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HESP 700- Hearing Aids > Lecture 6: Signal Processing & Compression > Flashcards

Lecture 6: Signal Processing & Compression Flashcards

1
Q

What is the difference between analog and digital hearing aids?

A

Digital Processor

  • In digital HAs, this processor converts acoustic signal to 0s and 1s
  • Numbers are converted back to sound with an analog to digital converter
  • This processor did not exist in analog hearing aids
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2
Q

Define hearing aid output.

A

An overall expression of the overall sound power

  • Expressed in dB SPL
  • Often referred to as maximum power output (MPO)
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3
Q

What is gain?

A

The hearing aid output - input

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4
Q

What is harmonic distortion?

A
  • The amplitude of the distortion produced by the hearing aid
  • The source of the distortion is the amplifier
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5
Q

What are the digital specifications in hearing aids?

A

1) Instructions per second (MIPS)
- The # of instructions that a digital processor can complete in a sentence
- The higher the instruction rate is, the higher the current consumption
- Higher instruction rates make it possible for the HA to perform complex operations (includes multiband processing, automatic feedback as supression)

2) Sampling Rate
- How many times per second the HA samples the input signal
- Must be greater than twice the highest frequency component that’s present in a complex signal
- The bandwidth of the signal processing is the limiting factor
- Power requirements can also limit the sampling rate

3) # of bits
- Derived from binary bits
- Greater than the # of bits, the greater precision of representing the signal
- If you have too few bits, the digital approximation is too coarse
- Too sloppy of a signal there is extra noise
- Use coding scheme that makes fewer bits produce as good of sound quality as more bits

4) Current consumption
- Consider battery life and size
- Dependent on the MIPS, the voltage requirements & the complexity of the integrated circuit

5) Processing Delay
- The amount of time that the input signal passes through the HA into the ear canal
- Too much time between input and output processing
- Longer delays makes for more sophisticated processing; makes the listener upset with their own voice quality
- Delay of less than or equal to 5 Ms is preferred

6) Physical size
- Physical size of the integrated circuit is still the limiting factor

7) Open Platform
- A closed platform means that you can’t fundamentally change how the compression works
- Open platform technology allows you to add updates to your hearing aids

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6
Q

What does the acronym ACT stand for?

A

Audibility
Comfort
Tolerance

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7
Q

What are the categories of signal processing?

A

1) Linear
- Referred to as compression
- One-to-one relationship between signal input and signal output
Output = input + gain
- Gain is constand for input up to a certain point

2) Nonlinear
- Both occur in analog and digital hearing aids

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8
Q

What are the limitations of linear amplification?

A
  • Harmonic distortion (from peak clipping)
  • Frequency volume adjustment or insufficient volume
  • The normal cochlea is a nonlinear system
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9
Q

What are the consequences of OHC damage?

A

1) Hearing loss
2) Linear basilar membrane response
3) Loss of gain for low-level inputs
4) Reduced dynamic range
5) Broad auditory filters

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10
Q

What are the purposes of compression?

A
  • Limit maximum output

- Amplify sounds to fit within the hearing aid user’s auditory dynamic range

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11
Q

Define compression.

A

The change in input level needed to produce a 1 dB change in output level

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12
Q

Compute the compression ratio & draw it on a graph.

A
  • See handout from class*
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13
Q

What are the characteristics of compression?

A

1) Static: how much compression is taking place

Compression threshold
- equals the input level in dB SPL above which gain decreases by 2 dB from linear gain (threshold knee point)

Compression ratio
- indirectly describes how much the gain decreases and is defined as the change in input level needed to produce a 1 dB change in output level

2) Dynamic characteristics: how quickly compression is occurring

  • Attack time: the time it takes for the compressor to react to an input signal that exceeds the knee point (less than 5 msec)
  • Release time: the time it takes for teh gain to return to the precompressed level (less than 20 msec)
  • Attack/release times are considered time constants
  • If the time constant is too fast, the listener will hear a pumping sound
  • If the time constants are too slow the listener may hear dead spots while the HA is recovering from compression
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14
Q

What is the difference between input and output compression?

A

Input compression
- the level where the compression begins (after the microphone but before the amplifier)

Output compression
- not until the signal leaves the amplifier is the signal compressed

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15
Q

How can you calculate compression ratio?

A

CR = change in input/ change in output

  • creates the slope on the compression graph
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16
Q

What is output limiting vs. WDRC?

A

OL does its work above the knee point

  • High compression knee point
  • High compression ratio

WDRC does it’s work below the knee point

  • Low compression knee point
  • Low compression ratio
  • Keep soft wounds audible
  • Keep moderate sounds comfortable
  • Keep intense sounds loud but not loud enough to cause discomfort
17
Q

What is BILL?

A

Bass increase at low levels

- As input increases, the gain at the bass will decrease

18
Q

What is TILL?

A

Treble increases at low levels

- As input increases, the gain at higher frequencies will decrease

19
Q

What is PILL?

A

Programmable increases at low levels

20
Q

What are the different types of compression?

A

1) Multichannel compression
- Variable across frequency ( to accommodate hearing loss) and variable dependent upon signal level

2) Syllabic compression
- Uses a fast-acting compressor to increase gain for weak syllables or phonemes, and to decrease gain for loud syllables or phonemes

21
Q

What is loudness normalization?

A
  • The goal is to make loudness seem the same for a person with hearing loss as it would be for a normal hearing person
  • Compression ratio decreases as input level increases
  • Different across frequencies
  • Low compression threshold
22
Q

What is loudness equalization?

A
  • To maximize speech intelligibility

- The goal is to make all frequency bands contribute equally to the volume of speech

23
Q

Define attack time.

A

How quickly the compression circuit responds to changes in input

24
Q

Define release time.

A

How quickly the compression circuit returns to “normal”

25
Q

What are the compression strategies?

A

1) Syllabic

2) Adaptive Dynamic Range Optimization (ADRO)
- applies multiple time constants and rules in sequence
- Gain is reduced to avoid exceeding loudness discomfort levels
- Gain is reduced to avoid upward levels of speech exceeding comfortable range
- Gain is increased to avoid softer levels of speech being inaudible
- Gain is never allowed to exceed levels that would cause feedback or excessive amplification of background noise

3) WARP Processing
- Takes into account the cochlea’s non-linear processing
- Uses fast syllabic attack and release times

26
Q

How can we optimize dynamic range and restore normal loudness perception?

A
  • Use AGI-1
  • Keep TK as low as possible to make weak sounds audible
  • Keep CRs low
  • AT/RT may be fast or slow, dependent upton patient
  • Multichannel compression better for sloping audiometric configuration
27
Q

What is expansion?

A
  • A noise reduction strategy for quiet environments
  • Minimizes soft environmental noises ( i.e., the sound of the refrigerator, the furnace, etc.)
  • Minimizes the sound from the circuit noise
  • Applied to inputs softer than the knee point (TK)
28
Q

What is a single channel?

A
  • Compression characteristics that are applied to the entire frequency response
  • Fine for relatively flat hearing loss
29
Q

What is a multichannel?

A
  • The difference between bands and channels
30
Q

What’s the difference between bands and channels?

A

Band: Section where gain can be controlled (frequency)

Channel: Section where compression can be controlled (processing)

  • There can never be more channels than bands
31
Q

What is AGCo vs. AGCi?

A

AGCo

  • Most important for managing maximum output
  • As you increase the volume control settings, the gain increases, the output increases, but the MPO remains the same

AGCi

  • Important for speech perception
  • As you increase the volume control settings, the gain increases, the output increases, and the MPO increases

HESP 700- Hearing Aids (9 decks)

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