SA3291_16 Datasheet, PDF (10/21 Pages) ON Semiconductor – Preconfigured Wireless DSP System

AYRE SA3291
environments and make the necessary adjustments to the
parameters in the audio path, such as ADM, ANR, WDRC,
FBC, in order to optimize the hearing aid settings for the
specific environment.
iSceneDetect will gradually make the adjustments so the
change in settings based on the environment is smooth and
virtually unnoticeable. This feature will enable the hearing
aid wearer to have an instrument which will work in any
environment with a single “memory”.
EVOKE Advanced Acoustic Indicators
Advanced acoustic indicators provide alerting sounds that
are more complex, more pleasing and potentially more
meaningful to the end user than the simple tones used on
previous products. The feature is capable of providing
pulsed, multi−frequency pure tones with smooth on and off
transitions and also damped, multi−frequency tones that can
simulate musical notes or chords.
A unique indicator sound can be assigned to each of the
ten system events: memory select (A, B, C, D, E or F), low
battery warning, digital VC movement and digital VC
minimum/maximum. Each sound can consist of a number of
either pure tones or damped tones but not both.
A pure tone sound can consist of up to four tones, each
with a separate frequency, amplitude, duration and start
time. Each frequency component is smoothly faded in and
out with a fade time of 64 ms. The start time indicates the
beginning of the fade in. The duration includes the initial
fade−in period. By manipulating the frequencies, start times,
durations and amplitudes various types of sounds can be
obtained (e.g., various signalling tones in the public
switched telephone network).
A damped tone sound can consist of up to six tones, each
with a separate frequency, amplitude, duration, start time
and decay time. Each frequency component starts with
a sudden onset and then decays according to the specified
time constant. This gives the audible impression of a chime
or ring. By manipulating the frequencies, start times,
durations, decays and amplitudes, various musical melodies
can be obtained.
Acoustic indication can be used without the need to
completely fade out the audio path. For example, the
low−battery indicator can be played out and the user can still
hear an attenuated version of the conversation.
Adaptive Feedback Canceller
The Adaptive Feedback Canceller (AFC) reduces
acoustic feedback by forming an estimate of the hearing aid
feedback signal and then subtracting this estimate from the
hearing aid input. The forward path of the hearing aid is not
affected. Unlike adaptive notch filter approaches, the Ayre
SA3291’s AFC does not reduce the hearing aid’s gain. The
AFC is based on a time−domain model of the feedback path.
The Ayre SA3291 third−generation AFC provides an
increase in added stable gain and minimal artefacts for music
and tonal input signals. As with previous products, the
feedback canceller in the Ayre SA3291 provides completely
automatic operation. The feedback canceller can be
activated in any front−end mode except for Telecoil−only or
DAI−only mode.
When the AFC is enabled, it is highly recommended that
you either have all channels with Squelch ON or all channels
with Squelch OFF. If you choose to have all channels with
Squelch ON then there is an additional requirement to have
all Squelch thresholds above the microphone noise floor. If
you require any assistance in determining what threshold
levels to set, please contact the applications department at
ON Semiconductor. Squelch ON/OFF does not incur any
current penalty. When Squelch and AFC are both ON, the
Squelch is limited to 1:2 expansion.
Feedback path
H
+
Σ
G
−
H’
Estimated feedback
Figure 5. Adaptive Feedback Canceller (AFC)
Block Diagram
Feedback Path Measurement Tool
The Feedback Path Measurement Tool uses the onboard
feedback cancellation algorithm and noise generator to
measure the acoustic feedback path of the device. The noise
generator is used to create an acoustic output signal from the
hearing aid, some of which leaks back to the microphone via
the feedback path. The feedback canceller algorithm
automatically calculates the feedback path impulse response
by analyzing the input and output signals. Following
a suitable adaptation period, the feedback canceller
coefficients can be read out of the device and used as an
estimate of the feedback−path impulse response.
Adaptive Noise Reduction
The noise reduction algorithm is built upon a high
resolution 128−band filter bank enabling precise removal of
noise. The algorithm monitors the signal and noise activities
in these bands, and imposes a carefully calculated
attenuation gain independently in each of the 128 bands.
The noise reduction gain applied to a given band is
determined by a combination of three factors:
• Signal−to−Noise Ratio (SNR)
• Masking threshold
• Dynamics of the SNR per band
The SNR in each band determines the maximum amount
of attenuation to be applied to the band − the poorer the SNR,
the greater the amount of attenuation. Simultaneously, in
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