RHYTHM SB3231

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8

During the Parameter Map creation, constraints are

applied to the compression parameters to ensure that the I/O

characteristics are continuous. Parameter adjustments

support two popular styles of compression ratio adjustment:

the upper threshold. As the compression ratio trimmer

is adjusted, high−level gain remains constant while the

low−level gain changes.

the lower threshold. Low−level gain remains constant

as the compression ratio trimmer is adjusted.

The squelch region within each channel implements a low

level noise reduction scheme (1:3 expansion) for listener

comfort.

This

scheme

operates

in

quiet

listening

environments (programmable threshold) to reduce the gain

at very low levels.

Automatic Telecoil

The automatic telecoil feature in Rhythm SB3231 is to be

used with memory D programmed with the telecoil or

MIC + TCOIL front end configuration. The feature enables

the part to transition to memory D upon the closing of

a switch connected to MS2. With the feature enabled and

a reed switch connected to MS2, the static magnetic field of

a telephone handset will close the switch whenever the

handset is brought close to the device, causing the hybrid to

change to memory D. The part will transition back to the

initial memory once the switch is deemed opened after

proper debouncing.

A debounce algorithm with a programmable debounce

period is used to prevent needless switching in and out of

memory D due to physical switch bounces when MS2 is

configured for automatic telecoil. Upon detecting a close to

open switch transition, the debounce algorithm monitors the

switch status. The debounce algorithm switches the device

out of memory D only once the switch signal has been

continuously sampled open over the specified debounce

period.

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, Rhythm

SB3231’s 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 6) allows for an

minimizing artefacts for music and tonal input signals. As

with previous products, the feedback canceller provides

completely automatic operation.

1. 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 6. 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 64−band filter bank (32 bands at 16 kHz

sampling) 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 64 bands.

The noise reduction gain applied to a given band is

determined by a combination of three factors:

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

each band, the masking threshold variations resulting from

the energy in other adjacent bands is taken into account.

Finally, the noise reduction gain is also adjusted to take

advantage of the natural masking of ‘noisy’ bands by speech

bands over time.

Based on this approach, only enough attenuation is

applied to bring the energy in each ‘noisy’ band to just below

the masking threshold. This prevents excessive amounts of

attenuation from being applied and thereby reduces

unwanted artifacts and audio distortion. The Noise