1. Figures for Chapter 6 Compression
Dillon (2001)
Hearing Aids

2. Varieties of compression
High Level
Wide Dynamic Range
Low Level
Weak
Moderate
Intense
Input level
Weak
Moderate
Intense
Weak
Moderate
Intense
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

3. Compression waveforms
Pressure
Input:
Output:
Time
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

4. Linear Ts
Compressed envelopes
Ta= Tr = 10Ts
Slow
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.
Ta = Tr = Ts
Medium
Ta = 0.1Ts
Tr = 0.3 Ts
Fast
Source: Dillon (2001): Hearing Aids

5. Feedforward compression
Delay
Detector
Figure 6.4 A block diagram of a feedforward, look-ahead compression control circuit.
Source: Dillon (2001): Hearing Aids

6. I-O curve and gain-input curve
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.
Source: Dillon (2001): Hearing Aids

7. Cuvilinear compression
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

8. Input controlled compression
Input (dB SPL)
Output (dB SPL) 40 50 60 70 80 90
100
110
Vol max
Vol min F
Input controlled compression
Input (dB SPL)
Output (dB SPL) 80 70 60 50 40 90
100
110
Vol min
Vol max
Output controlled compression F
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

9. Linear and compressed envelopes
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).
Source: Dillon (2001): Hearing Aids

10. 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

11. Loudness normalization
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.
Source: Dillon (2001): Hearing Aids

12. Loudness normalization (TILL) + (a)
Figure 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.
(b)
0.1
0.2
0.5
1.0
2.0
5.0
Frequency (kHz)
Gain (dB) 50 90
(c)
Source: Dillon (2001): Hearing Aids

13. Noise reduction Input Spectral Level (a) (b) Gain Output Spectral (c)
Signal
Noise
Possible masking
(a)
Noise reduction
Gain
(b)
Noise
Output Spectral
Level
Frequency
Signal
1 kHz
(c)
Figure (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

14. Noise reduction (BILL)+
Noise reduction (BILL)
Figure 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
5.0
Frequency (kHz)
Source: Dillon (2001): Hearing Aids

15. Output levels Linear Compression Output range Output level Input Range70
Input level
Figure 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