Figures for Chapter 6 Compression Dillon (2001) Hearing Aids

Figures for Chapter 6

Compression

Dillon (2001)

Hearing Aids

Figure 6.1 Three ways in which the dynamic range of signals can be reduced. In each case, the upper figure shows the spacing of different signal levels before amplification (the left end of the lines) and after amplification (the right end of the lines). The lower figure shows the same data, but as an input-output function.

Ou

tpu

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vel

Weak Moderate Intense

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put L

evel


Wide Dynamic RangeHigh Level Low Level

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Input level

Source: Dillon (2001): Hearing Aids

Varieties of compression

Time

Output:

Input:

Pressure

Figure 6.2 Waveforms that are input to a compressor and output from a compressor, showing the attack and release transitions that follow an increase and decrease, respectively, in signal level. The dotted line shows the envelope of the positive half of the signal.


Compression waveforms

Ta = 0.1Ts

Tr = 0.3 Ts

Fast

Ta= Tr = 10Ts

Slow

Ta = Tr = Ts

Medium

Linear

Ts

Figure 6.3 Envelopes for the output signal coming from a linear amplifier and compression amplifiers with different attack times (Ta), and release times (Tr) compared to the duration of each syllable (Ts) in the signal.


Compressed envelopes

Figure 6.4 A block diagram of a feedforward, look-ahead compression control circuit.

Delay

Detector


Feedforward compression

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Input Level (dB SPL)

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dB

SP

L)

I

O

Compression range

2 dB

LinearLimiting

Linear

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30 40 50 60 70 80 90 100


Ga

in (

dB)

Figure 6.5 Upper: input-output diagram showing the definition of several static compression characteristics. Lower: the graph of gain versus input that corresponds to the I-O curve above it.


I-O curve and

gain-input curve

80

90

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Out

put

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vel (

dB S

PL

)

Figure 6.6 Input-output characteristics corr-esponding to curvilinear compression(solid line) and a fixed compression ratio combined with compression limiting (dashed line).


Cuvilinear compression

Figure 6.7 Input controlled compression and output controlled compression: their block diagrams and the I-O curves for each as the volume control is varied from maximum to minimum positions.

Input controlled compression

F

Output controlled compression

F

Input (dB SPL)

Out

put

(dB

SP

L)

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100Vol max

Vol min

Input (dB SPL)

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(dB

SP

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40 50 60 70 80 90 100 110

Vol min

Vol max


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Leve

l (dB

)

(a)

-20

0

20

Gain

(dB

)

(b)

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110

Leve

l (dB

)

(c)

-20

0

20

Time

Ga

in (

dB

)

(d)

Figure 6.8 (a) Envelope of the signal The yellow flower has a big bud put into the hearing aid at two levels. The thick curve shows the envelope for linear amplification and the thin red curve shows the envelope for a compressor with a 3:1 compression ratio, attack time of 20 ms, and release time of 200 ms. Part (b) shows the gain applied by the compressor. Part (c) shows the envelope for linear amplification and for compression when the attack and release times of the compressor were increased to 1000 and 2000 ms respectively. The corresponding gain is shown in part (d).


Linear and compressed envelopes

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l (dB

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Linear

Medium levelcompression

WDRC

Figure 6.9 Input-output curves for medium level compression, wide dynamic range compression, and linear amplification, all combined with either compression limiting or peak clipping of high level signals.


0 20 40 60 80 100

Loud

ness

cat

egor

y

Very Soft

Comfortable

Uncomf. loudNormal

Impaired

SoftComf, slightly soft

Comf, slightly loudLoud but OK

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Inse

rtio

n ga

in (

dB)

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Input level (2cc dB SPL)

Out

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leve

l (

dB S

PL)

Figure 6.10 (a) Loudness growth curves for normal hearing people and a hearing impaired person with a 50 dB hearing loss. (b) Insertion gain needed for the impaired listener to receive a normal loudness sensation. (c) The corresponding I-O curve.


Loudness normalization

Figure 6.11 Block diagrams of (a) two-channel and (b) single channel processing schemes that can implement approximations of loudness normalisation, and (c) the resulting typical TILL gain-frequency response that increases in slope as the input level decreases from 90 to 50 dB SPL.

+

(a)

(b)

-100

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0.1 0.2 0.5 1.0 2.0 5.0Frequency (kHz)

Gai

n (d

B)

50

90

(c)


Loudness normalization

(TILL)

Figure 6.12 (a) Spectrum of the signal and noise input to a noise reduction hearing aid. (b) Gain applied to the signal and noise. (c) Spectrum of the signal and noise at the hearing aid output.

Gai

n (b)

Noise

Out

put

Spe

ctra

lLe

vel

Frequency

Signal

1 kHz

(c)

Inpu

t S

pect

ral

Leve

lSignal Noise

Possible masking

(a)


Noise reduction

Figure 6.13 Block diagrams of two-channel and single-channel processing schemes that can implement simple noise reduction strategies, and the resulting BILL response that decreases in slope as the input level decreases from 90 to 50 dB SPL.

0.1 0.2 0.5 1.0 2.0 5.0Frequency (kHz)

-10

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Gai

n (d

B)

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90

+


Noise reduction (BILL)

Input level

Out

put

leve

lLinear

Compression

70

InputRange

Out

put

ran

ge

Figure 6.14 Input-output functions for two different hearing aids adjusted to have the same output for a 70 dB SPL input signal.


Output levels

Documents

Figures for Chapter 6 Compression Dillon (2001) Hearing Aids