Transcript Document

Digital Signal Processing,
compression, linear and
nonlinear: terminology,
measurement and issues.
Richard Baker
University of Manchester
Outline
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A few common misconceptions
What is signal processing?
Advantages of going digital
Analogue to digital conversion
Compression – why and how?
Measurement issues
Common Misconceptions
• “Only digital hearing aids are signal processing
aids”
• “Digital is better than Analogue”
• “Wide dynamic range compression (WDRC) =
digital”
• “Nonlinear = digital”
• “Programmable hearing aids are the same as DSP
hearing aids”
• “Digital hearing aids cut out background noise”
What is signal processing?
• Signal processing is exactly what it says, it may
be:
– Amplifying
– Filtering
– Peak-clipping
– Compression: output limiting, WDRC, etc
– Frequency shifting
–…
– etc.
What is a digital hearing aid?
•
A digital hearing aid simply converts the signal
to a numerical form before processing it
ANALOGUE
AD
MIC.
•
LP
(Anti-aliasing)
ANALOGUE
DIGITAL
DSP
DA
LP
SPEAKER
(reconstruction)
It’s the signal processing algorithm that is
important
What is compression?
• Compression:
– the range of input sound intensities is “squashed”
into a smaller range of output intensities
– e.g. a range of input intensities from 0 to 100 dB
SPL may be compressed into an output range of
50 to 100 dB SPL
– The output “dynamic range” is reduced compared
to that of the input
Why do we need compression?
• Sensorineural hearing loss most often results
from damage to outer hair cells in the cochlear
• This results in:
– Loss of sensitivity at low sound intensities
– Abnormally rapid growth of loudness (recruitment)
– Loss of frequency selectivity (Hearing aids can’t
do much about this one at the moment)
Loudness Growth
• Typically, sensorineural loss results in
recruitment:
– Low intensity sounds are inaudible
– Moderate intensity sounds are heard as very quiet
– High intensity sounds are perceived as similar in
loudness to that normal hearing listener
• Implications for hearing aids
– High gain for low intensity input
– Low gain for high intensity input
– i.e. reduced dynamic range at output compared to
input
Compression
Normal
Impaired
Intense
Nonlinear
Moderate
Weak
Dillon (2001)
Hearing aid goals
• Audibility - be able to hear important sounds e.g.
speech
• Comfort - sounds comfortably loud
• Safety - sounds prevented from being too loud
• Intelligibility - maximise the intelligibility of
speech sounds
• Quality - maximise the perceived quality of the
sounds (e.g. little distortion)
• Consistency - same performance regardless of
listing conditions
• ...
• The same aims apply to both linear and nonlinear aids
Linear versus nonlinear
• Linear - gain is constant irrespective of input
level (if we ignore very high levels)
• Nonlinear - gain changes as input level changes
(may be compression or expansion)
• Remember, when talking in dB terms:
Output level = Input level + gain
Linear hearing aids
• Amplify all sounds by the same amount
• Problem – louder sounds become too loud to be
comfortable
• Solution – use some type of limiting to prevent
this
• e.g. clip the peaks off the waveform when it goes
too loud - peak clipping – causes distortion
Peak clipping
Peak-clipping
peaks clipped resulting
in distorted waveform
duration of increased signal intensity
0
0.05
0.1
0.15
0.2
time (ms)
0.25
0.3
0.35
The need for compression
• The problem with linear aids – the same gain is
applied to all levels of input signal
• we need high gain for low input levels, and low
gain for high input levels - compression
• we need some way of automatically turning down
the gain of the hearing aid as the input intensity
increases
• an automatic gain control or AGC
Automatic gain control (AGC)
• AGC parameters
• Attack-time – The time taken for the AGC to respond
to an increase in input level
• Release time – the time taken for the AGC to
increase the gain again when the input level
decreases
• Knee-point – below a certain signal intensity the
amplifier behaves linearly, above this intensity the
compression operates
• Compression ratio – above knee-point, output with an
increase in input is typically less than 1 dB per dB
change in input
Automatic gain control
AGC
attack time - time taken for gain to be turned down by AGC
release time - time taken for AGC to come
out of compression and restore original gain
duration of increased signal intensity
0
0.05
0.1
0.15
0.2
time (ms)
0.25
0.3
0.35
I/O functions, output spectra &
transfer functions etc.
• I/O functions - output vs input
– at one frequency
• Output spectra - output across frequency
– at one input level
• input/gain function - gain vs input
– at one frequency
• Transfer function - output/input (i.e. gain) across frequency
– at one input level
• All ways of plotting different aspects of hearing aid function
• Input-output function
• Output spectra
Types of compression
The main compression strategies fall into two
categories:
• Compression limiting – high knee-point, high
compression ratio (e.g. 10:1) – limits MPO
• WDRC – wide dynamic range compression, low
knee-point, low compression ratio (e.g. 2:1) –
aims to restore loudness perception in moderate
loss
• AVC - automatic volume control - slow acting compression
designed to adjust overall gain when moving from quiet to
noisy environment.
Output limiting
output (dB SPL)
output limiting
knee-point at 70 dB SPL input
120
output limiting
110
100
90
linear
+50 dB
80
+40 dB
70
+40 dB gain
+30 dB
60
+20 dB
+10 dB
+0 dB
50
input (dB SPL)
30
40
50
60
70
80
90
100
WDRC
output (dB SPL)
WDRC
knee-point at 40 dB SPL input
120
110
limiting at
105 dB SPL
output
100
90
+50 dB
compression ratio
of 2:1
80
+40 dB
70
+40 dB gain
+30 dB
60
+20 dB
+10 dB
+0 dB
50
input (dB SPL)
30
40
50
60
70
80
90
100
output (dB SPL)
50 dB
65 dB
80 dB
linear + PC
110
WDRC
100
90
80
70
Linear & WDRC aids set
to have same gain at 65 dB SPL
input.
+50 dB
Note. Linear aid under-amplifies
at 50 dB SPL input and
over-amplifies at 80 dB SPL input
+40 dB
+30 dB
60
+20 dB
+10 dB
+0 dB
50
input (dB SPL)
30
40
50
60
70
80
90
100
• Therefore need to test at different levels:
– 50 dB SPL input - quite speech level
– 65 dB SPL input - moderate speech level
– 80 dB SPL input - loud speech level
Multi-channel processing
Why multi-channel?
• different hearing losses at different frequencies
• different compression strategies required for
different frequency ranges
• theoretical reasons for differing frequency
response
• …
• … e.t.c.
From Killion et al, 1990
Test signals
• Pure-tone - single frequency component
• Swept-tone - pure-tone swept up or down in frequency
• Speech-weighted pure-tone sweep - swept-tone following the
spectral shape of an average speech signal
• White-noise - noise signal containing equal energy at all
frequencies
• Pink-noise - noise with energy decreasing with increasing
frequency
• Speech-shaped noise - noise with spectral shape of an average
speech signal
• Modulated Speech shaped noise - spectral AND temporal shape
similar to that of speech
Test signals
• Test signals can be either:
– Continuous - long(ish) duration with approximately
constant amplitude
– Fluctuating - varying up and down in amplitude
(usually designed to mimic temporal fluctuations in
natural speech)
• Least natural:
• Most natural:
continuous pure-tone
fluctuating speech shaped noise
Which signal to use?
• With a linear aid pure-tone test signals should
produce the same results as noise signals
• With non-linear aids, the aid can respond very
differently to different signals
Which signal to use?
• e.g. in some situations, pure-tones may produce
an artificially high measurement of low frequency
gain - “blooming”
– Suppose a compressor follows a high-pass filter
– A tone is swept upwards in frequency through the
cut-off region of the filter into the pass-band
– As the tone is in the cut-off region the input to the
AGC is low - thus the gain is high
– In the pass-band the input to the AGC is high so
the gain is low
– Result: Using a swept tone it appears that the lowpass filter isn’t working –
– use a broad-band signal!
blooming!
So, use a broad-band signal!
Which signal to use?
• e.g. swept-tone versus noise
– Pure-tone - single frequency component
therefore level well defined
– White-noise - many frequency components measured level is sum of frequency
components therefore level at one particular
frequency is lower
– Overall level with noise signal also depends on
analysis bandwidth
Implications of different signals
1. Output display for broadband signals is lower
than tones - use gain display!
2. Output display depends on analysis bandwidth
3. For multichannel aids swept tone gives higher
level signal through each band than broadband
noise
• At high levels tone may result in saturation
whereas noise doesn’t
• Nonlinear aids may have different gain for tones
& noise even though they are nominally the
same overall level
“extras”
• As well as different signal processing strategies
modern hearing aids are available with many
“extras” designed to improve their performance
• These also have implications for how the aids are
tested and the signals used…
“extras”
• Noise suppression/cancellation
– Algorithms attempt to “detect presence of speech”
and turn down the gain if no speech is present
– Note
• Need to use realistic speech like signal to
perform measurements – continuous noise will
be suppressed, so need to have speechshaped noise with fluctuating envelope (is such
a signal available?)
• Turn the noise reduction feature off
“extras”
• Multi-program/memory aids
– Can allow 2 or more different processing
algorithms to be used
– E.g. a second setting with extra gain for bouts of
OME
– Note
• Need to know what each of the memories are
supposed to do in order to test aid
“extras”
• Directional/Multi-Microphone technology
– Aims to improve signal-noise ratio by “picking out”
sounds from the front, and reducing those from
other direction
– Note
• Need to be careful how aid is positioned in a
test box to get accurate measurements
• Turn the directional microphone off!
“extras”
• Feedback management/cancellation
– Notch-filters or complex feedback cancellation
algorithms have been developed that can reduce
feedback and allow 10-20dB extra gain.
– This can allow additional gain, use of vents where
they are normally not possible etc.
– Note: awareness of notch-filters is necessary &
the feed-back suppression needs to be turned off
for measurement purposes (is this possible for
every situation?)
Feedback Management
Dillon (2001)
Feedback Cancelling
External leakage path
+

-
Internal
feedback path
Dillon (2001)
Implications
• conceptual complexity - difficult to understand
what the aid is doing
• complexity & adjustability - many different
parameters to adjust to set up the aid
• lack of user adjustability - some nonlinear aids
have no volume control - WDRC, in theory, should
do away for the need for it
• test signal - need to chose the right test signal
• lack of defined standards - no clearly defined
standards for measuring nonlinear aids
Ideal vs reality for testing aids
• Ideal situation:
– full test-box & programming facility, ability to turn
off “extras”, modulated speech-shaped noise as
test signal
• Likely situation for some (eg outreach or other
services?):
– “old” test-box, no programming facility, can’t turn
off “extras”, only continuous pure-tone or swept
pure-tone available
Summary
• Signal processing
• Compression
– Fits dynamic range of sounds into comfortable
range of hearing
– AGC
– Types of compression – output-limiting, WDRC
• Multi-channel processing
• Implications
– conceptual, complexity, test-signals
•
•
References
– Dillon, H. (2001) Hearing Aids, Thieme
– Sandlin, R.E. (2000) Hearing Aid Amplification, Singular
– Vonlanthen, A. (2000) Hearing Instrument Technonogy, Singular
– Venema, T. (1998) Compression for Clinicians, Singular
– Killion, M.C., Staab, W. & Preeves, D. (1990) Classifying automatic
signal processors. Hearing Instruments, 41(8), 24-26
– Seewald, R. C (2001), A Sound Foundation Through Early Amplification
2000, Phonak AG, ISBN: 3-9522009-0-5
– Seewald, R. C. & Gravel, J.C. (2002), A Sound Foundation Through
Early Amplification 2001, Phonak AG, ISBN: 3-9522009-1-3
Standards
– BS EN 61669:2001 Electroacoustics – Equipment for the measurement
of real-ear acoustical characteristics of hearing aids
– BS ISO 12124:2001 Acoustics – Procedures for the measurement of
real-ear acoustical characteristics of hearing aids