Figures for Chapter 7

Advanced signal processing

Dillon (2001)

Hearing Aids

Front(a)

Output

+ -T

Figure 7.1 (a) Block diagram of a subtractive directional microphone comprised of either a single microphone with two ports, or two separate microphones with one port each. The negative sign next to one of the inputs of the summer indicates that the two signals are subtracted. (b) A delay-and-add directional microphone array with four ports.

Source: Dillon (2001): Hearing Aids

Fixed directional arrays

Subtractive array

Front

(b) + T ++Output

TT

Additive array

-40

-30

-20

-10

0

10

100 1000 10000

Frequency (Hz)

Fro

ntal

res

pons

e (d

B)

300 300

20 mm

8 mm

Figure 7.2 Frontal sensitivity of a two-port (or two-microphone) subtractive directional microphone relative to the sensitivity of an equivalent single-port microphone. The parameter shown is the port spacing. The internal delay needed to produce a cardioid polar response has been assumed.

Source: Dillon (2001): Hearing Aids

Frontal sensitivity and port spacing

Figure 7.3 End-fire and broadside microphone arrays.

SourceArray

electronics

Output

Broadsidearray

Source

Array electronicsOutput

Endfire array

Source: Dillon (2001): Hearing Aids

Adapter

Front

Output

+ -T

Figure 7.4 A simple adaptive directional microphone with steerable nulls.

Source: Dillon (2001): Hearing Aids

Adaptive directional microphone

Figure 7.5 The Widrow Least Mean Squares adaptive noise reduction scheme, based on a reference microphone that picks up only the noise. The fixed delay compensates for the delay inherent in the adaptive filter.

Speech+

Noise

-

+

Noise

Delay

Source: Dillon (2001): Hearing Aids

Widrow LMS noise reduction

Figure 7.6 A Griffiths-Jim adaptive noise canceller, whereby the two microphone outputs are added in the top chain but subtracted in the bottom chain.

+

+

-

+Front

Left

Right

-

+Delay

Source: Dillon (2001): Hearing Aids

Griffiths Jim adaptive noise reduction

0

5

10

15

20

25

30

Conf room Living room Anechoic

SR

T im

pro

vem

en

t (d

B)

3 mics

7 mics

Figure 7.7 Improvement in speech reception threshold for an adaptive array relative to a single microphone. The experiment used frontal speech and a single noise masker at 45 degrees from the front in three simulated environments that differed in the amount of reverberant sound relative to the direct sound. From Hoffman et al (1994).

Source: Dillon (2001): Hearing Aids

Microphone array benefit

Figure 7.8 Blind source separation of two sources, S1 and S2, occurs when the two adaptive filters, G1 and G2, adapt to the response shapes that compensate for the room transmission characteristics, R1, R2, R3 and R4, from each source to each microphone. Note that everything to the right of the dotted line is in the hearing aid, whereas the blocks to the left are the transfer functions of the transmission paths within the room. When properly adapted, the response of G1 = R3/R1 and G2 = R4/R2. The output Y1 then does not contain any components of S2. The blocks G1 and G2 can alternatively be feed-forward blocks rather than feed-back blocks.

+S1

S2

+

+

AdaptorG

2

G1

R1

R4

R3+

+

+R2

Y1

Y2

Source: Dillon (2001): Hearing Aids

Blind source separation

Figure 7.9 A Wiener Filter incorporating a Fourier Transform (F.T) to calculate the spectrum of the combined speech and noise. A speech/non-speech detector classifies the spectrum as noise or speech plus noise, and thus enables the average spectral power of the speech to be estimated.

Averager

F.T.

Speech/non-speechdetector

Averager

-

+

Avg speech

Spectrum

Avg speechplus noisespectrum

Speechplusnoise

Switch

Input

÷

Noise

Source: Dillon (2001): Hearing Aids

Wiener filter noise reduction

Figure 7.10 A Spectral Subtraction noise reduction system incorporating a Fourier Transform to calculate the power spectrum, a speech/non-speech detector to enable the average spectral power of the noise to be estimated, and an Inverse Fourier Transform to turn the corrected spectrum back into a

waveform.

Speech/non-speechdetector

Switch Averager-

+F.T.

Avg noiseSpectrum

Phase

I.F.T.Magnitude

Source: Dillon (2001): Hearing Aids

Spectral Subtraction

0

10

20

30

40

Frequency (Hz)

He

arin

g a

id g

ain

(d

B)

8000400020001000500250125

Figure 7.11 The gain-frequency response of a (hypothetical) four-channel hearing aid, where feedback oscillationhas been avoided by decreasing the gain of the band from 2 kHz to 4 kHz (solid line) from the original response (dotted line).

Source: Dillon (2001): Hearing Aids

Feedback management

-40

-30

-20

-10

0

100 1000 10000Frequency (Hz)

Loop

gai

n (d

B)

300 3000

-180

-90

0

90

180

100 1000 10000Frequency (Hz)

Loop

pha

se (

degr

ees)

300 3000

Figure 7.12 Gain-frequency and phase-frequency response of the complete feedback loop for an ITE hearing aid. Redrawn from Hellgren et al., (1999).

Source: Dillon (2001): Hearing Aids

Feedback-loop response

Figure 7.13 Internal feedback path added to cancel the effects of the external, unintentional leakage path.

Internal feedback path

-

External leakage path

+

Source: Dillon (2001): Hearing Aids

Frequency

Inte

nsity

Frequency

Inte

nsity

1000250 4000

1000250 4000

Figure 7.14 Input and output spectra for a frequency transposition scheme in which the output frequency equals half the input frequency. The amplifier also provides some high frequency pre-emphasis. The arrows show the reduction in frequency of each formant.

Source: Dillon (2001): Hearing Aids

Transposition

Time (seconds)

Spectral enhancement

Figure 7.15 Spectrograms of the syllable /ata/ (a) unprocessed and (b) spectrally enhanced, showing more pronounced formants (Fisher, Dillon & Storey, in preparation).

Fre

quen

cy (

kHz)

(b)

Time (ms)

0

2

4

0

2

4

0 200 400 600 800

(a)

Source: Dillon (2001): Hearing Aids