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CSD9518 > 12_Hearing Aid Hardware & Software > Flashcards

12_Hearing Aid Hardware & Software Flashcards

1
Q

Name 1 component of hearing aid hardware

A
  • microphones
  • receivers
  • telecoils
  • wireless coil and radio IC
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2
Q

Name 1 part of analog-to-digital conversion

A
  • sampling

- quantization

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3
Q

Name 1 aspect of digital signal processing

A
  • fast fourier transform

- digital filtering

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4
Q

In a general hearing aid schematic, what comes first:

  • digital signal processor using intelligent formulae to process electrical signals, OR
  • microphone converting sound from acoustic energy to electric energy
A

Microphone converting sound from acoustic energy to electrical energy

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5
Q 
In the diagram of a digital hearing aid, there are 5 general steps:
Sound pick up
\_\_\_\_\_\_\_\_\_\_\_
Environment classification
\_\_\_\_\_\_\_\_\_\_\_\_
Signal Out
A

Sound cleaning

Audibility and loudness

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6
Q

Name 2 things “sound cleaning” refers to

A 
Adaptive Beamformer
Pinna simulation
Mic matching
Feedback cancellation
Noise reduction (stationary noise, impulsive noise, wind noise, and reverberation)
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7
Q

Name 2 things Audibility and Loudness refer to in a digital hearing aid schematic

A 
Gain calculation (incl. amplitude compression)
Frequency compression
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8
Q 
Which of the 5 "stages" of a digital hearing aid includes the FFT:
Sound PIck Up
Sound Cleaning
Environment Classification
Audibility and Loudness
Signal Out
A

Sound Pick up (analysis filterbank)

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9
Q

A hearing aid microphone is built based on which principle?

A

Capacitance

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10
Q

What does MEMS stand for?

A

Micro electromechanical systems

  • same principle as electret, but built using semiconductor manufacturing techniques
  • advantage is the size
  • most recent technologies use this
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11
Q

What is the microphone noise floor due to?

A

Random motion - thermal energy creates a small amount of random motion of the electrons w/in the electrical circuitry (electrical noise) and in the air volume adjacent to the diaphragm (acoustic noise)
- the mic noise floor in modern HAs is around 25 to 30 dB SPL

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12
Q

What does the term “saltshaker” refer to?

A

A diaphragm with pores

- they make it more robust in terms of frequency response

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13
Q

What effect does adding microphones have on the noise floor?

A

Adding mics increases the noise floor

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14
Q

Describe the balanced armature principle? Which aspect of a hearing aid is based on this principle?

A

Wire from the electrical circuit is wound around the armature, magnetizing it depending on the direction of the current.
- This is the principle behind receivers

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15
Q

For more SPL, do you need a bigger or smaller receiver? Why?

A

Bigger - in order to move more air, the diaphragm itself needs to be bigger

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16
Q

What do the peaks of a HA receiver frequency response indicate?

A

The EM resonances that come from the construction of the receiver

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17
Q

What does bandwidth refer to (for HA receiver frequency response)?

A

Where the flat part deviates from the mean by more than 3 dB

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18
Q

What are 2 ways to drive the receiver in a Class D Amp?

A

Pulse width modulation
- depends on width of pulse (lower level input -> narrow pulse width -> little movement of receiver)

Pulse Density modulation
- depends on number of pulses (low level input -> fewer pulses per second -> less movement of diaphragm)

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19
Q

What are the 2 differences b/w induction based wireless tech and regular telecoil?

A

1) 2nd coil does not use the magnetic field at the speech frequencies (uses higher frequency carrier, like modulation)
2) Telecoil pickup is ANALOGUE. Wireless coil has magnetic field at much higher frequency, which is modulated to induce e.g. 300 kHz current in HA (=DIGITAL process)

20
Q

What is an radio IC?

A

Dedicated integrated circuit (IC) chip that allows hearing aid to directly communicate with a source using a wireless protocol

  • if HA doesn’t have this receiver, uses intermediary dongle, which has BT chip and induces current in secondary wireless coil
  • antenna essentially connects to radio IC
  • 2.4 GHz (or other) wireless protocol
  • Bluetooth low latency
21
Q

What does DAI stand for? What does it do?

A

Direct Audio Input

  • “boot” that plugs into the bottom of HA
  • almost obsolete
  • analogue (needs to be digitized)
  • can be an FM receiver
22
Q 
Place the following items in order regarding A/D/A setup:
DAC
Anti-alias Filter
Reconstruction Filter
ADC
S/H (x2) (Sample and Hold)
A 
Anti-alias Filter
S/H
ADC
DAC
S/H
Reconstruction Filter
23
Q

What is the Nyquist criterion?

A

Regarding sampling, the sampling rate must be greater than twice the highest frequency of the input signal

24
Q

What do you do if trying to sample a signal, and the highest frequency is unknown?

A

Use a low pass filter to remove unwanted frequencies (anti-aliasing filter)

25
Q

What is oversampling?

A

Sampling at rate much higher than Nyquist
- used to reduce the number of bits while maintaining the same quality
bit rate = sampling rate x bits per sample
- DSP will need to be much faster to manage the increased number, so you can use fewer bits if you increase your sampling rate

26
Q

A signal is sampled at 8 kHz. What is the effective bandwidth of the digitized signal?

A 
4 kHz (half the sample rate)
- sample rates vary b/w manufacturers (mark as audio quality)
27
Q

What has a typical sampling rate of 96000 samples/sec?

A

DVD player

28
Q

What has a typical sampling rate of 8000 samples/sec?

A

Telecommunication systems

29
Q

What is quantization?

A

During digitization, when continuous values are transformed to digitized numbers (essentially “mapping”)

30
Q

How do you determine how many bits are used for quantization of a sample?

A
  • Determine the number of “levels”
  • solve for: 2^n = number of levels
    where n = bits
31
Q

In a 3 bit sample, sample values are quantized into corresponding 3 digit values of 1’s and 0’s (e.g. 1-0-1). What do each of the 3 places represent?

A

2^2 - 2^1 - 2^0

so…

4 - 2 - 1

e.g. 5 = 4x1 + 2x0 + 1x1

32
Q

How many bits is a typical hearing aid?

A

12-24 bits

33
Q

What is the equation for the dynamic range for the quantizer?

A

6*B (where B = number of bits)

Dynamic range is important b/c will tell you the range of dB SPL the quantizer can handle
e.g. if you want 30-100 dB, you need a dynamic range of at least 70 dB (at least 12 bits)

34
Q

What is a “floating dynamic range”?

A

When the dynamic range can be shifted in real time, depending on the input signal

  • e.g. 12 bit quantizer can shift its 72 dB dynamic range to access 20-92 dB SPL, 40-112 dB SPL, etc.
  • fixed dynamic range -> inputs beyond the max value will be clipped
35
Q

What is quantization noise? Does it increase or decrease with increase number of bits?

A

The difference between the true value of the signal and the quantized value.
- it decreases with increased number of bits (e.g. 16 bit has more “resolution” than 12 bit -> less quantization noise)

36
Q

Do quantization errors accumulate?

A

Yes (that is why it is important to keep quantization error low before starting any DSP operations)

37
Q

Since all of our operations are frequency specific, we tend to use either FFT or _________ to split input signal into frequency specific operations

A

filter banks

38
Q

With digital filters, the value of the coefficient determines the ____ __ _____

A

Type of filter

39
Q

The shape of a digital filter is typically expressed by the filter output when presented with a very brief input signal. What do engineers call this?

A

The system’s “impulse response”

40
Q

What does the Fourier analysis do to an audio signal?

A

It decomposes the varying time domain series of audio samples into a series of functions representing sinusoids (pure tones)

41
Q

What is Fourier synthesis?

A

The recombination of simple components into a complex waveform

42
Q

The Fourier Transform is an analogue function. What is the equivalent Fourier transform in the digital realm referred to as?

A

Discrete Fourier Transform (DFT). The digital counterpart to Fourier synthesis is referred to as inverse FFT (or reverse FFT)

43
Q

With FFT, what determines the resolution with which we look at the frequency response?

A

The number of points

Frequency resolution = sample rate/number of FFT points

44
Q

Approximately how much of a delay will cause a perceptual impact in hearing aids?

A

> 10 ms

45
Q

Which have shorter delays: analogue or digital hearing aids?

A

Analogue (virtually no signal processing)

46
Q

Name 2 things that the amount of delay in hearing aids is dependent on?

A
  • speed of the processor
  • number of computations required
  • presence of specialized hardware to handle computations
  • type of filterbank used

CSD9518 (13 decks)

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