RHYTHM R3910
www.onsemi.com
10
more biquad filters, post3 and post4, and the peak clipper.
The last stage in the signal path is the D/A H−bridge.
White noise can be shaped, attenuated and then added into
the signal path at two possible locations: before the volume
control (between the wideband gain and the volume control)
or after the volume control (between post 4 and the peak
clipper) as shown in Figure 1.
Functional Block Description
iSceneDetect 1.0 Environment Classification
The iSceneDetect feature, when enabled, will sense the
environment and automatically control the enhancement
algorithms without any user involvement. It will detect
speech in quiet, speech in noise, wind, music, quiet and noise
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 aid 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 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 AFC does not
reduce the hearing aid’s gain. The AFC is based on
a time−domain model of the feedback path.
The third−generation AFC (see Figure 5) allows for an
increase in the stable gain (see Note) of the hearing aid while
minimizing artefacts for music and tonal input signals. As
with previous products, the feedback canceller provides
completely automatic operation.
NOTE: Added stable gain will vary based on hearing aid
style and acoustic setup. Please refer to the
Adaptive Feedback Cancellation information
note for more details.
Figure 5. Adaptive Feedback Canceller (AFC)
Block Diagram
G
Σ
H’
H
+
−
Feedback path
Estimated feedback
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.