Transcript Noise reduction algorithms

NOISE REDUCTION STRATEGIES
Wave 4 SFR 17/05/04
MCHAS
Modernising Children’s Hearing Aid Services
NOISE REDUCTION STRATEGIES
Multi-microphone technology
Noise Reduction Algorithms
Expansion/Squelch
Multi-channel compression i.e. BILL
MCHAS
Modernising Children’s Hearing Aid Services
What’s The Problem?
 Interference from background noise represents the
problem
 To improve speech perception in noise, hearing aids
need to improve the signal-to-noise ratio
 Hearing impaired individuals require a greater S/N ratio
than normal hearing listeners
 Hearing aids with omnidirectional microphones amplify
sounds from all directions in the listeners environment
equally
MCHAS
Modernising Children’s Hearing Aid Services
Multi-Microphone Technology
Directional amplification is the most effective
way to improve signal-to-noise-ratios in hearing
aids.
Percent words correct
Effect of decreasing noise
100
80
60
40
20
0
-4
-3
-2
-1
0
1
2
3
Signal to noise ratio (dB)
4
Dillon 2001
BKB sentences: Moore, Johnson, Clark & Pluvinage, 1992
MCHAS
Modernising Children’s Hearing Aid Services
Directional microphones enhance the
hearing instrument’s sensitivity to sounds
from desired direction over sound from
other directions.
It is the only technology that can
effectively distinguish between desired
and undesired speech signals. It is
therefore quite effective in so called
‘cocktail party’ situations.
MCHAS
Modernising Children’s Hearing Aid Services
Directional Microphone
Rear sounds
entering back
port are
electronically
delayed so…….
….. rear sounds
entering the front
port impinge on the
diaphragm at the same
time. Hence the
diaphragm does not
move.
Important
information
Adapted from
Dilion 2001
MCHAS
Modernising Children’s Hearing Aid Services
Dillon 2001
Dual Microphones
 Similar principle to single
directional microphone
but has two microphones
with 2 separate ports.
 Important users have the
option to switch between
omni-directional &
directional amplification.
MCHAS
Modernising Children’s Hearing Aid Services
The 2 microphones can be either:
– 2 omni-directional ones whose outputs are
coupled with electronic time delay and
subtraction components to make them
directional. When omni- direction is needed
just one of the microphones is used.
– 1 directional microphone (which has a front
and rear port) and the other an omnidirectional microphone.
The directional sensitivity of a microphone
is usually indicated on a polar plot.
MCHAS
Modernising Children’s Hearing Aid Services
MCHAS
Modernising Children’s Hearing Aid Services
Microphone Arrays
FIXED MICROPHONE ARRAYS
Current hearing aids implement variable
polar patterns in one of two ways
The first method introduces circuitry that
allows either the user or the hearing aid
programmer to select the desired
directional pattern (presently, none of the aids
on contract have this option)
Such a system means that there will be
the same polar pattern in all situations
MCHAS
Modernising Children’s Hearing Aid Services
Microphone Arrays
 ADAPTIVE MICROPHONE ARRAYS
 The second method is designed to
adaptively switch between polar patterns
in response to the listening environment.
 In noisy environments adaptive
instruments automatically shift polar
patterns in an attempt to maximize S/N
ratio in the presence of other noise
sources.
MCHAS
Modernising Children’s Hearing Aid Services
Microphone Arrays
 It may appear that adaptive directional processing has
the potential to improve speech recognition across noisy
environments
 However, little evidence to suggest more beneficial than
fixed directional processing
 Ricketts & Henry (2002) found advantages only in
specific environmental listening situations
 Bentler et al (2003) found directional better than
omnidirectional but adaptive no better than fixed.
MCHAS
Modernising Children’s Hearing Aid Services
A Word of Caution
Increased circuit noise in directional mode
Less effective as speaker-listener distance
increases
Echoes/reverberation
Microphone drift
Dirt!
MCHAS
Modernising Children’s Hearing Aid Services
Patient selection
Everyone benefits from increased SNR.
Younger children must be able to hear
sounds from all directions so must have
access to omni-directional microphone.
Directional depends on maturity
Those with good low frequency hearing
will hear low frequency noise through
vents therefore the benefit of the
directional microphone will be reduced.
MCHAS
Modernising Children’s Hearing Aid Services
Phonak Aero 211, 311 & 413 AZ
 Provides omnidirectional and fixed directional
(supercardiod) microphone performance. Either
microphone can be implemented in specific programs.
MCHAS
Modernising Children’s Hearing Aid Services
Danalogic 283D
 Provides omnidirectional and fixed directional
(hypercardiod) microphone performance. Accessible in
all programs.
MCHAS
Modernising Children’s Hearing Aid Services
Prisma 2M & 2D SP
Directional
microphone
accessible in both
channels.
Fixed supercardiod
Self calibrating to
avoid microphone
drift (uses 2 omnidirectional mics
coupled together)
MCHAS
Modernising Children’s Hearing Aid Services
Oticon Spirit II D
Directional in program
2 only, and can be
disabled.
Fixed supercardiod
array.
In program 2, can
access different
degrees of LF cut.
MCHAS
Modernising Children’s Hearing Aid Services
Noise Reduction Algorithms
 2 types of noise reduction systems
Modulation Detection & Synchrony
detection
We will concentrate on modulation
detection as it is far more common at
present.
MCHAS
Modernising Children’s Hearing Aid Services
Modulation Detection
Underlying algorithms may vary between
manufacturer’s but overall aim is to
provide improved listener comfort and
possibly improved speech intelligibility in
background noise.
Common to all algorithms is this need to
identify which signal is noise and which is
speech.
MCHAS
Modernising Children’s Hearing Aid Services
Modulation Detection
A speech/non speech detector analyses
fluctuations in signal amplitude.
Speech & noise envelopes fluctuate in a
well characterised manner:
– Speech modulations tend to be slow and have
big amplitude fluctuations.
– Noise modulations tend to be more constant
with rapid and smaller fluctuations.
MCHAS
Modernising Children’s Hearing Aid Services
Envelope of speech + noise
MCHAS
Modernising Children’s Hearing Aid Services
Modulation Detection
 The precise way in which the hearing aid
processes the two signals will differ
depending on the algorithm.
 One common approach is to:
1. Estimate when the speech signal is present
in each channel.
2. If the amplitude envelope at a channel’s
output is characteristic of speech the gain in
that channel remains unaltered.
MCHAS
Modernising Children’s Hearing Aid Services
Modulation Detection
3. If the envelope is relatively constant then
the signal is assumed to be noise and the
gain for that channel is reduced.
4. If more noise is detected than speech at a
certain channel, then the gain will also be
reduced.
MCHAS
Modernising Children’s Hearing Aid Services
Important points to note:
– If the unwanted signal has similar fluctuations to
speech, then it is unlikely to be attenuated.
– Best results occur with steady state noise that has a
narrow frequency band, or is of low frequency (to
reduce the risk of upward spread of masking)
– If any other ‘constant’ amplitude signals (e.g. pure
tones) are identified then they will also be treated as
a ‘noise’ signal and be attenuated. This means that
pure tones should not be used to either set or test
the hearing aid, unless the noise reduction feature
has been turned off.
MCHAS
Modernising Children’s Hearing Aid Services
Modulation Detection:
Advantages and Limitations
ADVANTAGE: Channel specific gain
reductions to ensure comfort whenever
noise is present
LIMITATION: Cannot distinguish between
noise only versus speech plus noise
Tryanski & Pogash
(AAA 2204)
MCHAS
Modernising Children’s Hearing Aid Services
Synchrony Detection
Assumption: you can find speech versus
noise but you cannot separate them
Designed to look for the unique structure
of speech (energy)
Maintains full response whenever speech
is present….only goes into comfort mode
when speech is no longer present
Tyranski & Pogash
(AAA 2004)
MCHAS
Modernising Children’s Hearing Aid Services
Synchrony Detection:
Advantages & Limitations
ADVANTAGE: Can protect audibility of
speech whenever speech is present
LIMITATION: Anytime speech is present
there will be no comfort-based changes,
even if noise present and high level
Tyranski & Pogash
(AAA 2004)
MCHAS
Modernising Children’s Hearing Aid Services
The Future of Noise Reduction?
One company have recently launched a
system that combines the two……??
MCHAS
Modernising Children’s Hearing Aid Services
Conclusions
 Very little research on NR systems and none at all with the
paediatric population
 Alcantara et al (2003) found there to be no improvement in sound
quality or listening comfort with noise reduction turned on
 Boymans & Dreschler (2000) found no extra benefit with NR &
Directional when compared to directional alone
 Therefore, research published to date would suggest that directional
microphone systems lead to a better improvement in the S/N ratio
MCHAS
Modernising Children’s Hearing Aid Services
Phonak Aero 211AZ, 311AZ & Supero 413
digital Noise Canceler (dNC)
 Light indicates up to 3 dB gain



reduction
Moderate indicates up to 6 dB gain
reduction
Strong indicates up to 12 dB gain
reduction
OFF deactivates the dNC in any
program
MCHAS
Modernising Children’s Hearing Aid Services
Danalogic speech/ noise analysis
in 14 frequency bands
Noise Noise Speech Speech Speech Speech Noise
Noise Speech Speech Speech Speech Speech Noise
reduced
gain
normal gain
MCHAS
Modernising Children’s Hearing Aid Services
reduced
gain
Danalogic 283D Noise Reduction
Uses modulation or level fluctuations to identify ‘speech’ from
noise.
Noise Reduction either
ON or OFF
MCHAS
Modernising Children’s Hearing Aid Services
Prisma 2M & 2D SP
 Can be deactivated.
 Reduction of gain can be
altered to ‘min=12 dB’,
‘med=18 dB’ and
‘max=24 dB’. Defaults to
min.
 Gain reduction is
prioritised in channel 1 &
4.
Off
Min
Med
Max
C1
min
med
max
C2
min
med
C3
min
med
C4
min
med
max
MCHAS
Modernising Children’s Hearing Aid Services
Starkey Strata 312
Off = 0dB reduction
Min = Up to a 10dB
channel gain
reduction (default)
Max = Up to a 20dB
channel gain
reduction
MCHAS
Modernising Children’s Hearing Aid Services
Oticon Spirit II
No control over settings.
Identifies speech & noise signals based on
modulation index.
If signal characteristic of noise the release
time will be delayed by up to 4 times. If
characteristic of speech there will be no
change in release time.
MCHAS
Modernising Children’s Hearing Aid Services
Expansion/Squelch
Expansion or squelch occurs when the
compression ratio is less than 1:1,
therefore gain decreases as the input level
decreases.
It is useful for decreasing the audibility of
very low level sounds. Good as long as
ALL inaudible sounds are unwanted e.g.
microphone internal noise, computer hum.
MCHAS
Modernising Children’s Hearing Aid Services
Expansion & Squelch
By removing low level electronic noises it
can be thought of as a low level noise
reduction strategy.
Important to distinguish between ‘true
noise reduction algorithms’ and squelch.
NHS DSP Aids with expansion/squelch:
– Phonak Aero, Oticon Spirit II, Starkey Strata.
MCHAS
Modernising Children’s Hearing Aid Services
Multi-channel compression- BILL
BILL compression circuits were designed to
improve speech intelligibility in background noise
by providing more gain for soft low frequency
sounds than for loud low frequency sounds.
Based on the premise that background noise is
predominantly low frequency, quite loud and
hence would be attenuated while high frequency
sounds would remain unaffected.
MCHAS
Modernising Children’s Hearing Aid Services
Multi-channel compression- BILL
Any aid that has……
– Multi-channel compression
– Adjustable kneepoints
– Adjustable compression ratios
………. could be configured to a BILL
response.
MCHAS
Modernising Children’s Hearing Aid Services
Questions?
MCHAS
Modernising Children’s Hearing Aid Services