedback creating an out-of-phase clone of its signal
What are some limitations with digital feedback cancellation?
May result in brief feedback until the out-of-phase signal is generated
Any sustained tone can activate it, patient can hear clone without the actual feedback
Can attenuate or distort parts of speech signal
Shortens battery life
Does wind noise mask speech?
Yes
Wind cover are only a temporary solution
What is digital wind noise reduction?
3 methods
LF filtering applied
Audio signal from least affected mic streamed to the affected side
Directional mic switched to omni in LF channels
Do steeply sloping hearing losses pose a unique challenge to hearing aid gain?
Yes
The HA receiver may not be capable of providing enough output at the highest frequencies
Venting also limits HF gain
What is used as an attempt to solve the issues with fitting steeply sloping losses?
Frequency lowering
What are the three types of frequency lowering?
Linear frequency transposition
Nonlinear frequency compression
Spectral envelope warping
What is linear frequency transposition?
Moves the high frequency band one octave down to a lower frequency region
Transposed frequency components are mixed with any low frequencies present
What is nonlinear frequency compression?
A range of high frequencies is compressed into a lower frequency range
Frequencies below the start and end frequency are not affected/altered
Tonotopic order is maintained
What is spectral envelope warping
Copy and keep
High frequency signals are transposed into a lower frequency band but simultaneously remains present in its original tonotopic position
Should frequency lowering be turned on during an initial fitting?
No
Should frequency lowering be present in a music program?
No
What are some frequency lowering outcomes for speech recognition?
For some, FL improves speech recognition of fricatives
For most, speech recognition did not improve
For some, FL degraded speech recognition
When do frequency lowering benefits increase?
With the severity of the hearing loss
Might also require more time to adjust to FL
What are some possible benefits of FL?
Reduced listening effort
Speech production and vocal quality
Improved ability in noise
When is frequency lowering immediately turned on for the pediatric population?
When the child lacks audibility of /s/ or /z/
What scenes will an algorithm detect and switch programs?
Environmental noise level detection
Automatic telephone detection
Music detection
Does wireless binaural signal processing improve digital decision making?
Yes
What are different types of wireless binaural signal processing?
Volume control synchronization
Program change synchronization
Bilateral output for telephone signal
Bilateral wind noise management
Signal microphone directionality in a CIC (each mic port functions as one of the directional mic ports, they communicate with each other)
Does binaural wireless improve spatial hearing though better representation of ILD?
Yes
WDRS takes away ILD by amplifying soft sounds more
Restores the difference
What improves with the restoration of ILD?
Localization and hearing in noise
What does data logging give you?
Objective data on device use
Can help identify counseling topics and identify ways to customize devices to meet lifestyle needs
What is digital self-learning?
An automatic way that the devices can learn your patients volume control preferences
Can make automatic changes based on these preferences
Can hearing aids be teleprogrammed?
Yes by an audiologist
Can be done remotely
