Figures for Chapter 6
Compression
Dillon (2001)
Hearing Aids
Varieties of compression
High Level Wide Dynamic Range Low Level
Output Level
Output Level
Output Level
Weak Moderate Intense Weak Moderate Intense Weak Moderate Intense
Input level
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.
Source: Dillon (2001): Hearing Aids
Compression waveforms
Pressure
Input:
Time
Output:
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.
Source: Dillon (2001): Hearing Aids
Ts
Compressed envelopes
Linear
Slow
Ta= Tr = 10Ts
Figure 6.3 Envelopes for the
output signal coming from a linear
amplifier and compression
amplifiers with different attack
Medium
times (Ta), and release times (Tr)
Ta = Tr = Ts compared to the duration of each
syllable (Ts) in the signal.
Fast
Ta = 0.1Ts
Tr = 0.3 Ts
Source: Dillon (2001): Hearing Aids
Feedforward compression
Delay
Detector
Figure 6.4 A block diagram of a feedforward,
look-ahead compression control circuit.
Source: Dillon (2001): Hearing Aids
130 I-O curve
Limiting
and
Output Level (dB SPL)
Linear
120
gain-input curve
110 2 dB
DO
100 DI
90 Compression range
Linear Figure 6.5 Upper: input-output
80 diagram showing the definition of
30 40 50 60 70 80 90 100 several static compression
Input Level (dB SPL) characteristics. Lower: the graph of
60 gain versus input that corresponds
to the I-O curve above it.
50
Gain (dB)
40
30
20
30 40 50 60 70 80 90 100
Input Level (dB SPL)
Source: Dillon (2001): Hearing Aids
Cuvilinear compression
130
Output Level (dB SPL)
120
110
100
90
80
30 40 50 60 70 80 90 100 110
Input Level (dB SPL)
Figure 6.6 Input-output characteristics corr-esponding to curvilinear
compression(solid line) and a fixed compression ratio combined with
compression limiting (dashed line).
Source: Dillon (2001): Hearing Aids
Vol max
100
Output (dB SPL)
90
F 80
70
60
50 Vol min
Input controlled compression 40
40 50 60 70 80 90 100 110
Input (dB SPL)
Vol max
100
Output (dB SPL)
90
80
F
70
60 Vol min
50
Output controlled compression 40
40 50 60 70 80 90 100 110
Input (dB SPL)
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.
Source: Dillon (2001): Hearing Aids
110
(a) Linear and compressed
90
envelopes
Level (dB)
70
50
Figure 6.8 (a) Envelope of the
30 signal The yellow flower has a big
20 bud put into the hearing aid at two
Gain (dB)
levels. The thick curve shows the
0 envelope for linear amplification
(b) and the thin red curve shows the
-20
110 envelope for a compressor with a
(c) 3:1 compression ratio, attack time
90 of 20 ms, and release time of 200
Level (dB)
ms. Part (b) shows the gain
70
applied by the compressor. Part
50
(c) shows the envelope for linear
amplification and for compression
30 when the attack and release
times of the compressor were
20
(d) increased to 1000 and 2000 ms
Gain (dB)
0 respectively. The corresponding
gain is shown in part (d).
-20
Time
Source: Dillon (2001): Hearing Aids
110
Linear
100
Output Level (dB SPL)
90 WDRC
80
70
Medium level
60 compression
50
40
30 40 50 60 70 80 90 100 110
Input Level (dB SPL)
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.
Source: Dillon (2001): Hearing Aids
Loudness category
Uncomf. loud Loudness
Loud but OK Normal
Comf, slightly loud normalization
Comfortable Impaired
Comf, slightly soft
Soft
Very Soft
0 20 40 60 80 100 Figure 6.10 (a)
50 Loudness growth curves
gain (dB)
Insertion
for normal hearing
30 people and a hearing
10 impaired person with a
0 20 40 60 80 100 50 dB hearing loss. (b)
Insertion gain needed
110
Output level
(dB SPL)
for the impaired listener
90 to receive a normal
70 loudness sensation. (c)
50 The corresponding I-O
0 20 40 60 80 100 curve.
Input level (2cc dB SPL)
Source: Dillon (2001): Hearing Aids
(a) Loudness
normalization
+ (TILL)
(b) Figure 6.11 Block
diagrams of (a) two-
channel and (b) single
channel processing
schemes that can
60 implement
50 (c) 50 approximations of
40 loudness normalisation,
and (c) the resulting
Gain (dB)
30
typical TILL gain-
20 frequency response that
90
10 increases in slope as the
0 input level decreases
-10 from 90 to 50 dB SPL.
0.1 0.2 0.5 1.0 2.0 5.0
Frequency (kHz)
Source: Dillon (2001): Hearing Aids
Possible masking
Noise reduction
Input Spectral
Level
(a)
Signal Noise
Gain
(b)
Output Spectral
Noise
(c) Signal
Level
1 kHz Frequency
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.
Source: Dillon (2001): Hearing Aids
Noise reduction
+ (BILL)
Figure 6.13 Block
diagrams of two-channel
and single-channel
processing schemes
that can implement
simple noise reduction
50 strategies, and the
resulting BILL response
Gain (dB)
that decreases in slope
30 50
as the input level
decreases from 90 to 50
10 90 dB SPL.
-10
0.1 0.2 0.5 1.0 2.0 5.0
Frequency (kHz)
Source: Dillon (2001): Hearing Aids
Output levels
Linear
Compression
Output
range
Output level
Input
Range
70 Input level
Figure 6.14 Input-output functions for two different hearing aids adjusted to
have the same output for a 70 dB SPL input signal.
Source: Dillon (2001): Hearing Aids